Diodes in parallel sharing current in reverse polarity protection

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Sergio34

Joined Nov 10, 2018
52
Hello, I used this configuration for reverse polarity protection in my new project. The picture is generic downloaded, in my case I used 1.6A FAST fuse and as I had only 1N4003 diodes that can handle 1A each I put two in parallel. I know that low resistors should be connected in series with each diode but the values are so small that I wonder why the wire is not just enough to behave like low resistor. And this is what I did, I connected each anode to negative side through 26awg wires 5cm each. My question is if thermal runaway can happen in the short time that takes the fuse to blow. I am trying also to decipher the graph in the datasheet of the diode about peak surge current and I suspect that one 1N4003 could withstand 2A for one second, I am not sure. (2A/1sec figure is worst case scenario of the fuse I guess). What do you think?
 

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Thread Starter

Sergio34

Joined Nov 10, 2018
52
I thought some experiments wouldn't hurt (me...). Fun with fuses. I did three shorts, first with two diodes in parallel and 1.6A fuse, second again two diodes in parallel but 2A fuse and last but not least one diode only and 2A fuse. Dead short with 24V from lead acid battery. Fuses blew instantly in all cases and both diodes are fine. Was it luck all three times? I am trying to figure out what I do not understand, on one side it's you which I respect with my modest hobbyist knowledge and on the other side it's this experiment and others on youtube where they pass through components something like 10A-15A and still the components take some seconds to melt. They conduct huge currents for 1-2 seconds. But I am not interested in huge currents, this is about 2A for a fraction of a second. I am tempted to buy higher or slower fuses to see when the diodes will collapse.
 

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MisterBill2

Joined Jan 23, 2018
6,304
I thought some experiments wouldn't hurt (me...). Fun with fuses. I did three shorts, first with two diodes in parallel and 1.6A fuse, second again two diodes in parallel but 2A fuse and last but not least one diode only and 2A fuse. Dead short with 24V from lead acid battery. Fuses blew instantly in all cases and both diodes are fine. Was it luck all three times? I am trying to figure out what I do not understand, on one side it's you which I respect with my modest hobbyist knowledge and on the other side it's this experiment and others on youtube where they pass through components something like 10A-15A and still the components take some seconds to melt. They conduct huge currents for 1-2 seconds. But I am not interested in huge currents, this is about 2A for a fraction of a second. I am tempted to buy higher or slower fuses to see when the diodes will collapse.
You are all set as long as the fuse is the rated value. But if some dummy puts in a 20 amp automotive fuse all bats are off. I once got a premium CB radio to repair that had that exact thing happen. Not only the protection diode blew, but also several electrolytic capacitors, and then the RF output stage transistor. So the two diodes in parallel is a smart choice. If your circuit can afford the voltage rop from a series diode that will be the ultimate protection.
 
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