Simple Short circuit protection circuit....

Thread Starter

bigjoncoop

Joined Feb 1, 2019
123
Hey fellaz,

placing a order with JLBPCB and I'm trying to get together a bunch of different pcbs to order at once to save on shipping. I have a bunch ready to go but want to make one more...

I had previously asked a while back about a simple circuit to protect against short circuits and reverse polarity. I'm big into building DIY racing quadcopters. there's a large group of us in my area that all fly together. you can buy pre-made "shortsavers or smoke stoppers". some of them are a little more complex than others and some just use a resettable fuse. but I would like to make some of my own pcbs that I can hand out to everybody instead of spending $20 on a premade solution. I would like to just make it very simple and don't mind going the resettable fuse route.... but I've also seen some that use 2 resettable fuses... which I think is just there in order to light up a error indicating LED

below I have included a few pictures. A couple or of pre-made smoke Stoppers. and I've included my schematic of a simple one just using a resettable fuse and a schematic that I was given on my old Forum post when I asked about a short circuit protection which are using to npn or to PNP transistors or mosfets but they are not labeled.

again I think I'm just going to use the resettable fuse approach since that is what we use already but I would like to maybe Implement reverse polarity protection as well. but if the other two schematics offer a way better or safer solution I wouldn't mind going that route as well but again I don't know what transistor or mosfets to use

so somebody could take a look at my schematic just to verify that it will work which it should since I just copied it off a premade solution and how hard it might be to implement reverse polarity protection. and if I would be better off using a different method included in the other schematics.Smokestopper reset.JPG200px-Ограничение_извести_с_помощью_PNP-транзисторов.png200px-Ограничение_извести_с_помощью_NPN-транзисторов.pngl
51mcxNtCmkL._AC_SL1100_.jpgCapture.JPG
 

crutschow

Joined Mar 14, 2008
25,267
Those circuits are current-limiters, not fuses (current going to zero after the fuse trips).
So do you want a fuse or a current-limiter?

Over-current, and reverse-voltage protection are two different animals, and I don't know how to do both with the same circuit.
Reverse-voltage protection can be done with a single P-MOSFET (Vs to drain, gate to ground, and source to load)
 

dendad

Joined Feb 20, 2016
3,595
We generally just use a Polyswitch in the power feed, and a unidirectional Tranzorb after it. That way, spikes are clipped, and the Tranzorb conducts as a diode on reverse polarity and trips the Polyswitch.
Maybe not the best, but works well.
 

MisterBill2

Joined Jan 23, 2018
6,736
I agree that a system using an adequately sized FET as a switch that only comes on with correct polarity is the best anti-reverse solution. But the natural selection method of somebody destroying their system will be a lesson that most folks will learn from. Not all folks, but most of them. An adequately sized reversed shunt diode after a fuse will also protect against reversals, and the fuse will likewise protect agains short circuits. That is the simplest system.
 

dendad

Joined Feb 20, 2016
3,595
It has been my experience that trying to make a device idiot proof just underestimates the deviousness of some idiots.
For instance, on one of our boards, the +24V feeds in via a 1A Polyswitch with the reverse diode (Tranzorb) to protect it. Then, the protected +24V is available on a couple of connectors to feed proxes or similar input sensors. Not only did the customer connect the power reversed, but was using the +24V out as the input power so bypassing the Polyswitch altogether. So the old standby protection worked. Blown track!
 

MisterBill2

Joined Jan 23, 2018
6,736
It has been my experience that trying to make a device idiot proof just underestimates the deviousness of some idiots.
For instance, on one of our boards, the +24V feeds in via a 1A Polyswitch with the reverse diode (Tranzorb) to protect it. Then, the protected +24V is available on a couple of connectors to feed proxes or similar input sensors. Not only did the customer connect the power reversed, but was using the +24V out as the input power so bypassing the Polyswitch altogether. So the old standby protection worked. Blown track!
At some point I tend to back off and let the fools destroy their equipment. Bad experience has a painful school indeed, but a fool will learn in no other.
 

Tonyr1084

Joined Sep 24, 2015
4,912
Reverse polarity protection

Short circuit protection
1595259551941.png
The relay won't come on until you press the "Set (on)" button. Once on the relay will stay on until a short occurs or until you push the "Reset (off)" button. The down sides to this circuit is the drain of the relay and the need to shut it off when not in use. Otherwise you will drain the battery. And if the load is large enough that, too, will drop out the relay. Typical drop-out is at 30% rated voltage. If it's rated for 12 volts then it will typically drop out at around 4V.

There's probably a solid state way to do this as well. Less wasted energy.
 

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MisterBill2

Joined Jan 23, 2018
6,736
he circuit shown relies on the internal resistance of the battery to produce a voltage drop in the event of an overload. so it is not precise, but a fairly effective protection against shorted circuits.
Unfortunately it does have all of the disadvantages described.
 

MisterBill2

Joined Jan 23, 2018
6,736
Certainly the Polyswitch is a wonderful invention, and they are able to prevent things from burning up most of the time. But they are not fast, and they do put a resistance in the line all of the time. So there are several trade-offs, the most notorious and least mentioned is that they reduce efficiency and introduce an added voltage drop. Keep in mind that the only way for them to operate is with heat, and it is current through resistance with a voltage drop that can produce that heat. Not the goal of battery powered flying devices.
 

MisterBill2

Joined Jan 23, 2018
6,736
That power limiting circuit in post #11 is interesting indeed, although it is rather complex and uses some resistors that must be fairly accurate. Certainly the Q1 transistor needs to be adequately heat-sinked. So while it would probably provide the functions requested it would be neither simple nor cheap, and it would not protect against gross stupidity, although it could limit the damage a bit.
 
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