Checking fast recovery diode

[bbjunkie]

Got an unnamed LCD that uses Vestel 17ips16-3 PSU unit, making a very quiet clicking noise and wont come out of standby.

I have located a diode in the PSU which I feel may be faulty, just want to run it by you guys before I order a replacement.

It's a BA159 - a fast recovery diode datasheet at http://html.alldatasheet.com/html-pd...2/1/BA159.html
and is D810 on the board if anyone is looking at a schematic.

In one direction open circuit, other direction 5.9M Ohms both measurements taken out of circuit.

This is faulty, right?

Jim

[Toasty]

Re: Checking fast recovery diode

What type of meter (DMM/Analog) and what scale are you using to test?

EDIT: Grabbed the diagram from elektrotanya.com for you.

EDIT 2: <sigh> Sheet is for 17IPS16-1 or -2 not -3 as on their site...

[Toasty]

Re: Checking fast recovery diode

Looks like there are several of these on the board. Does this one differ from the others?

[bbjunkie]

Re: Checking fast recovery diode

Haven't checked the others, good point i'll lift another and compare them. D801 and D803appear to be the same component. Will check against them.

[retiredcaps]

Re: Checking fast recovery diode

In one direction open circuit, other direction 5.9M Ohms both measurements taken out of circuit.

I normally use the diode test function to test diodes. A good diode should test between 0.5 and 0.8V one way and "out of range".

[bbjunkie]

Re: Checking fast recovery diode

Did this too, reading .440 or so (cant recall exactly, will re-test when im back in the workshop) but the extremely high resistance had me wondering.

The whole reason I pulled it to test in the first place was that it was measuring 95k ohms in both directions in circuit.

[bbjunkie]

Re: Checking fast recovery diode

Ok, checked and D800 801 802 and 803 are in fact not the same. Perhaps this is one of the differences in the -3 version.

There dont appear to be any other BA159 diodes on the board. In fact there are only 5 on the primary side, 4 rectifier diodes and the D810 which is in circuit with a few other components across pins 1 & 3 on the primary side.

I will potter around some more with it and see how I get on. Any further problems and I will start a repair thread and post some pics.

[tom66]

Re: Checking fast recovery diode

I don't think you're dealing with an open diode problem - a shorted diode would cause the symptoms described

I've never seen a diode fail open, but I suppose it could happen.

Bad SMPS controller, bad startup cap?

[PeteS in CA]

Re: Checking fast recovery diode

Diodes hould be checked using the diode checker function. An ohm meter can detect shorts or abnormally low resistance. But a healthy diode will usually read high resistance in the forward direction and extremely high or out of range in the reverse. An exception to that latter is that I have seen some schottkies read onthe order of 200 ohms in the forward direction. Different diode types have different forward voltages, which can be seen with a diode checker. Schottkies are usually low, .2V-.5V; regular GP diodes perhaps .5-.7V; FR and ultrafast perhaps .8-1.2V. I have seen highvoltage ultrafast parts as high as 1.5V.

Sounds to me like your power supply is in a burp-mode current limit state. That diode could be bad - that forwaard voltage sounds low to me, though different meters can read differently. Maybe check to see if the output is shorted by something in the load, or maybe a bad output filter capacitor.

[bbjunkie]

Re: Checking fast recovery diode

Ok, it has beaten me - i've started a repair thread: https://www.badcaps.net/forum/showth...463#post214463