For ATX power supplies (standard PC power supplies):
- Fuses generally aren't needed for PC ATX-type power
supplies, because these usually have built-in overload protection
- The built-in protection in an ATX supply is typically
provided by a thermal "resettable fuse" that only cuts power temporarily,
and automatically resets itself after the overheated parts cool off
Fuse selection quick reference
Here are some recommendations for fuses for the most common pin cab
devices. For each device, we list the electrical specs you should
look for, and provide an example of a fuse that meets the
requirements. You don't have to use that exact fuse, just one that
meets the specs listed. We'll explain what the specs mean and how we
came up with them later in the chapter.
Fuses and ratings marked with asterisks (*) are examples only, because
they're for component type that vary so much that no one fuse will be
right for every example.
LedWiz output port (no boosters)
Bel Fuse 2JQ 500-R
Zeb's LedWiz booster board output port
Bel Fuse 2JQ 4-R
Pinscape power board output port
Bel Fuse 2JQ 5-R
USB relay board (e.g., Sainsmart) output port
|5A* (Use Max Amps for relay switch)||≥50VDC**||Normal
Bel Fuse 2JQ 5-R *
Replay knocker & other pinball coils
Bel Fuse 3JS 1.5-R TR
OEM/eBay/generic power supply
|10A* (Use PSU's Max Output Amps)||≥PSU output volts DC||Normal
Littelfuse 0AGC010.V *
|5A* (Use converter's Max Output Amps)||≥Converter output volts DC||Normal
Bel Fuse 2JQ 5-R *
* means that this is an example only, because this type of equipment
varies. Check the actual Max Amps ratings on your equipment in these
cases and substitute appropriate fuses if necessary.
** I used a blanket 50VDC recommendation for the sake of simplicity.
This is high enough for anything commonly used in a pin cab, including
knocker coils and other real pinball coils. It's actually difficult
to find fuses rated for such high DC voltage, though; it's much easier
to find 32VDC fuses, since that rating is used for virtually all
automotive fuses. You can safely use 32VDC-rated fuses in any circuit
where your actual power supply voltage is 32VDC or below.
How to choose a fuse
Fuses aren't one-size-fits-all. You have to choose fuses according
to the electrical specs of the circuits they're protecting. Each
circuit has its own requirements, so you might need different fuses
for different circuits.
Current (Amps). This is the most important number when choosing
a fuse. The whole purpose of a fuse is to limit the amount of current
that's allowed to flow in a circuit, so choose a fuse for each circuit
according to the maximum safe current for the components in the
A fuse's current rating tells you maximum the fuse will allow without
blowing. For example, a fuse rated for 5A will allow up to 5A to
flow, and should never blow as long as the current stays at or below
5A. If the current goes above the rated level, the fuse will blow -
eventually. Not necessarily immediately. If the current is
only a little above the limit, the fuse might not blow for minutes or
even hours. The higher the current goes above the limit, the faster
the fuse will blow.
In a pin cab, most of our fuses are for protecting delicate
electronics, like LedWiz outputs. These can be damaged very quickly
by overloads, so we don't want the fuse to think about it for too long
if the current exceeds the safe level for the device. A rule of thumb
in these cases is to choose a fuse rated for about 75% of the maximum
current for the device. For example, LedWiz outputs can be damaged
above 500mA, so you might look for a fuse rated at around 375mA.
Voltage. The voltage rating on a fuse is a maximum. You don't
have to match your circuit's voltage; you just have to make sure the
fuse is rated for at least the circuit voltage. The voltage
rating has nothing to do with the current limit, so it's fine to use a
fuse with a higher voltage than your circuit uses. For example, a
125VDC fuse is fine to use in a 24VDC circuit.
You should select fuses with DC voltage ratings, since pin cab
circuits are almost all DC. Many fuses are only rated for AC
voltages. You might think this doesn't matter, but the fuse
manufacturers warn that DC ratings are more stringent than AC, so a
fuse that hasn't been rated for DC use might not properly stop current
flow if used in a DC circuit. All of the fuses linked in this chapter
Speed. Fuses come in two main speed classes: normal fuses that
act quickly, and "slow-blow" fuses that act on a time delay.
