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Circuit Protection - A Brief Overview

Copyright (c) 2007 by Steve Sawtelle - All Rights Reserved.
Copyright (c) 2007 by Slot Car Corner L.L.C. - All Rights Reserved.
 
Introduction
 
Circuit protection is an area many racers tend to overlook when wiring their track. It's like an insurance policy - you hope you never have to use it (but you're glad you have it if you do...). The information below is intended to provide readers with a brief overview of common circuit protection options and some considerations when selecting and sizing circuit protection for your wiring project.
 
Power Supplies with Built-in Circuit Protection
Let's start with the power supply. Many power supplies do include some sort of circuit protection; however, it is important to keep in mind the protection has been designed/sized to protect the power supply - NOT your controllers, track wiring, track, etc.. For example, the Pyramid PS26KX (a very popular power supply with 1/32 racers) DOES include circuit protection; however, it generally doesn't take effect until a circuit fault is present. While it may prevent damage to the power supply, the full rated current of the power supply (about 22 amps in this example) is flowing to your track when a circuit fault is present. Without some form of properly sized circuit protection at your driver's stations, this large amount of current will likely damage your controllers, your track wiring (if not sized properly) and in a worst case scenario, possibly cause an electrical fire. Therefore, my recommendation is NOT to rely on the power supply’s circuit protection to protect your driver’s stations, controllers, track, cars, etc..
 
Common Types of Circuit Protection for Slot Car Applications
The type of circuit protection used depends on factors such as the power supply and types of cars/motors you plan to race. There are 2 broad types of circuit protection which are most commonly used - fuses and thermal protection devices. The latter includes thermal circuit breakers (magnetic circuit breakers also exist; however, they are considerably more expensive so I have not incuded them in this discussion) and "polymeric positive temperature coefficient devices” (PPTC's). Let's take a look at each in a little more detail.
 
Fuses
Fuses are easy to incorporate into your track wiring - fuses holders are readily available at automotive and electronic stores everywhere. If you are using wallwarts or power supplies with a low current rating (say 5 amps continuous or less), fuses are probably your best (and safest) choice. This will become clearer during the discussion of thermal circuit protection devices below. When sizing fuses:
 
  • Use fuses with the smallest current rating needed for the "loads" (created by your car's motors...) you expect.
  • ALWAYS use a fuse with a current rating which is LESS than the current rating of your wallwart or power supply!! If your load demands more current than your wallwart or power supply can supply, DO NOT size the fuse based on the load! Instead, upgrade your power supply (or only run cars with motors your current power supply can safely handle…)! Failure to do so may prevent the fuse from “blowing” when a circuit fault occurs – as described earlier, damage will likely occur.
 
Fuses are less convenient than thermal circuit protection devices – you must open the fuse holder, replace the fuse, etc.. There’s also the cost of fuses to consider – over the life of your track, buying replacement fuses can add up.

At a minimum, include a fuse holder/fuse on the "+" (power supply) side of your driver station wiring using the sizing guidelines above. If your budget permits, include a second fuse holder/fuse on the "-" side of your driver station wiring as well. Since the "-" side of the circuit (brakes) will carry less current, you can use a small fuse. The general rule of thumb I use (YMMV - if you choose to follow this, you do so at your own risk) is 1 to 1.5 amps or 1/2 the "+" fuse current rating (based on the sizing guidelines above) - whichever figure is LESS.
 
Note: High-quality fuses holders are available for purchase at the end of this article (scroll to the bottom of this page) or in the Online Store.
 
Thermal Circuit Breakers
Thermal circuit breakers differ from fuses in a couple of important ways. First, trip times are generally slower than fuses (particularly when compared to “fast blow” fuses). How quickly the circuit breaker will trip is a function of how much current is present over and above the circuit breaker rating when a circuit fault occurs. The more current that flows when a circuit fault is present (think heat), the faster the circuit breaker will trip. The general rule of thumb I use (YMMV - if you decide to use this rule of thumb, you do so at your own risk) is a minimum of 3:1 when sizing breakers. In other words, size the breaker for the expected load (your "hottest" car motors) and multiply this by 3. If the power supply is not rated at this figure (or higher), you should use a fuse instead. Some examples:

Example 1 – I plan to run 1/32 cars with RTR motors which draw no more than 1 amp, I might size the breaker for 1.5 amp (to give me just a little "wiggle" room...). Multiplying by 3 I would need to ensure the power supply I planned to use was rated at 4.5 amps continuous or more.

Examples 2 – I plan to run cars with motors that draw 2 amps maximum current. I have a power supply rated at 3 amps continuous. If I sized the breaker for 2.5 amps, it should trip since the power supply is rated at 3 amps, right? It might; however, it might literally take minutes or hours before tripping. Much too long (and risky…) for our application. On the other hand, a 7.5 amp power supply (3x) would trip the breaker quickly.

