my circuit protection is getting HOT!

[bigfarmerdave]

I've got a little 12 volt DC circuit that I built which gets its power from the battery on the tractor. I guess the function of the circuit isn't of importance here so I won't go into detail. Anyway, what I wanted to do was protect it from a reverse polarity hook up and also protect it if a direct short were to develop. So, what I did was put a 7 amp polyswitch and a 6 amp, 200 volt rectifier diode in series between the battery and my circuit. When I reverse the polarity to my circuit, the diode does its job of protecting the circuit. When I directly short something within my circuit, the polyswitch drops the voltage from roughly 12.5 volts to about 1.2 volts and the current draw is limited to 7 amps. The problem is that the diode gets VERY hot. My question is this, is the diode getting hot because it is only rated at 6 amps and there is a 7 amp draw or is the voltage being dropped so low that there isn't sufficient voltage to forward bias the diode? I thought that the min voltage required to forward bias a diode was somewhere in the neighborhood of .4 to .7 volts depending on the diode. I'm thinking my problem is that I simply need a higher amperage rated diode and not the latter. Anyone agree?

 

[gerty]

This is where a circuit diagram would come in really handy. Do you have the diode located before or aftter the polyswitch? Why did you opt for a polyswitch instead of fuse/circuit breaker? How much current does the circuit draw without shorts ?

 

[mneary]

I don't see a problem. When you sort circuit the supply, things get hot. This is not unusual.

 

[bigfarmerdave]

I put the 6 amp 200 volt rectifier diode after the 7 amp polyswitch. If I used a fuse instead of a polyswitch, I know what would happen the first time the fuse blows. Someone will put a bigger fuse in to "fix the problem" best case scenario. Worst case scenario is they put a piece of copper in the fuse holder. I circuit breaker would be an option but these are readily available on my bench so they were the first thing that came to mind. Typical current draw will be in the 2-4 amp range depending on the load we have on the circuit.

 

[gerty]

Sounds like you could use a lower amperage polyswitch.
Like mneary said , when you have a short things get hot, a lower amperage poly would still let the circuit work but not let the diode get as hot.

 

[Nigel Goodwin]

Put the diode in series with the device, then it just won't work if it's the wrong way round.

 

[Hero999]

Polyswitches are slow blow devices, they take a long time to act and will briefly pass 50 times their rated current if you short circuit them. The polyswitch takes long enough to cause the diode to heat up before it totally cuts off.

 

[bigfarmerdave]

To answer one comment made previously, the diode is in series with the device and it works just as it should. To answer another comment made implying that polyswitches are slow to react, I do realize that. But, the diode doesn't heat up the instant that you short the circuit. It takes a few minutes before it becomes hot to the touch. I think I simply need a lower amperage polyswitch.

 

[Roff]

I looked at the datasheet of a 6 amp rectifier. The forward voltage at 7 amps is typically about 0.85V. Seven amps times 0.85V is 6 Watts. Six Watts in a tiny package will get HOT!!!.
Your diode may not have the same specs, but it will be similar.

 

[Mikebits]

I like this reverse polarity protection circuit. Of course you would need a better relay for you app, but this circuit will not draw power when reversed. It also does not solve any short circuit protection either.

https://www.chris.org/Modifications/reverse-polarity-protection.html

 

[Nigel Goodwin]

To answer one comment made previously, the diode is in series with the device and it works just as it should.

No it's not, it's in parallel with the device - if it was in series it wouldn't get hot at all if connected the wrong way, and you wouldn't need a polyfuse.

 

[Hero999]

I don't think it gets hot under reverse polarity, just when it short circuits.

To answer one comment made previously, the diode is in series with the device and it works just as it should. To answer another comment made implying that polyswitches are slow to react, I do realize that. But, the diode doesn't heat up the instant that you short the circuit. It takes a few minutes before it becomes hot to the touch. I think I simply need a lower amperage polyswitch.

The diode will have a certain thermal time constant, a delay between the heat being generated at the PN junction and it arriving at the case.

Try leaving it on for a much longer period of time and it might cool down.

Are you using a heatsink for the diode?

It sounds like you should be.

 

[ste5442]

It gets hot because you have 7 amps flowing through the diode.
P=IV so 7*0.6 = 4.2Watts dissipated as heat from the diode.

Maxim (www.maximic.com) do a good app note on automotive power supply protection and ST do a good protection IC called an RBO40.
I would attach the data but I cannot figure out how to add files to a post :-(

S.