Slow-blow fuses are designed to withstand brief current overloads, for
a period of several seconds to a couple of minutes, depending on how
big an overload occurs. Normal fuses, in contrast, act quickly when
the current goes over the limit.
Some manufacturers also make extra-fast fuses that operate even
more quickly than the "normal" type, to protect especially sensitive
circuits that can't tolerate even brief current surges.
The terminology for "fast" and "slow" can be confusing when shopping,
because the terms aren't always used consistently. Some sellers use
"fast" to refer to the extra-fast type, whereas others simply use
"fast" for everything that's not "slow". When extra-fast fuses are in
the mix, sellers will usually also offer "medium" or "normal" fuses,
which refer to the regular fast-blow type, as opposed to extra-fast.
If a site you're shopping at only offers "fast" and "slow" categories,
you can probably assume that the "fast" ones are only the normally
Which speed class is best for a pin cab? It depends on the type of
circuit you're protecting. For most circuits, normal fuses (not
time-delayed and not extra-fast) will work well. You might consider
extra-fast fuses in a few situations where small IC chips are part of
the power circuit, specifically for LedWiz controllers, since the
small driver chips on those devices can fail quickly when overloaded.
Slow-blow fuses are useful for circuits driving pinball solenoids,
such as replay knockers or chimes. Pinball coils are specifically
designed to operate at intermittent "overload" levels, which is
exactly what slow-blow fuses are good for, since they only act when an
overload is sustained for an extended time period. We'll offer some
advice on selecting slow-blow fuses for solenoid circuits in the
section on coils below.
What if a fuse keeps blowing?
If a fuse in your cab blows repeatedly in normal use, one of two
things could be going on. One possibility is that something's wrong
with the circuit that's causing an intermittent overload. The other
is that you just need a fuse with a slightly higher limit.
To debug this kind of situation, I'd always start by assuming the
worst - that there's a fault in the circuit that's causing a real
overload situation. Carefully check the circuit for potential
problems. If the fuse blows seemingly at random, check for the sorts
of things that can cause intermittent problems, such as loose wires.
If you can't find anything wrong, the next thing I'd do is
double-check the current load of the circuit to make sure it's within
the expected limits. For example, if this is a solenoid feedback
device, use a multimeter to check the current drawn by the solenoid,
and make sure that it's below the limit for the output controller
port. If the device draws more current by design than the output
controller allows, you'll have to either get a beefier output
controller or substitute a smaller solenoid.
Assuming that you don't find anything wrong with the circuit, and that
everything is within the expected limits, you might simply have to use
a fuse with a slightly higher rating. For an inductive device like a
motor or solenoid, if you're currently using a normal fuse, try
switching to a slow-blow fuse with the same current rating. If you're
already using a slow-blow fuse, try increasing the current rating on
the fuse. Be conservative; raise it by maybe 25% of the original
rating. Don't iterate this process indefinitely, though: it defeats
the whole purpose of using a fuse if you have to use a fuse with such
a high limit that something else in the circuit blows before the fuse
does. If the problem doesn't clear up with a modest increase in the
fuse rating, I'd go back and check again for faults, and failing that,
I'd consider substituting a different device (a smaller solenoid, for
Fuses come in several physical package types. For pin cabs, I
recommend one of the types that plugs into a socket or holder, since
this lets you replace a blown fuse by simply pulling the old one out
of the socket and plugging in a new one. There are two main options
for these: cartridge fuses and blade fuses.
I prefer cartridge fuses. They're the most widely used type in
electronics in general, so they have the greatest range of options
Note that sizes for these fuses vary. There are about ten size
classes each for the cartridge fuses and the blade fuses. The fuses
linked in this chapter are all cartridge fuses in the 5x15mm (also
known as 2AG) and 6.3x32mm (3AG) sizes. I apologize for not sticking
to just one size: I was hoping to do that to keep things simpler, but
unfortunately I wasn't able to find good matches for everything in one
size, so I ended up with a mix.
