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.
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.
Just use a high speed diode like a 1N4148, BAS16 or 1N914. It will certainly block 12 volts and won't conduct any appreciable reverse current when the heavy duty 12 volt supply is activated. I said use a fast diode because reverse recovery time for those stated above and in the single figure nano second range.
Andy's answer is simple and correct, I only wish to add to it (since you asked about how to select a diode and what specs to look at).
Standard diodes conduct when forward-biased and block (mostly) when reverse-biased - you already know this. Bias is expressed entirely as a voltage, so a diode can block quite a bit of current if the voltage is low enough. To use the water-in-pipe analogy, without sufficient pressure a one-way valve will stop backflow indefinitely, no matter how much water is in the pipe.
When choosing a diode for general use, you'll want to look at the following:
- Diode type - standard "silicon" diodes have high threshold voltages (~0.7V) and high reverse breakdown voltages (>50V). Schottky diodes have much lower threshold voltages (~0.2-0.4V) and faster switching times, but also generally lower reverse voltages (~30-60V). Zener diodes conduct in reverse when reverse-biased above their "Zener voltage", essentially operating in both a normal forward mode and a precise, controlled breakdown mode.
- Forward voltage - determines the needed voltage for the diode to be fully conducting (real diodes conduct some current when they're forward biased at all).
- Reverse voltage - determines what amount of reverse-bias the diode can repeatably withstand before it enters breakdown and begins to conduct in reverse. Real diodes allow a very small amount of leakage current (on the order of up to a few micro-Amperes) when reverse-biased, but generally not enough to cause problems.
- Continuous forward current - if you need a specific current throughput when your diode is "on", you'll need to make sure it's rated to allow that much current.
- Reverse current - the amount of "leakage" current when the diode is reverse-biased. In your case, you aren't overly concerned about a few uA slipping by, but some applications may have a maximum allowable reverse current. "Low-leakage" diodes may have a reverse current on the order of pA.
- Recovery time - the amount of time a diode needs to switch between "conducting" and "blocking". High-speed diodes can make this transition in nanoseconds, Schottky diodes faster still.
- Temperature ratings
There are other specs for diodes that specific applications may need to be in certain ranges, but for fairly basic applications these specs should suffice.
In your specific application, a Schottky diode or high-speed silicon diode would work perfectly to block the 12V reverse.
There are two parameters for that diode you need to consider.
You need to be able to light the LED through it, say at least 20mA. Cheap 'cooking grade' diodes like 1N4148 will easily do that. You'd have trouble finding one that would not do that.
At first glance, it looks that 12v would be sufficient, or 7v looking at the difference between the 12v rail and the 5v rail.
However, the fuse current could be up to 30A, and we don't know what the inductance of the wires from the 12v source to the fusible link is. This large current could store a lot of energy in the inductance, which when the current is broken rapidly, could generate hundreds of volts. The 75v PIV of a 1N4148 might not be able to cut it, or it may.
You could hope and use a 4148, or aim higher and use a 1000v diode.
You might expect that the fusible link would arc well below that voltage and keep the voltage below 1000v. A transzorb placed across the fusible link would certainly limit the high voltage.