Testing a diode

[skippie101]

I am testing several Diodes in a circuit and they all read the same values or close to it one way and then no reading at all the other way when reversing the digital meter probes.

I have read that a high reading on one way should be 10% difference from the low reading the other way. If not, then the diode is bad.
Also, read that a high reading both ways indicates a blown Diode and a low reading both ways indicates a shorted Diode.

So, if I am getting a reading of 7.43 one way and Zero the other is this a good Diode or not because of the 10% rule.

Thanks so much.

 

[KeepItSimpleStupid]

7.3 what? Peaches? Ohmmeter diode tests are somewhat unreliable. The ohmmeter partially turns on the junctions of surrounding semiconductors. Withany in circuit ohmmeter tests you have to know what's surrounding the diode.

Hopefully your ohmmeter reads OL for open, ~ 0 for shorted. Ohmmeter tests for diodes used as rectifiers are generally open in one direction when tested out of circuit.

A more valid test, is the diode test where a current is passed through the diode and the voltage drop is measured, but again, in circuit tests depend on the surrounding components.

 

[skippie101]

Hey KISS,

Peaches, funny stuff. I try to be as detailed as possible when trying to describe something in writting. I guess I goofed.

Anyway, Yes, Ohms. I have a basic digital multimeter that tests Ac/Dc volts, Ohms, and continuity. I did unsolder one leg from the circuit board before I began testing the Diodes. As for the Diode acting as a Rectifier, how would I know that?
I am telling you I need one of those books Electronics for Dummies.

 

[Reloadron]

A diode is a uni directional current devive so it allows the flow of current in only one direction. So when testing a diode using a basic ohmmeter you will see a high resistance in one direction and a low resistance in the opposite direction. Different DMMs will bias the diode under test in different ways and different diodes will respond in different ways. Thus it is difficult to say the forward bias resistance will be xx.x ohms or the reverse bias will be xx.x ohms. The simple rule being that in one direction very high resistance and in the other a low resistance. That is about all an ohmmeter will tell you, makes it easy to find a shorted diode or open diode (most simple rectifier diodes fail short but not all). Using an ohmmeter to test a diode will not for example tell you if the diode is breaking down prematurely as to its ratings but gives a basic indication as to good or bad. More extensive testing of a diode requires a device like a curve tracer. This is based on doing as you have done and lifting a leg.

Ron

 

[KeepItSimpleStupid]

0 and 7.3 is probably bad. Take a 9V battery and a 10K resistor in series . Put it across the diode and measure V across the diode in both directions.

Units and significant figures have been drilled into my head so much that my head hurts. Sometimes 1E18 and 2E18 are virually the same number.

 

[skippie101]

Thank you some good advise.

 

[skippie101]

A diode is a uni directional current devive so it allows the flow of current in only one direction. So when testing a diode using a basic ohmmeter you will see a high resistance in one direction and a low resistance in the opposite direction. Different DMMs will bias the diode under test in different ways and different diodes will respond in different ways. Thus it is difficult to say the forward bias resistance will be xx.x ohms or the reverse bias will be xx.x ohms. The simple rule being that in one direction very high resistance and in the other a low resistance. That is about all an ohmmeter will tell you, makes it easy to find a shorted diode or open diode (most simple rectifier diodes fail short but not all). Using an ohmmeter to test a diode will not for example tell you if the diode is breaking down prematurely as to its ratings but gives a basic indication as to good or bad. More extensive testing of a diode requires a device like a curve tracer. This is based on doing as you have done and lifting a leg.

Ron

Thank you Ron.

Let me clarify what you said in my own terms and correct me if I am wrong.

Testing a simple 2 leg ceramic diode, I should obtain a high value on one side and lesser value on the other.
Although, the lesser value should not be zero unless the diode is used as a rectifier. Value obtained not necessarily important when doing this kind of test as long as the high value is not greater than 10% or less than 5%.
A rectifier will be several diodes in a housing and will have several legs soldered to a circuit board.
I have read that testing a ceramic diode if both sides are high it is blown and if both side are low readings it is shorted.
Thanks again

 

[crutschow]

The lesser reading will never be zero for any rectifier or diode unless it is bad.

To clarify, there is no fundamental difference between a solid-state diode and a rectifier, it just depends upon the application. A rectifier is just a high current diode that is typically used at higher currents to rectify AC power to get DC power. It's called a (signal) diode when it's a low current device (capable of perhaps a few hundred mA max.) used for small signal applications such as signal detection or rectification.