Cartridge fuses are designed to plug into sockets or holders, so
you need the holders to complete your installation. There are
several options available; search on Mouser or other electronics
sites for "fuse holder". The type I like is a one-piece plastic
holder, like these:
Note that there are several different sizes of cartridge fuses, so
you'll need holders that match the size you're using. Here are
example holders for the most common sizes (these cover all of
the fuses linked in this chapter):
These have a screw hole for mounting to just about any surface, so you
can mount them directly to your cabinet wall or floor or to a separate
piece of plywood that you can later mount in the cabinet. If you're
using an output controller, I recommend mounting the controller plus
the fuses for all of is outputs on its own plywood carrier. That
lets you do all of the initial wiring on your workbench rather than in the
confined spaces inside your cab. If you're using an LedWiz or Pinscape
board, you'll have a lot of fuses to install, so it makes the work
a lot easier.
Once it's all set up, you can then mount the whole thing in your
cabinet with a couple of screws. This also lets you remove the whole
setup later if you ever need to do any repair or upgrade work.
Note that a key element of a modular setup like this is pluggable
connectors. You'll want to use some kind of mating plug-and-socket
connector to connect all of the wires coming out of the fuses to
the devices inside the cabinet. You can see one of the connectors
I use in the lower right of the picture above; here's a closeup.
That's a connector from the Molex
series, which I found useful for many of the interconnects
in my cabinet. There's more information on these and other options in
. The thing I like about pluggable connectors
like this is that they help avoid dumb mistakes. You only have to
plan out and wire the connectors once, and from then on it's just a
matter of plugging the mating connectors back together any time you
have to do any work that involves removing something. You don't have
to match up the individual wires again, since they're bundled into
connectors that only fit one way.
What to protect
If you're not experienced with electronics (and even if you are), it
can be tempting to add fuses anywhere and everywhere. But every fuse
you add has costs: not just the monetary cost of the fuse, but the
space it takes up, the time it takes to wire, and the additional point
of failure. It's best to limit yourself to circuits that really
benefit from fuse protection.
Let's look at the places in a typical pin cab where fuses are most
If you're using any sort of output controller to attach feedback
devices (such as solenoids, contactors, lights, motors, etc.), it's a
good idea to place a fuse in each individual output circuit. This
is probably the most important place to use fuses in the whole cab,
because it's the place where things are most likely to go wrong.
In this case, the point is to protect the controller. We're
protecting the controller from two things. First, from simple short
circuits. Feedback devices are scattered around the cabinet, so they
often have long wire runs, leaving lots of openings for accidental
shorts. Second, we want to protect the controller from the attached
device. Every output controller has limits on how big a load it can
handle, so we want to make sure that the attached device doesn't draw
too much power, either routinely or due to a malfunction. A fuse
accomplishes that by shutting down the circuit if the power draw goes
over the limit.
In most cases, we don't need to be concerned with protecting the
feedback device itself (the light, motor, etc). That device is
usually the threat, not the victim. If anything else in the circuit
malfunctions, the worst that usually happens as far as the device is
concerned is that it gets turned on at full power. But that's what
it's designed for in the first place, so this usually isn't a threat.
The exception to this rule is pinball coils, which we'll come to
Since we're protecting the output controller, we need to choose a fuse
based on the current limit of the controller:
- LedWiz (with no booster board): 500mA (0.5A) per output.
- Pac-Drive (with no booster board): 500mA (0.5A) per output.
- Pinscape power board: 5A per output.
- Pinscape main board flasher & strobe outputs: 1.5A per output.
- Pinscape main board flipper button LED outputs: no fuses are
necessary, because these outputs have built-in current limiters.
- Pinscape DIY MOSFET output circuit: the limit depends primarily
on MOSFET you choose, but 5A is a safe (conservative) choice for all
of the options we recommend in our circuit plans.
- Zeb's booster board for LedWiz: 5A per output. You don't
need a separate fuse for the LedWiz in this case, because the
booster board isolates the LedWiz.
