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Understanding Fuses and Fuse Values in a FAST Nano-controlled pinball machine

Wiring, high voltage, and electricity can be dangerous. Read this first!

The voltages and electricity discussed here can be dangerous and could cause property loss or death. It is your responsibility to ensure you are comfortable doing the things discussed here. Furthermore your local jurisdiction may have regulations or rules which differ from what we discuss here, including wiring colors, standards, techniques, etc. Seek professional guidance from someone local to you if you are unsure of anything you're doing here. Also there could be errors, omissions, or typos here. Use this content at your own risk.

This guide is for FAST Nano-powered machines only

This wiring guide is for pinball machines powered by a FAST Nano Controller. If you have a FAST Neuron Controller, please see the Neuron wiring guide.

One of the most common questions people have when designing and building a pinball machine is what type and value of fuses they should use. This guide provides some background information and guidance on important topics.

From a practical standpoint, fuses exist to protect equipment and the surrounding environment. From a more pragmatic standpoint, the idea is that if there's a situation that might become dangerous (short, over current, etc.), you want the fuse to blow before something else that's more expensive blows or before something heats up and becomes dangerous. As Dave (FAST Pinball Engineer) likes to say, "Replacing a fuse is cheaper than replacing what it's protecting."

Understanding fuses

It's important to understand that fuses are both timing and current sensitive. For the slow blow type fuses which are generally used in pinball machines, they will allow more current than their ratings for a longer period of time versus fast blow fuses. (Here's more information on slow-blow versus fast-blow fuses.)

Overloading a fuse for a period of time is generally fine. For example, if the average current draw for an LED strand is 2.5A, but every LED on full brightness results in a 5A draw (maybe during some fun strobe mode), you might still end up choosing a 3A fuse. If it ever blows, you could look at choosing a 4A fuse, as long as you first investigated the actual loads your machine is drawing and verified that everything in the chain supports that load.

This timing delay before a fuse blows is why you don't want to the fuse to exactly match what your wiring and devices can handle. If your wire gauges and devices can handle 5A and if you protect that with a 5A fuse, that fuse will allow more than 5A before it blows, and in that time you may have done some damage downstream to something that couldn't handle more than 5A.

Fusing and fuse selection is one of the topics that's very difficult to provide generic guidance on since there are so many things to take into consideration in your specific machine. The most important thing is to TEST, TEST, TEST! All fused circuits should be tested (use your meter to check what the actual current draw is during typical and maximal scenarios, and even in scenarios where something fails or shorts since you really want to make sure a short blows the fuse). Then once you think you know what your current draws and current limits are, verify the specifications for every piece in the chain to ensure they can all handle the current loads you're planning. (e.g. look at the specs for the wire, the connectors, the boards, the LED chips, etc.)

Really your goal for fuse sizing is that when you hear a popping sound or see an "unexpected light show" (which will most likely happen many times when building your own pinball machine), you want that sound or flash to be a 5-cent fuse blowing and not something else that's much more expensive and that will delay your fun by several days as you wait for replacement parts to arrive.

Fuses and power supply relationships

Pinball machines are extremely complicated (on many levels), and the electrical design is certainly part of it. For example, the specific type of power supply has a big effect your pinball machine, both in terms of its safety as well as how it performs when stressed.

An important thing to know about power supplies is what they do when they're overloaded. The 48V 6A power supply we carry on our web site is specifically designed to go into constant current mode when it's overloaded. This means that once it hits its limit and a load wants more power, the power supply will drop the voltage so it can maintain the current. We selected this type of power supply because the main flipper windings take more than 6A when enabled. So when they're on and asking for more, the 48V power supply will keep providing the 6A but will drop its voltage, with huge capacitors on the power filter board providing the additional power. That lets the power supply ramp back up to its full 48V while providing the full 6A the entire time.

This is why we don't recommend just using whatever power supply you find in your basement or on eBay unless you investigate how it operates (which is more than just looking at its rated voltage, amps, and watts.)

