2N3055 failure (power supply)

"Ben Jackson"

** Yes - change all those 0.1 ohm resistors to 0.33 ohms.

Also, use all *new* 2N3055s from the same batch to effect the repair - or, if funds allow, use all new MJ15003s.

Otherwise device current sharing will likely be haywire.

BTW

Love the old Natsemi LM723 reg IC - versatile as all hell.

....... Phil

Not having any resistance between the common base node and the output bothers me. This gives the gang of 3055s a slow turn off and a voltage overshoot when a .load is step reduced.

Something between 10 and 100 ohms might improve this.

The 0.1R resistors are there to promote current sharing between the five

2N3055 transistors. If the circuits that the power supply is connected to, are not real sensitive to over-voltage then you might replace the 0.1R resistors with fusable resistors. If you do that, then you might want to put a resistor and LED circuit across each fusable resistor (to let you know when one is blown). If you need more protection than that, then you might want to put a voltage sensing circuit on the output and a fuse in the collector circuit. Have the voltage sense circuit trigger a SCR, that blows that fuse in the collector circuit.

Brian Ellis

In addition to the other suggestions, I recommend that you consult the 723 datasheet and investigate the foldback current limiting suggestions given there. It's well worth the trouble if there's a possibility of a short on the output. Could save your power supply and an expensive repair job on connected equipment.

Cheers!!!!

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Some days you\'re the dog, some days the hydrant.

put in a SCR crowbar circuit to clamp and pop the fuse when it over voltages?

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

"Brian"

** For the benefit of the OP - the simplest crowbar circuit would consist of a 1 watt zener diode, a 56 ohm resistor and a 10 to 15 amp rated SCR ( TO220 pack).

The zener conduction voltage is chosen and tested to be the same as the supply's nominal output voltage (ie 13.8 volts ?).

Connect the zener from the supply output to the G (gate) terminal of the SCR, the 56 ohm goes from G to K (cathode ) while the A (anode ) terminal connects also to the output positive rail. The K terminal also goes to the output common or minus terminal.

The ( 20 amp ? ) fuse may be in the unregulated DC supply or low voltage AC feed to the rectifier.

....... Phil

I cant read this circuit very well, it appears to be an emmiter follower. If thats the case then you have 2 basic problems. Firstly your output is not inside the regulation loop so your regulation will be poor. Secondly you have no short circuit protection, or indeed any current control. Both these problems are easily solved by looking at the application notes for the 723.

** Nonsense.

The OP supplied a partial schem and STATED that it was a current limited supply.

** The OP very likely has a schem like figure 4 here:

Rsc is probably one of the 0.1 ohm ballast resistors shown in his schem, resulting in a 6.5 amp limit per 3055 device.

....... Phil

The partial schematic I drew > ** Yes - change all those 0.1 ohm resistors to 0.33 ohms.

Good idea, but I'm not sure this supply (BK Precision 1688) has room for the 10W resistors I'd need. It was definitely not built for maintenance. The final assembly consisted of pushing the PCB onto the lugs of all the front panel stuff (outputs, meters) and soldering it in place!

Yes, I'm h> In addition to the other suggestions, I recommend that you consult

The supply does include current limiting, but the nature of the failure really took the 723 out of the equation. That's one of the reasons I was looking for mod suggestions, which brings me to:

Jaime wrote and Phil Allison expanded:

That seems like the best plan. The max in-spec voltage is 14.5V (I think this was meant for automotive applications).

Thanks all.

--
Ben Jackson AD7GD

http://www.ben.com/

That's the way I read it too.

The Op stated: "The net result was that the failure caused the supply to jump to 22V+, far higher than its typical max voltage, and with NO CURRENT LIMITING.

Unless you are seeing something that I'm not seeing, I don't see how you can think that. In the partial schematic that the OP showed, the 0.1 ohm resistors are being used to make sure the power transistors in parallel, are sharing the current (close to equally).

Brian Ellis

"Ben Jackson"

** How many amps will this supply deliver ?

The reason for the suggested change, BTW, is that 0.1 ohm ballast resistors only work well with carefully Vbe matched devices.

If your new ones are all suitably Vbe matched, then use 0.1ohms.

...... Phil

When the output power transistor shorts out, there is no current (or voltage) control.

Brian Ellis

"Brian"

** Indicating that the current limiting inherent in the supply was rendered ineffective by the faulty 3055.

The OP has now confirmed this IS in fact correct.

** Look at the schem again and see what is blindingly obvious.

Any one of the 0.1 ohm ballast resistors can be used as Rsc.

Then the max output current = 5 times the current limit set by that resistor.

........ Phil

"Brian"

** Never in dispute.

Your earlier false assertions were.

....... Phil

Well i guess Phil, one of us cant read.

Maybe but since he didnt state it its just speculation on your part.

That would be too bad if it was since the 2n3055 wont survive a short at that current for long.

You act like he finally came to that conclusion. He stated that from the very beginning.

Even if that "should" be the case, so what, it has nothing to do with his problem

Of course it is.

The operative word here is "can". But is it? It doesn't matter, it has nothing to do with his problem.

It's a 20A supply, so 4A per 2N3055, the .1R should dissipate about 1.6W max and the .33 would be a little over 5W so I'd need the bigger package. I would also have to trace the rest of the circuit and see what else might be affected by the change (there's an LM339 looking at at least temperature and maybe other things that will shut down the 723).

I ordered 10 so maybe I'll measure them and choose the best 5.

--
Ben Jackson AD7GD

http://www.ben.com/

"Brian"

** You have a serious reading comprehension disability.

What the OP has confirmed is the supply IS indeed current limited.

** You have a serious reading comprehension disability.

Plus an utterly ASD f***ed brain.

Go top yourself anytime - ASSHOLE.

........ Phil

** If you were not such a complete ASSHOLE might feel sorry for you. ** An educated speculation that has been now confirmed as correct.

Enjoying your sour grapes ????

** Its within the DC safe area curve for a 2N3055.

A temp cut out switch on the heatsink would prevent failure.

PSU design is obviously way over your pointy head - ASSHOLE.

........ Phil