- PacLed, i-Pac Ultimate I/O: these have built-in current limiters
per output, so fuses aren't required.
- Zeb's booster board for PacLed: 5A per output. You don't
need a separate fuse for the PacLed.
- USB relay boards (e.g., Sainsmart): Check the specs for your board
for the DC current limit for the relay switch. It's usually about 10A.
The wiring for a fuse in an output circuit is the same for all of the
controllers, so we'll just show the LedWiz as an example. The basic
plan is to interpose the fuse between the output controller port and
First, connect a wire between the output port on the controller and
one end of the fuse. Next, connect a wire from the other end of the
fuse to the feedback device (the light, motor, etc). If the device
cares about polarity, this is the negative or "ground" terminal
on the device.
In the diagram above, we used an LED as the example output device, so
there's actually an extra step involved, because LEDs usually require
resistors (that's a whole separate subject, explained in
Feedback Devices Overview
). In this case, the resistor goes between
the fuse and the LED, so we connect the wire from the fuse to one end
of the resistor, and connect a wire from the other end of the resistor
to the output device. For anything but an LED, there's no resistor,
so the wire from the fuse goes straight to the device.
Fuses don't care about polarity, so it doesn't matter which direction
it goes. Resistors don't care either.
Unlike most other feedback devices, real pinball coils can benefit
from fuse protection. As we mentioned above, most other feedback
devices don't need fuses for their own sake; the fuse is to protect
the output controller, not the device itself. But pinball coils are
different. They're designed to be activated only in split-second
bursts. If you turn one on and leave it on, it'll quickly get so hot
that its internal wiring melts, destroying it.
To protect against this danger, we can use a special type of fuse
called a "slow-blow" fuse. "Slow-blow" means that the fuse takes
longer to blow than a regular fuse does. A slow-blow fuse allows a surge of
current to flow briefly, but if the current is sustained for too long,
the fuse blows. This is exactly what we need to protect pinball
coils, which are likewise designed for brief bursts of power, but
can't withstand sustained use.
Why are we even worried about this? We only use these coils to
simulate bumpers and other things that fire momentarily, so why would
the software ever leave them on for long periods? Normally, it
wouldn't. The danger is that the software doesn't always work
perfectly. Sometimes it crashes, and if it crashes at the wrong
moment, it can leave an output stuck on. This isn't just a
theoretical possibility, either: this has actually happened to other
cab builders. It might seem incredibly improbable that the software
would crash at such a perfectly wrong moment, but it's actually an
especially likely time to crash, because it takes special code to fire
a coil in the first place. That code can contain an error that makes
the program crash right after the coil turns on, so the code that was
supposed to turn the coil back off never gets a chance to run, leaving
the coil stuck on.
As an alternative to using fuses, or as a second layer or protection,
you can use a special time limiter circuit. These circuits contain
their own timers that turn the coil off after a couple of seconds,
even if the software doesn't. See Coil Timers
on how to build one of these. If you're using a Pinscape main
expansion board for your knocker, it has this type of timer built in
to the knocker output. All of the outputs on the Pinscape chime board
also have these timers. In my own cab, I use both the timer and the
fuse for my knocker coil. I think of the timer as the primary
protection, but I still like having a fuse as a last resort in case
the timer fails.
The types of pinball coils that can benefit from fuses include:
- Replay knockers
- Chime units
- Bumper assemblies
Flippers are more complicated; more on them below.
How do you choose the right slow-blow fuse to protect a pinball coil?
It takes a little research and some guesswork.
The first step is to figure the normal operating current for the coil.
You need two numbers to figure the current: the voltage you're going
to use to operate the coil, and the coil's electrical resistance. The
voltage is easy: it's the voltage of the power supply you're going to
use with the coil. The resistance is something you can measure with a
multimeter: set your meter to the "resistance" or "ohms" setting and
measure across the coil's terminals. You should measure the coil
while it's not connected to anything else, of course. Here are the
specs for some commonly used knocker coils:
- AE-26-1200: 10.9 ohms
- AE-23-800: 4.2 ohms
Once you have the voltage and resistance, determine the amperage
as Volts/Ohms. For example, if you have a replay knocker with an
AE-26-1200 coil that you'll operate at 35V, the current is
35V/10.9Ω = 3.2A.