 

[alec_t]

As for the 10% rule, wherever did you find that? For a diode out of circuit (or one leg lifted) you would expect a MUCH greater than 10% difference between the forward and backward resistance measurements if the diode is ok.

 

[skippie101]

I read that on a website from a google search "ehow.com".
That is also where I read about the shorted and blown information.
Thanks.

 

[KeepItSimpleStupid]

The term "rectifier" is "probably" a left-over tube terminology. e.g. Selenium rectifier or vacuum tube rectifier. The rules would be different. The Se rectifier is about 1" x 1"and has lots of fins and stinks when it goes bad. The rules are also different for high voltage solid state rectifiers. In all cases though, a short in both directions means it's bad.

A diode is a "rectifier", but is sometimes used as a "detector" in RF circuits. The half-wave rectifier, full wave rectifier and full wave bridge rectifiers are topologies on how diodes are used to produce DC from AC.

The plating industry calls a DC power supply a "rectifier": https://www.caswellplating.com/kits/rect.htm

 

[KeepItSimpleStupid]

Beware, "You can test if something is bad, but it's difficult to test if something is good.". Testing a fuse with an ohmmeter out of circuit bit me twice.

 

[skippie101]

Beware, "You can test if something is bad, but it's difficult to test if something is good.". Testing a fuse with an ohmmeter out of circuit bit me twice.

Yes, I can see this would be true.

I am just trying to get my Reel to Reel player operating correctly. Nothing earth shaking, like high voltage or big. I have been looking at the schematic and just now understanding a bit more than I did before. Even though I can actually tell what is what and where it is on the circuit board, it will not really help me unless I can test the parts correctly.
Thank you for your time and information.

 

[carbonzit]

So what's wrong with your tape recorder that makes you think the rectifier diodes are to blame?

 

[skippie101]

So what's wrong with your tape recorder that makes you think the rectifier diodes are to blame?

Well it was working fine, except for the capstan motor, so I changed a transistor, I goofed when I changed the transistor for the capstan motor. I had forgot to put the plastic shield behind the transistor when I mounted it back on the metal plate. So, the metal plate in contact with the metal frame of the transistor shorted together. Even though it did not affect the operation of the capstan motor, that is working fine, I know have a problem with the reel operation. When I turn the unit on, it automatically goes into the fast forward mode and I can not stop it from running, no matter what other operation button I press. Sometimes but not always when I press the stop button it will blow the main fuse.

 

[carbonzit]

I'm not an expert electronics troubleshooter.

But I might suggest taking a more systematic approach to your problem than just randomly testing parts you suspect to be bad.

If you think you might have fried part of the power supply by shorting that transistor case to its heat sink, the thing to do would be to find the power supply "rails" (i.e., the lines that supply power to various parts of the unit) to see if they're alive. This can be done without yanking out any parts, using a voltmeter. Takes a bit of tracing, and obviously would be a lot easier with a schematic.

Is the power supply on a separate circuit board? if not, is it clearly segregated from the rest of the unit, so you can see where its outputs are?

 

[skippie101]

The supply is on its own board. I did find the points where it connects to the PCB and it tested 120v.
I do have a schematic and just really learning how to navigate through it and identify the electronic components.
I know what you mean by a systematic approach, which I am trying to stick to but I find myself saying well let me try this.
It does get quite confusing, for me anyway. I am not giving up just yet, though.

 

[carbonzit]

The 120V (AC) is the input to the power supply; you want to test the outputs.

If you have a schematic, that should be very do-able.

How many different voltage outputs does the schematic show for the power supply?

Your job is to find each output, put your voltmeter on it and see if it's delivering what it's 'spozed to.

Sometimes the output voltages are silkscreened right on the PCB.

 

[KeepItSimpleStupid]

Step one is usually look for visible damage. Use smell.
Step two is usually check for supply voltages.

Then there is what's wrong and verify what;s wrong. You have an additional piece of info, like what shorted to what?

 

[carbonzit]

Step one is usually look for visible damage. Use smell.

Yes, very important: I forgot that.

In other words, look for obvious signs of damage. Burnt or discolored components or circuit boards. Bulging electrolytic capacitors. Stinky components (hot parts often give off a distinctive "electronic-y" smell). Possibly even blown-apart components (unlikely, but it does happen).