Thermal circuit breakers are more convenient to use than fuses - there is nothing to replace and they can be reset quickly (push a reset button); however, they are getting harder and harder to find (and therefore, more and more expensive). More and more applications are using PPTC’s instead. Note: I do NOT recommend thermal circuit breakers, no matter how “small” (current rating) if you plan to use wallwarts – use a small fuse instead!

At a minimum, include a thermal circuit breaker on the "+" (power supply) side of your driver station wiring using the sizing guidelines above. If your budget permits, include a second circuit breaker on the "-" side of your driver station wiring as well. Since the "-" side of the circuit (brakes) will carry less current, you can use a small circuit breaker. The general rule of thumb I use (YMMV - if you choose to follow this, you do so at your own risk) is 1 to 1.5 amps or 1/2 the "+" circuit breaker current rating (based on the sizing guidelines) - whichever figure is LESS.
 
 
 
PPTC’s
A “polymeric positive temperature coeffecient device”, commonly called a PPTC, differs from fuses and circuit breakers in that they don’t actually “break” (open) a circuit when a fault is present. Under normal operating conditions, they are in a very low resistive state. When a circuit fault is present, they change to a highly resistive state; however, a very small amount of current still flows through the device to keep it latched in the highly resistive state. Once the circuit fault is corrected, the PPTC cools and returns to its very low resistive state. Unlike thermal circuit breakers, PPTC’s do not need to be reset manually (no buttons to push) – they automatically reset themselves when the circuit fault is corrected. PPTC’s are also much cheaper than thermal circuit breakers and much easier to source.

When sizing PPTC's, be sure to select a PPTC with a voltage rating suitable for your application (there are models rated for 16V which work for most 1/32 applications and others rated 30V which would cover both 1/32 and H.O.). Sizing them for current rating can be a bit confusing since there are 2 key spec’s you need to understand. The first, is the hold current and the second is the trip current. The hold current represents how much current the manufacturer rates the device to carry without “tripping” (going into the high resistance state). This is the figure you will typically see in catalogs and on websites when ordering. The trip current is typically 2x the hold current (you should check the manufacturer's specs for the specific PPTC you plan to use). The trip current represents how much current will cause the PPTC to trip (go into the high resistance state). The area between the hold current and trip current is like no man’s land – the device may continue to let current flow or not. From a sizing perspective, I work with the trip current and apply a 2:1 factor (YMMV - if you decide to use this rule of thumb, you do so at your own risk) to ensure your power supply will “trip” these devices quickly when a circuit fault occurs. Let’s look at some examples:

Example 1 – The maximum load (current required by motor) I expect is 1 amp. I am considering a PPTC rated a 1.25 amp hold and 2.5 amps trip current. To ensure the PPTC “trips” and does so quickly, my power supply should be rated at 5 amps of continuous current or more.

Example 2 – The maximum load (current required by motor) I expect is .5 amp. I am considering a PPTC rated a .75 amp hold and 1.5 amps trip current). My power supply should be rated at 3.0 amps of continuous current or more.

Example 3 – I have a “wallwart” rated at 700 milliamps (.7 amps). My cars draw .5 amps or less of current so I plan to use a PPTC rated at .5 amps hold and 1.0 amps trip. Since the .5 amp hold figure is less than the .7 amps my wallwart can produce, I should be fine, right? Again, I recommend a power supply rated at least 2x the trip current (not the hold current) – in this case, a minimum of 2 amps. Note: I do NOT recommend PPTC’s, no matter how “small” (trip current rating), if you plan to use wallwarts – use a small fuse instead!

At a minimum, include a PPTC on the "+" (power supply) side of your driver station wiring using the sizing guidelines above. Because PPTC's are very affordable, I would strongly recommend including a second PPTC on the "-" side of your driver station wiring as well. Since the "-" side of the circuit (brakes) will carry less current, you can use a small PPTC. The general rule of thumb I use (YMMV - if you choose to follow this, you do so at your own risk) is 1 to 1.5 amps or 1/2 the "+" PPTC current rating (based on the sizing guidelines) - whichever figure is LESS.
 
Note: An extensive selection of PTTC's are available for purchase at the end of this article (scroll to the bottom of this page) or in the Online Store.
 
Summary
Again, this is not intended to be an exhaustive discussion of circuit protection options but rather a brief overview of common circuit protection solutions to consider when wiring your track. If you are not sure what type of circuit protection is right for your application (or how to “size” the circuit protection), please don't hesitate to contact us.

 

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