Step two is to decide on a time limit. This is balancing act. We
want to pick a time limit that's short enough that the fuse will blow
before the coil overheats, but long enough that the fuse won't blow
during normal operation. The complication is that slow-blow fuses
have inexact timing. They don't give you an exact delay time, just a
range of times.
Pinball coils on real machines fire for a fraction of a second,
anywhere from a few milliseconds to about half a second. We can
consider times in this range to be safe for the coil, so we don't want
our fuse to blow within the first half-second. But how long is too
long? Unfortunately, I don't have any hard data on that. I haven't
been willing to sacrifice a bunch of coils to methodically measure
burn-out times experimentally, and as far as I know, neither has
anyone else. So this is where a bit of guesswork comes in.
Anecdotally, I've heard from a few people who've had their coils get
stuck on and burn out. From those reports, it appears that coils will
pretty reliable overheat after about a minute, maybe two. I've also
heard one or two reports of failure after much shorter times, around
10 seconds. Based on these reports, it seems best to choose a fuse
that will blow after perhaps 10 to 20 seconds.
Step three is to choose a fuse that matches the combination of the
current and time we came up with in steps one and two. This is
another research step, because slow-blow fuses aren't sold with
simple, fixed time limits. Instead, the time limit is a function of
the current. The manufacturer presents this function with a chart
in the data sheet, like this one, taken from the data sheet for the
Bel Fuse 3JS data sheet:
Each green curve represents the current/time relationship for one type
of fuse, labeled at the top.
Here's the way I use this type of chart to pick a fuse. Let's say we
want to pick a fuse for a replay knocker with an AE-26-1200 coil that
we're running at 35V. As we calculated above, that gives us 3.2A as
the normal operating current for the coil, and as we guesstimated
above, we'd like a fuse that blows in perhaps 10-20 seconds. So let's
go to the chart, and find the intersection of 10 seconds and 3.2A. I
marked that spot with a red dot. That happens to fall right on one of
the green lines - if it didn't, it would be between two lines, so we
could pick the closest. I highlighted the line we're on to make it
easier to follow. Now trace the line to the top of the chart to see
which fuse it's for: it's the 1-1/2A (1.5A) fuse.
It seems a little strange that we're going to use a 1.5A fuse for a 3A
coil, but the amp rating on the fuse is only nominal. The timing
chart tells the full story, and in this case the timing chart says
that the nominally 1.5A fuse will let our 3 Amps flow for up to about 10
seconds. In case you're still not convinced, I've been running with
this fuse protecting my own AE-26-1200 replay coil, and it hasn't
ever blown on a false alarm.
Since this is such a common coil in virtual pinball machines, I'll
save you the trouble of finding the 1.5A version of this family.
Here's the part number and a Mouser link: Bel Fuse 3JS 1.5-R TR
Wiring the fuse for a knocker is just like wiring a fuse for any
other output device. The fuse goes between the output port on
the controller board and the knocker coil. Here's a diagram;
we use an LedWiz with booster board as an example, but it's
the same for any other type of output controller.
First, run a wire from the output port on the controller board to one
end of the fuse. Next, run a wire from the other end of the fuse to
the knocker. This connects to the negative side of the knocker
coil, usually signified by a black wire. (The coil itself doesn't
care about polarity, but it should have a diode attached, and the
diode does care.) Fuses have no polarity, so the direction you
connect the fuse doesn't matter.
Flipper coils are more complicated than most other types of pinball
coils, but in a way that actually simplifies things for our purposes
here. In most cases, you won't need a fuse for a real flipper coil.
The reason is that flipper coils for real machines are built to
tolerate being activated for long periods. They have to be, because
players routinely hold a flipper up to trap a ball. So unlike other
pinball coils, these coils are designed to withstand continuous power,
and thus don't need to be protected from getting stuck on.
