Diodes

[mull982]

Hello

I was wondering if anybody can help me out with some diodes I am troubleshooting in an SCR. It is my understanding that diodes will only allow voltage and current to flow in one direction? When I have now power across the diodes (480V) and I measure the resistance across the diodes, I get different resistance values when I reverse the positive and negative leads of my meter across the diodes. Does anyone know why this is? Across two of the SCR leads even without power, the resistance reading fluculates back and forth between closed and open. Should this flucuation be happening even without power on the device?

Would appreciate any help.

Thanks

Mull982

 

[EV607797]

In general, diodes will only pass DC current in one direction and block it in the opposite direction. If you put an ohm meter across it, you will get a virtual dead short across the leads with the polarity in one direction and wide open in the opposite. Is the 480 volts you are working with AC or DC? This will make a big difference since the diode will allow current to pass through every half cycle on AC.

 

[mull982]

The voltage I am working with is 480V AC. I follow what you are saying about the different resistance readings depending on the polarity of the meter leads, but is this because the meter produces a small amount of voltage and it will only conduct through the diode in the one direction and not the other which leads to the two readings you are talking about?

When i take a resistance reading across two of the diodes in the SCR the meter reading bounces back and forth between open and closed. Should this reading be jumping around like this? What causes the resistance reading to go back and forth between open and closed when using the meter and no other power is connected?

Also I am seeing a high resistivity value across each of the diodes in the kilo-ohms range. Is this a typical reading I am seeing, or should it be some other value?

 

[dennis schaffert]

diodes

diodes

It is possible that if the diodes are still apart of the circuit you are back reading through some other component unfortunately isolating a diode usually means a solder gun and at least one side of the circuit open

 

[dereckbc]

but is this because the meter produces a small amount of voltage and it will only conduct through the diode in the one direction and not the other which leads to the two readings you are talking about?

Diodes and SCR are similar but different animals. Diodes are have an anode and cathode, where as a SCR has a third junction called a gate.

To answer your question; no the meter does not send a voltage, it sends a small DC current, and then reads the voltage developed across the resistance the current passes through. If you have one of the modern DVM, it should have a diode tester, which is basically a ohmmeter calibrated to diodes. When you attach the negative lead to the cathode you should read the forward resistance of .7 volts, and when reversed it should read OL.

This will not work on a SCR because the SCR gate has not been turned on. You have to take an additional step to forward bias the gate and your DVM is not capable of doing that.

 

[rattus]

Fyi:

Fyi:

For the record, SCR means Silicon Controlled Rectifier. It may be used as an electronic relay. It is often used in switching power supplies to regulate the ouuput voltage. A similar device, the TRIAC, is used in AC applications, e.g., dimmers.

In the old days, an electronic tube known as a Thyratron behaved like an SCR, but not nearly as well.

 

[Twodollar]

You need to test diodes on the diode setting of your meter. The ohmmeter setting does not supply enough voltage to turn on a diode. And really you probably only want to know if it is working, not the actual resistance which would require an ac measurement. The forward voltage will be about 0.7V and the meter should say that. Your SCR will have three terminals, A, K, G and you will only see diode action by putting the positive lead on G and the negative lead on K. No other combination works.