For example, many power supplies when overloaded simply disconnect and try again later. Obviously having flippers turn off when things are going wild in a pinball machine is not an option.

For the 5V power supply which is most likely powering your LEDs (which require a lot of current), if you have a 6A fuse on a 6A 5V power supply, that fuse will probably never blow. (Which is a bad thing. Remember we want the fuse to blow when it needs to so something else doesn't.) A 6A fuse on a 6A power supply that doesn't blow means the power supply will over current before the fuse blows. This will cause the power supply to continuously retry causing heat and stress on your wiring or shorted area.

So, for example, using a 4A fuse on the 5V 6A power supply will still let you use 6A of current (which the power supply can provide) for a short period of time—it's just not something that can be sustained for very long. Most likely your average current is much lower than 3A or 4A which is why those fuse values should work (again you need to test). And if you have a ton of LEDs and 3-4A is not enough, then most likely you'll want to buy a larger power supply that can provide more current (maybe a 12A supply instead of 6A), and then you'd run multiple 5V branch lines each with their own 3-5A fuses, since even though you have 12A at the power supply, your wire, connectors, LEDs, etc. can't handle that much current. (Just remember that all the values of all the fuses on your branch lines need to be less than the power supply current maximum. Remember if we want a 3-4A fuse for a 6A supply, then you wouldn't want more than 9-10A total with a 12A supply.)

Practical guidance for fuses in a pinball machine

The main location for fuses in a FAST Pinball machine is at the Power Filter Board. We also have a standalone Smart Fuse Block which can be used if you need additional fuses.

As a refresher, the FAST Power Filter Board has five separate power taps, each protected by their own fuse (which you need to figure out the value of).

  • HV (High voltage, typically 48V), enable switch protected, with 2 huge capacitors, fused
  • 5V fused
  • 12V fused
  • V1 (Additional Voltage 1), enable switch protected, with 1 huge capacitor, fused
  • V2 (Additional Voltage 2), fused

The purpose of the HV, 5V, and 12V lines are pretty obvious, but the V1 and V2 lines are a mystery at first. The V1 line is intended to be a second high-voltage line. It has its own giant capacitor (that's what the third capacitor is for), and it is also disabled when the enable switch is open (e.g. the switch you wire into your coin door open switch). So, for example, you could put your flippers on the HV line and a magnet and other loads could be on the V1 line. That way you could fuse them separately as appropriate. It's also okay if you run all the coils on the main HV line, again depending on your setup, and then if you have a magnet or something else specific you could run that from the V1 line.

If both HV and V1 are the same voltage, then all three capacitors get tied together in parallel.

(The HV tap does not have to be 48V. It can be anything from 9-70V.)

The V2 line is intended to be a second low-voltage line, which in many cases is used as an additional 5V line in scenarios where you have more LEDs than a single 5V branch can handle.

Now let's look at each of the voltages specifically through the lens of fusing, remembering that across the board, the fuses (or total amps of all fuses when you have multiple branches off the same power supply) should be LESS than the rated amps of the power supply.

48 volts

For the 48V (HV) fuse value, assuming you're using the 48V, 6A power supply we sell, the fuse for that 48V line will most likely be 3A or 4A slow-blow. (Again, this will depend on your loads and what you're driving.) But to repeat what we've written many times already, we can say for sure that if you have a 6A supply, you do not want a 6A fuse.

If you put a 6A fuse on a 6A power supply and the load increases above 6A, most likely the fuse will not blow, the voltage will drop, and whatever is shorted will smoke (or worse). Selecting fuse values is the one time you want to go lower, not higher. Seriously, you should brag about how small your fuses are. Start with 2A, if it blows try 2.5A, then 3, then maybe 4, etc...

12 volts

For the 12V supply, the power supply we resell is rated for 3A on its 12V output, so use 2A or 2.5A fuse as a starting point. Again, the exact fuse selection needs to be based on the wire size, specifications of your devices, and expected (and test verified) loads.

