Capacitor question

rkcstr · Sep 12, 2008

yeah, if you use a "diode test" on a multimeter, it should show a reading in one direction, but not the other.

GooeyGus · Sep 12, 2008

I know my meter has diode test... but I have no idea how to use it lol...

Edit:

I put my meter on the function that has the diode symbol next to it (which also happens to be the 2kOhm setting) and I get 1ohm both ways. I should also note that even if I just touch the two multimeter leads together, I get 1ohm, so that single ohm is probably not a function of the diode, but of the meter.

GooeyGus · Sep 12, 2008

Does anyone think I would cause issues if I removed this diode and then powered the unit on for a split second, just to see if the breaker blows, and then promptly turned it off?

Hemlock_Mike · Sep 12, 2008

Gus --
The components may already be stressed. Do you have a Variac? Bring the voltage up slowly with that. Read the ?dead? diode first. Is it shorted ? If yes, that WILL blow fuses.

Mike

rkcstr · Sep 12, 2008

Yeah, don't test it without the diode. If they are the AC rectifiers, they are part of what is converting the AC to DC voltage. Without one of the diodes, the DC voltage is going to be off every half-cycle, which may damage the PSU.

here's a page that explains the basic PSU with what the rectifiers and caps do:
http://www.kpsec.freeuk.com/powersup.htm

Also, thought I'd mention... if the rectifiers blown, it may be just a bad part, but it may also be something else that caused an over draw of current, which the rectifier was the first to blow.

As Mike suggested, a variac would be a good way to slowly raise the AC voltage to the PSU to watch for excess current draw suggesting a short rather than just powering it up and watching for something to blow.

GooeyGus · Sep 12, 2008

No variacs here :'(

But now that I saw the description on how the rectifiers work, it makes even more sense that it's one of these diodes. I think anything past these diodes would be an obvious failure, since it's basically running DC at that point. I'll double check them to make sure they are all the same part number, and I'll just replace all four at once. If one is dead I'm sure there are others that aren't far behind. Judging from the heat sinking on these diodes, I'm sure they're put through quite a bit of stress while running. Since this PSU was made in '91, it couldn't hurt to replace all of them.

rkcstr · Sep 12, 2008

Yeah, replacing all 4 wouldn't hurt... just make sure you get the same model or at least one with the same specs and package type.

I see its a Motorola MR2404. Unfortunately, Motorola no long manufactures parts, they are now made by Onsemi (which doesn't offer it). Most results on google give a datasheet for it from Micro Commercial, which I've never heard of. But, either way it looks like its likely an obsolete part, but I did find it here:

http://www.onlinecomponents.com/product/3592562/

Unfortunately, they also have a $35 minimum order :-/

There's also a seller on Ebay with them, but they charge $5 EACH! Plus $5.95 shipping.

http://cgi.ebay.com/MR2404_W0QQitemZ160078553012QQcmdZViewItem?_trksid=p3286.m20.l1116

Bionic-Badger · Sep 12, 2008

I've got an ExTech multimeter that can test diodes, but you have to switch to the sub-diode mode before it works. I've attached an image-tutorial on how to do it with the model I have. Yours is probably much the same. Can't hurt to test it before investing money in new diodes.

If some are indeed broken, these would probably be suitable replacements. Here's a list of other ones.

GooeyGus · Sep 12, 2008

I didn't find anything on mine to 'switch' modes into diode mode, but I did find that when I put it on 2kohm (which has the diode symbol by it) it kicks out more voltage than any of the other ohm settings.

rkcstr · Sep 13, 2008

Well, not sure what multimeter you have, but if you can't find or figure out the diode tester, you can just measure resistance. You should see some resistance with the negative lead (black) attached to the side the arrow is pointing on the diode (or the side with the stripe) and the positive on the other lead. In the other orientation you should see no measurement of resistance, open circuit (initially you may, but let it settle). A blown diode (rectifier) will likely show some resistance in both directions, meaning its shorted.

