[Solved] Identification of resistor on EXTECH 382202 power supply.

[rfengg]

Hi there,
Need your help on identification of a wire wound resistor on EXTECH 382202 power supply since I can't find the schematics/service manual.
The power supply output was always stuck between 19-35V when the voltage controlling potentiometer was varied and I found a 2N3055 shorted between collector-emitter.
One of the resistors in the pic is charred and both of them read open on a multimeter.

I believe both may be the same value but I cannot decipher the markings which read RX-21 6W RS3J on one of them.
Assuming that its a 6W rated wire wound resistor, should the value be 0.3ohms or is it 3 ohms?

thanks

[mikerj]

The 'R' in RS3J will indicate the decimal place, so most likely 0.3 ohms and the J will indicate tolerance (+-2%). The only thing I've not seen before is the 'S' which makes me a little wary.

If these are current sharing resistors in series with the two 2N3055 emitters then 0.3 seems feasible, that gives approximately 1.5 amps per resistor at maximum current, dropping 0.45v and dissipating about 0.7watts.

Be careful of where you buy 2N3055's, there are huge numbers of fake parts sold through Chinese sellers that have a fraction of the power rating of genuine parts.

[rfengg]

The 'R' in RS3J will indicate the decimal place, so most likely 0.3 ohms and the J will indicate tolerance (+-2%). The only thing I've not seen before is the 'S' which makes me a little wary.

If these are current sharing resistors in series with the two 2N3055 emitters then 0.3 seems feasible, that gives approximately 1.5 amps per resistor at maximum current, dropping 0.45v and dissipating about 0.7watts.

Be careful of where you buy 2N3055's, there are huge numbers of fake parts sold through Chinese sellers that have a fraction of the power rating of genuine parts.

Thanks mikerj......I have been bitten once by the fake 3055's off ebay and have stuck to Mouser/Digikey ever since.
Just confused why they have a put a 6W resistor if they power dissipation is only 0.7W? Resistor seems 10X over rated?

[mikerj]

They do seem a bit overrated for this application, perhaps the PCB is also used in a higher current design.  I assumed these were emitter resistors for the 2N3055 pass transistors, is that the case?

[SeanB]

Yes likely 0R3, and probably a fusible resistor, thus it cooked with a failed 2N3055. I pretty much do not like 2N3055 these days, preferring to use a 2N3773, as it is a lot better in all parameters, and is a lot more robust as well, plus has a better gain under high current, unlike the ancient 2N3055, which often these days is some random die in the case, as the spec to exceed a 2N3055 are so lax that pretty any modern power NPN device can easily exceed most of them  even on the production line rejects that are not totally shorted out.

[rfengg]

Thanks Sean, unfortunately I did order a couple of 2N3055's already from Digikey but will keep your advise in mind.

Do you know why they would use a 6W rated resistor if the resistor is indeed 0.3ohms? The max current thru the resistor under "normal" conditions should be 1.5A and so the power dissipated in the resistor should be 0.675W.....why would they use a resistor rated a order of magnitude higher?

[SeanB]

Current rating more than anything else, and also it will run cool, so the resistance will not vary with current as it heats up, thus allowing use of a much cheaper higher temperature coefficient resistor, as it only heats up a little, so the resistance change is tiny, so it will stay at the rated resistance no matter the current. Would recommend replacing both of the output transistors, along with both resistors, and check the driver transistor as well, as it could also be cooked, though I did not with one simple power supply, just removed the 2N3055, and insulated the leads, as I did not need the full 10A output, only 2A, so only having one was fine.

Output got shorted by accident, killing the one transistor as it ran into current limit, and with 35V input voltage it went well out of SOA in a few seconds. Just replaced the 15A fuse, that finally blew about a half second after the transistor.

[rfengg]

A weekend fully dedicated to repairs  .....managed to get the power supply up and running today morning.....procured two 2N3055's and two 0.33ohm/7W resistors from Digikey and soldered them in.
The power supply is back up and running and was loaded the supply to its max 3 amps capability and everything seems to be normal.
Thanks to everyone who helped me out with advice and suggestions 

[CJay]

Yes likely 0R3, and probably a fusible resistor, thus it cooked with a failed 2N3055. I pretty much do not like 2N3055 these days, preferring to use a 2N3773, as it is a lot better in all parameters, and is a lot more robust as well, plus has a better gain under high current, unlike the ancient 2N3055, which often these days is some random die in the case, as the spec to exceed a 2N3055 are so lax that pretty any modern power NPN device can easily exceed most of them  even on the production line rejects that are not totally shorted out.

You're as likely to buy a fake 2N3773 as you are a 2N3055 unless you buy from a reputable supplier, fakes of both have been a thing for at least 10-15 years.