How to select a right diode for large current reverse bias and small current forward bias?

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Usiela
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How to select a right diode for large current reverse bias and small current forward bias?

Postby Usiela » Mon Mar 05, 2018 1:25 pm

I'm designing a circuit where I want to pass a small test current from one leg, but block a large "firing" current (rocketry) coming from another leg in reverse bias.

I have a 5vdc indicator line (with about 1-2 vdc and <.5 amp once it would get to the diode) I want to pass through a forward biased diode to indicate the fuse is still viable without burning the fuse, but need that same diode needs to block 12 vdc at as much as 30 amps in reverse bias for a short duration (maybe as long as 5 second until the igniter lets go and opens the circuit and the "firing" voltage is turned off manually). How do I select the proper diode? What am I looking for on the diode spec sheet to be sensitive enough to allow the low current forward bias and strong enough to block the large current reverse bias. I know that is what diodes do, but how to select the right one escapes me.

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Re: How to select a right diode for large current reverse bias and small current forward bias?

Postby OverTheTop » Mon Mar 05, 2018 3:16 pm

You just need something that will allow the forward current of the LED without destruction, and block the reverse voltage (at almost no current) when the 12V is applied.

Almost any regular rectifier diode will do in that situation. A 1N4148 (or 1N914, or similar) "signal diode" will have a sufficient current rating to run a small LED quite easily. Ratings are around 75V reverse voltage, and 300mA forward current (but I wouldn't push one that hard :D )

The standard rectifiers like 1N4002-1N4007 are also suitable, with forward current of up to 1A and reverse voltages up to 1000V (depends on the exact part number). Any of that series will suit your application in series with the LED.

Forward current on a LED is usually quite small, so any of the above suggestions should work.

Note that most of the igniters we use have a minimum fire current of around 200mA, so I would be getting very nervous if your circuit put anywhere above about 50mA (allowing for tolerances) into the igniter.

Not a very critical part in the circuit do just look at the reverse voltage and forward current ratings, and keep that LED current down!

I assume the diode in series with the 12V is a different device and rated to quite a few amps.
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Re: How to select a right diode for large current reverse bias and small current forward bias?

Postby Voyager » Thu Mar 08, 2018 10:43 am

Yep! What OTT said.
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Re: How to select a right diode for large current reverse bias and small current forward bias?

Postby strud » Thu Mar 08, 2018 3:17 pm

Maybe you should consider using a transistor to amplify a very small sensing current (say 0.1mA) to a large current (say 20mA) to drive the LED?

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Re: How to select a right diode for large current reverse bias and small current forward bias?

Postby Voyager » Thu Mar 08, 2018 10:23 pm

At some point you will need to switch the +12V / 30A firing supply through the load (fusable link - or igniter?). Personally, I wouldn't bother with the diodes and just use a momentary SPDT switch (or relay) to select the desired path. The LED could be in the NC (normally closed) line from the switch; the +12V / 30A firing supply could be in the NO (normally open) line from the switch; the load would be in the common line from the switch.

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The LED current can be limited to about (say) 5 mA with the appropriate series resistor; 2k for a +12V supply, or 680 for a +5V supply. I use a similar, but more complex circuit for our club launch controller in the continuity test circuit based on a SPDT ignition relay.

Note: You may need to adjust the series resistor value depending on the particular LED you use (colour and efficiency). The forward voltages vary from about 1.5V to 3.5V.

However, this all assumes that you don't have a specific requirement to use the diodes in your proposed circuit.
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Re: How to select a right diode for large current reverse bias and small current forward bias?

Postby Voyager » Thu Mar 08, 2018 11:21 pm

You might find the LED is a bit dim at 5 mA. Up to 20 mA is quite safe for any igniters/ematches. Adjust series resistor accordingly.
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