positron · Sep 13, 2008

rkcstr said:
Well, not sure what multimeter you have, but if you can't find or figure out the diode tester, you can just measure resistance. You should see some resistance with the negative lead (black) attached to the side the arrow is pointing on the diode (or the side with the stripe) and the positive on the other lead. In the other orientation you should see no measurement of resistance, open circuit (initially you may, but let it settle). A blown diode (rectifier) will likely show some resistance in both directions, meaning its shorted.

you could also use a LM317 circuit or similar as a constant current source to pass a small current through the diode and measure the voltage drop across the diode and if you try that in both directions, that is pretty similar to what the multimeter does on the diode test setting I think.

also for testing capacitors, they have a couple of properties that need to be tested to be sure they are ok, they have capacitance value uF and ESR. You can measure the ESR with a ESR meter like one of these: http://members.ozemail.com.au/~bobpar/esrmeter.htm

laserguys · Sep 13, 2008

Want something simple to test a diode???? ;D

Take a 9volt battery.. a 1k resistor.. an LED... and your Diode to test.
Put them all in series.

To test the basic circuit don't use the Test diode and the LED should
be "ON"

With the Diode under test connected with its Cathode (the Bar end)
towards the Neg of the battery... the LED should come "ON".

With the Diode connected in the reverse direction the LED should be "OFF"

I've used this in the past and it works on diodes as small as a 1N914 small
signal diode all the way up to a D400 400 amp 1200v power diode.

GooeyGus · Sep 13, 2008

laserguys said:
Want something simple to test a diode???? ;D

Take a 9volt battery.. a 1k resistor.. an LED... and your Diode to test.
Put them all in series.

To test the basic circuit don't use the Test diode and the LED should
be "ON"

With the Diode under test connected with its Cathode (the Bar end)
towards the Neg of the battery... the LED should come "ON".

With the Diode connected in the reverse direction the LED should be "OFF"

I've used this in the past and it works on diodes as small as a 1N914 small
signal diode all the way up to a D400 400 amp 1200v power diode.

Cool I'll do this tonight and see what I come up with. My biggest concern is that I wasn't sure that the 9v would be enough juice to make the diode do what it's supposed to do, but since you've done it all the way up to 1200v 400A I should be fine! I think mine are 400V and ~25A

daguin · Sep 13, 2008

GooeyGus said:
Cool I'll do this tonight and see what I come up with. My biggest concern is that I wasn't sure that the 9v would be enough juice to make the diode do what it's supposed to do, but since you've done it all the way up to 1200v 400A I should be fine! I think mine are 400V and ~25A

When you get done with this one, I have a Lasos PSU (basically a German-made Spectra-physics clone) that you can play with (in all of your spare time)

Peace,
dave

GooeyGus · Sep 13, 2008

daguin said:
[quote author=GooeyGus link=1221086698/20#28 date=1221333768]
Cool I'll do this tonight and see what I come up with. My biggest concern is that I wasn't sure that the 9v would be enough juice to make the diode do what it's supposed to do, but since you've done it all the way up to 1200v 400A I should be fine! I think mine are 400V and ~25A

When you get done with this one, I have a Lasos PSU (basically a German-made Spectra-physics clone) that you can play with (in all of your spare time)

Peace,
dave[/quote]

whats wrong with it?? After tinkering so much with this one, I have a much better understanding of how argon PSU's work. At least the smaller air cooled ones haha. I think my 3-phase beast will be a different story....

I ordered two new diodes from ebay so hopefully I'll be patched up soon. And if those don't work, at least I'll have another thing ruled out.

daguin · Sep 14, 2008

GooeyGus said:
whats wrong with it?? After tinkering so much with this one, I have a much better understanding of how argon PSU's work. At least the smaller air cooled ones haha. I think my 3-phase beast will be a different story....

I ordered two new diodes from ebay so hopefully I'll be patched up soon. And if those don't work, at least I'll have another thing ruled out.

It No Va! The lights come on, but the fan doesn't spin and the laser doesn't glow. It will be academic for at least a bit though. I hooked the laser up to the Cyonics PSU and the indicator LED and the optical monitoring diode circuit blew. At least two chips on the circuit board fried

I'll see if Chris has a replacement or knows of a work around. Then I guess I begin speaking German

I wonder If I can get any German frogs . . . . . :

Peace,
dave

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