If you need more than 3A of 12V power, you can go up to 6A on the same feed if you get a higher-rated PSU.

5 volts

The vast majority of 5V power in your pinball machine will be for LEDs. The Nano controller also runs on 5V, and you'll probably end up with a few smaller things here and there, but most things that take a lot of power run on 12 or 48V, meaning that 5V planning is mostly just LEDs.

The 12V/5V power supply we resell is rated for 6A on its 5V output. Since you want to leave some headroom in your fuses versus what the max output of your PSU is, that means you're looking at probably 5A max fuse size for your 5V fuse.

5A should be plenty of power for the 250 LEDs, though that could vary based on how bright you're running them, how much resistance is in your wires, etc. (See the guide to adding more power for more on LED power consumption.)

If you have fewer LEDs, you can start with lower fuse values, maybe a 3A and then move up in 0.5A increments as they blow (up to the max of 5A). If you do find that you require more than 5A of 5V power (which is rare, but possible), then you can get a higher-powered 5V PSU (maybe move up to 12A) and then have two feeds of 5A each—the 5V and V2 feeds. See the guide to adding more power for details.

Note that the V1 switched auxiliary feed on the power filter board has a minimum voltage of 9V (due to the MOSFET which acts as the switch), so V1 is not available as an extra feed for your 5V line.

AC line fuse

There's one more fuse value to consider, and that's the fuse that's on the mains side (the wall socket side by the plug) which is going into the AC input side of your power supplies.

For this, 8A is probably good for North American 120V, while 5A should do it for 240V countries.

Key takeaways

  • The power supplies we resell are sized for most pinball machines, but if you're doing a lot in your machine then you might need a supply with more current.
  • The fuse needs to be lower than the max output of the supply or it may never blow.
  • You can use more current than the fuse is rated for, you just can't do it for very long.
  • Do not "solve" a frequent blowing fuse by just adding a larger fuse without testing, measuring, and calculating what the actual value should be.
  • TEST, TEST, TEST your actual loads and fuse appropriately.
  • Blow some fuses! The only way you can really know your fuses will work is to test them. Buy one of those $10 packs with hundreds of fuses of different values from Amazon and spend a night playing around with different ways they blow. Then try shorting out various lines and drawing too much current and make sure your fuses blow when and how you think they should. (Reach out to us for guidance if you're unsure how to do this. We don't want you to blow out an expensive board!)
  • It is impossible to provide concrete guidance without knowing the details of your machine.
This guide is part of our complete series on wiring your FAST Nano-controlled pinball machine. Click to see the rest!

Wiring guides for FAST Nano-controlled pinball machines

We have many guides and a complete wiring walk-through for your entire pinball machine powered by a FAST Nano Controller. Please read and understand all of the wiring guides before you start planning and physically wiring your machine.

Baseline wiring skills & knowledge

Important wiring and electrical background information you need to know before you start planning your machine's wiring.

FAST Nano-controlled Pinball machine wiring guides

The guides below walk you through a complete machine wiring, section-by-section. The numbers in the drawing match up to the numbers in each diagram. We assume you follow these in order. Click the image to zoom in.

  1. AC line in & power supply wiring
  2. Earth ground wiring
  3. Power filter board wiring
  4. Fuse Theory & Selection
  5. FAST Controller & I/O Board wiring
  6. Driver & coil wiring
  7. Flipper wiring
  8. Switch & opto wiring
  9. LED wiring
  10. Adding more power
  11. Cabinet wiring & power distribution

And remember, the bottom line is to be safe! If you're confused on anything, reach out, and we'll help. (And we'll add/update our docs as needed!)

Documentation Feedback? Requests? Confused?

Hi! I'm Brian, and I'm responsible for the documentation at FAST Pinball. If you have any feedback, requests, corrections, ideas, or any other thoughts about this documentation, please let me know! You can email me at brian@fastpinball.com. Thanks!

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