How do they accomplish this, when other coils can't? Their trick is a
clever two-coil design. Flipper coils are really two coils in one:
two separate coils of wire wrapped around the same core. One coil is
the high-power "lift" coil, which generates the strong initial force
that lifts the flipper from the rest position and propels it (and the
ball) through the flipper's arc. The other coil is the low-power
"hold" coil, which only kicks in when the flipper reaches the top of
its arc. The hold coil only has to exert enough force to hold the
flipper in place; it doesn't have to propel the flipper or the ball.
This allows the hold coil to operate at much lower power - low enough
that it can be left on indefinitely without overheating. The flipper
assembly toggles power between the two coils by way of an
end-of-stroke switch, which the flipper trips mechanically when it
reaches the top of its arc.
If you're planning to use a real flipper coil in your virtual cab, you
should make sure that it's part of a full flipper assembly
that has the end-of-stroke switch in place and properly adjusted. The
end-of-stroke switch is critical because it's what prevents the lift
coil from getting stuck on. If the lift coil gets stuck on, it'll
overheat like any other pinball coil.
The dual-coil design isn't universal. Newer Stern machines (2000s
onward) dispense with this somewhat complex electro-mechanical design
and use a somewhat complex software system instead. On these
machines, the flipper coil is just an ordinary coil with a single
winding. The flipper button is connected to the CPU, not directly to
the flipper. When you press the button, the CPU turns the coil on at
full strength. But this lasts only for a split second, long enough
for the lift stroke. Once that initial time period has passed, the
software reduces power to the coil using PWM, or pulse-width
modulation. PWM is a method for controlling power by switching the
voltage on and off rapidly (hundreds of times a second).
The single-coil Stern design isn't suitable for virtual cabs, because
it requires the specialized software system to manage the power. None
of the current software or hardware in the pin cab environment can do
this. So if you want to use a real flipper assembly, you should use the
traditional dual-coil type, not the newer Stern single-coil type.
Solenoids might or might need the same protection as pinball coils
(see above) against being left on for long periods.
To find out whether you need a fuse for your particular solenoid,
start with the data sheet, if one is available. Look for information
on maximum continuous "on" time.
If you can't find a data sheet, or it has nothing to say on the subject,
you can do some testing of your own. Apply power to the solenoid for
a couple of seconds, then cut power. Check to see if the solenoid
feels hot. If not, try again, leaving it on a little longer. Repeat,
extending the time on each trial, until the solenoid starts feeling
hot to the touch. If you can leave it energized continuously for
several minutes without it getting too hot, you probably don't have to
worry about a special fuse for it. If it does start getting hot
within a couple of minutes, though, you should add a slow-blow fuse
using the same procedure explained above for pinball coils.
If you're using a solenoid to simulate any sort of rapid-fire device,
like a bumper, slingshot, replay knocker, chime, bell, etc., the same
rule of thumb for timing that we used for pinball coils should work
well here. However, you might want to extend the maximum time a bit,
like taking it up to 30 to 60 seconds, assuming your solenoid didn't
overheat that quickly in the tests above. The reason is that bumpers
and the like get a lot of use in some games - they fire briefly, but
many times in a row. Many short bursts in a short time add up, since
this is all about heat dissipation. So choosing a fuse with too short
a time limit might result in the fuse blowing unnecessary during
bursts of activity during normal play.
Wire the fuse for a solenoid the same way you would for a pinball coil.
PC/ATX power supplies
Good news! You probably don't need any fuses for PC ATX power
supplies. These almost always have built-in thermal overload
protection that temporarily shuts them down if they overheat. That's
the same function a fuse performs, so there's no need for a separate
fuse; the built-in protection is all you need. The thermal protection
circuit in an ATX power supply should automatically reset itself when
the temperature returns to normal, so you won't have to open it up to
replace a fuse, or even push a reset button, if you ever trigger an
overload. Just unplug everything for fifteen minutes or so to let the
power supply cool down. (And while you're waiting, you might also
want to fix whatever caused the short circuit or overload in the first
OEM power supplies
Pin cab builders
often use a mix of power supplies that includes one or more generic,
no-name power supplies from eBay that look similar to the ones shown
at right. These are often sold on eBay as LED strip PSUs or OEM PSUs,
since they're primarily designed for sale to other manufacturers
to embed in consumer devices.
