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Old 08-18-2014, 01:02 PM
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Default Diode Question

Does anybody know about diodes? I have a IR 10E 3H diode in my charge light relay (It's the four terminal charge relay fuse on a 84 pickup) that I pretty sure is bad. I tested with my diode tester function on my multimeter and it showed OL both ways. If I test it on the resistance function, it will test 2.3M Ohms one way and OL with leads reversed. The coil in the relay tests at 92 Ohms which seems to be in specs of a coil. Also it I apply voltage to the correct terminals, I can hear the switch close.

I don't know a lot about the specifics of diodes, but doing a search means that IR stands for international rectifier.

I figured I would try replacing it first instead of buying the relay. I thought about going to Radio Shack and asking them.
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Old 08-18-2014, 01:34 PM
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Are you testing the diode in the circuit or have you removed it?
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Old 08-18-2014, 01:38 PM
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In the circuit
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Old 08-18-2014, 02:50 PM
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Do you know if the diode is a transient suppressor in the this circuit? I assume it is, which would mean it's in parallel with the coil with the cathode (the line side) to positive.

Your measurement troubles don't surprise me, most meters have issues with diodes and/or in circuit test of and with them. OL just means the diode isn't turned on, doesn't mean it's failed. Particularly when set to measure resistance many meters use too low of a voltage to turn on the diode or may partially turn on the junction, which is even more confusing.

With a suppression diode when you measure using the resistance setting you will see the coil value with the plus lead of the DMM to the plus side of the coil. With the leads reversed, plus lead to the nominally ground side of the coil you should see the diode turn on and a short if the meter can do it. Again, not all can. My Fluke does but it's supposed to and cost more than a few bucks.

The diode check function probably won't work with the diode in the circuit, the coil will shunt current and it'll look more or less like the diode is working unless it's failed totally and a short. You should see the forward voltage drop of the diode measured if it's good and nothing (open, OL) reverse biased.

In any case most of the time diodes fail shorted and that would be obvious when you test it with a meter. My guess is the diode is probably not bad unless you've seen a short at some point measuring it.
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Old 08-18-2014, 03:11 PM
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The diode is in parallel with the coil to suppress the voltage spike from going back to the regulator.
I did some reading on my multimeter manual and it seems that I was testing incorrectly. I had the diode test function selected but I did not switch it over to the small volt test. I tested it in circuit and got .575 volts with the plus lead on the cathode side and the negative side to the anode. Which is testing now seems like it would be a good diode. What do you think Dave?
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Old 08-18-2014, 03:28 PM
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A drop of 0.575V sounds right but the leads are reversed for a regular diode. You'd want red (+) to the anode (fat side of the triangle) and black (-) to the cathode (line side) to measure forward voltage drop. With the (+) to the cathode you should see a reversed bias junction and no current flow. In circuit it's possible the diode is failed open and you're measuring the voltage drop of the coil at whatever current the meter sources. If the coil resistance is 92 ohms a 0.575V would be 6.25mA, which sound like reasonable for a DMM. They don't generally have beefy current sources.

If you're comfortable tinkering you can connect a battery to the coil connections and just measure voltage drop across the diode to see if it's working. Connect it backwards (plus to ground on the coil) with a 4.99K resistor in series (to current limit) and see if you measure ~0.7V across the diode. If you hook it up right you wouldn't see anything at the diode unless it's failed short and then the voltage would be just above zero with maybe smoke coming out of the diode. One nice thing about that would be it's a definite indicator of diode condition. They don't work once you've let the smoke out.

BTW, do you happen to see anything black or does the diode smell really bad? Seriously. If not it's either still good or failed open if not.
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Last edited by DaveInDenver; 08-21-2014 at 07:38 AM.
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Old 08-18-2014, 03:36 PM
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Sorry, my mistake. You are right, I typed it out wrong. Well looks like this relay is good. Back to the wiring schematic for me.
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Old 08-21-2014, 12:58 AM
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Bill, don't worry about it. If the diode is open it won't cause any harm. It is for protection, like having an umbrella in the rain. If you get wet it isn't a problem, it is just uncomfortable.

As was written, it is to prevent reverse current from going back into your feed circuit which can eventually cause harm. Potentially. Good electronic design practices has all of us using clamp diodes across any magnetic load such as solenoid coils. When you turn off the juice going into a coil, physics says that the collapsing magnetic field sends a dump of that energy back to where it came from, and the reverse oriented diode just shunts it harmlessly back to a little energy dissipation. The real harm from these reverse pulse events is from excessive voltage on the semiconductors they are connected to. In the real world, it is rare and not something to lose sleep over.
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