These power supplies might or might not have built-in overload
protection. Check the site where you bought your to find out if they
say anything about it. Also check to see if it has its own
replaceable fuse accessible from the exterior of the case (this is
If there's no built-in overload protection, you might want to protect
the power supply with a fuse.
Choose a fuse that matches the rated maximum output amperage for the
unit. If the unit's output limit is only stated in Watts, you can
determine the maximum amperage by dividing Watts by Volts, using the
voltage on the output side. For example, if your power supply has 12V
output and a maximum power output of 48W, the maximum amperage output
is 48W/12V = 4A. So you'd choose a 4A fuse.
(Earlier, we talked about a 75% rule of thumb, where we use a fuse
rated for 75% of the maximum for what we're protecting. That applies
when we're protecting a transistor or IC chip. Power supplies
shouldn't need this adjustment, since the components they use aren't
as vulnerable to brief current surges.)
Connect a power supply fuse as shown in the diagram below. (We show a
24V supply as an example, but the principle is the same for any
voltage.) Run a wire from the power supply's positive (+) output to
one end of the fuse. Connect the other end of the fuse to whatever
devices the power supply will be powering.
Step-up and step-down voltage converters
Pin cab builders sometimes use step-up and step-down voltage
converters to get special voltage levels that you can't easily get
from a PC power supply or eBay/OEM unit. For the purposes of
fuses, you can treat these converters the same as power supplies.
Look at the instructions or spec sheet for the converter to determine
its maximum output current. Select a fuse that matches the maximum
Note: If you're only using a converter to power a single device
(e.g., a knocker coil or a shaker motor), and you already have a fuse
in the circuit, you don't need a separate fuse for the converter. The
first fuse will protect the whole circuit. Just make sure that its
amperage limit is below the converter's maximum output amperage
Wire the fuse for a converter the same way as with a power supply.
Connect a wire from the converter's positive (+) voltage output to
one end of the fuse. Connect the other end of the fuse to each
device that the converter will be powering.
Fuses vs resettable devices
Fuses have the downside of being expendable: when a fuse blows, you
have to throw it away and replace it.
It's possible to find resettable (non-expendable) circuit protection
devices in the range of currents we use in a pin cab. I don't have
experience with any of these devices myself, and I haven't found any
options that look ideal for our purposes. But I wanted to mention
them in case you don't like the idea of expendable fuses and wanted to
look into reusable alternatives.
One thing you could look at is mechanical circuit breakers. These are
similar to the ones you might find in the electrical panel in your
house. There are options for these with suitable specs for a pin cab.
The cheapest are a few times the price of an equivalent fuses, so
they're more expensive if you never blow a fuse, but could end up
being cheaper if the same circuit overloads several times.
Another possibility is PPTC (polymer positive temperature coefficient)
devices. These are essentially temperature-dependent resistors that
develop very high resistance when they got hot. High current levels
heat them, triggering the rise in resistance, effectively cutting off
current (or at least greatly reducing it). These devices have the
advantages of being very compact, and being passive: you don't have to
reset them after a fault (the way you do with a mechanical circuit
breaker), since they return to normal resistance when they cool.
PPTCs are widely used in PC equipment, including ATX power supplies.
The big problem I see with circuit breakers and PPTCs is that they're
slow. They tend to have timing curves similar to slow-blow fuses.
That makes them good for fire prevention, but bad for protecting
semiconductors - which is the primary function we need fuses for. For
circuits where we need to protect transistors and ICs from short
circuits and overloads, we need something that interrupts the current
flow almost instantly on overload. The only thing I've found that
does this is traditional, expendable fuses.