Understanding a split-mode power supply.

As drawn, Q1 is intended to limit current in the 8R2 emitter resistor, when the main switch is functional. The event that killed the main switch and fused the emitter resistor, could have fried Q1 in the process. I think you'll find that Q1 is intentionally inverted (collector and emitter swapped). When it functions, it resembles a temperature-dependant voltage reference of ~ 1V4.

This is one of the simpler self-oscillating flyback circuits popular below 25W (in switch-mode power supplies) since about 1970.

Without the appropriate tools, training and test procedures, you cannot ensure that this unit, even restored to an apparently functional condition, is safe to use.

If you know the supply's output requirements, you would probably be better off replacing the whole assembly.

RL

The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction.

Sylvia.

Hmm....

I further surmise that the failure of Q1 doesn't explain the failure of the 8.2 Ohm resistor, because even with its specified maximum beta of

40, the switching transistor wouldn't pass enough current to burn out the resistor.

The implication is that some other event has done for the both of them, but surprisingly not destroyed the switching transistor.

I thought I'd understood the sequence of events, but clearly not :(

Sylvia.

"Sylvia Else"

** In an earlier post you state the PCB had been subjected to water ingress and insect attack - so almost anything is possible.

Odds are, the switching transistor was forced hard on by the above, taking out the 8.2 ohms and Q1 in quick succession.

Relace whatever parts you find are damaged, clean the PCB carefully with detergent and a brush (as you would washing glass ware etc in the sink ) rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair dryer).

Then cross you fingers and try it again.

If all is well, coat the PCB is clear acrylic lacquer to help it survive.

..... Phil

It's true that I mentioned brushing away a dead spider.

I wish I'd noted exactly where the spider was. It conceivably was around the 200K resistor, which could explain both the failures, and the demise of the spider.

Anyone know the resistance of a small spider before it dies from shock?

Sylvia.

** If you see one with three red stripes across its abdomen

- means it is 2.2 kohms, +/- 20 % .............

..... Phil

Power rating?

And what's the pinout?

Dave.

The power rating is certainly an issue. At 2.2 kohms, it would be dissipating about 50W. I would expect to find it splattered around PS cover, an effect which is noticeably absent.

At higher resistances, significantly more power would be dissipated in it than in the 8.2 ohm resistor. It's hard to see how the resistor can burn out without there being obvious signs of arachnid distress.

Sylvia.

You might find it on the web ...

"David L. Jerkoff"

** Depends on the size of the damn spider - f****it.

** Mobile octal - of course.

.... Phil

"Bob Parker"

** Cobblers ......

........ Phil

system.

Someone else , last month, asked a similar question here.I have now found the text book that I found useful for understanding SMPSs

Simplified design of Switching Power Supplies by John D Lenk, 1995

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Thanks for the reference.

Sylvia.

You seem to have access to some tools and a fair understanding of basic electronics. There's nothing preventing you from undertaking a repair by component substitution, for your own interest's sake, providing you are aware of, and use, use safe procedures around powered circuitry.

This assembly was designed to fail in a safe manner - no fire, explosion or shock hazard was intended to result from a single fault resulting in the fusing of the emitter resistor. However, this does not mean that the repaired unit will meet the same standards. The events that occurred may have affected the safety isolation of magnetic isolators in such a way that a second similar event may not be as benign.

If the main switch is not damaged (a big if), damage to the magnetic component's internal insulation is also unlikely. With the introduction of foreign particles, anything is possible. It should at least be subjected to a basic hipot stress test procedure, after the repaired and cleaned assembly is burned in and still warm.

RL

How about Q1 being a thyristor as a crow bar on the bias of that second transistor?

In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc..

That's just a guess of course.

"

To me it looks like a safety circuit that is designed that requires you to pull the plug , so that Q1 being a thyristor in my case of thinking will clamp down on the circuit from an over voltage, and remain that way until you pull the plug and wait. The 8.2R could just be burnt out or it maybe located in a thermo area intentionally to burn out (thermo Fuse) in which case, Q1 would also clamp.

Either scenario will prevent the coil to be energized, if that is what T1 is?

Since you haven't supplied any component part numbers, it's hard to say really, if that is what's going on.

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"

http://webpages.charter.net/jamie_5"

**What are the type numbers of the two transistors?

--
Trevor Wilson
www.rageaudio.com.au

**Scratch that. You already told me.

--
Trevor Wilson
www.rageaudio.com.au

In the self-oscillatin flyback, base current is supplied regeneratively by the feedback winding, to provide over-drive for saturated switching. Beta is assumed to be somewhere between 4 and 12.

Note that the drive connection is drawn across the EB jn, so fusing of the resistor would not turn the switch off - the voltage on the emitter and base terinals could then be quite large - popping locally connected parts like Q1.

Some re-examination of the actual board connections and further examination of the feedback section (usually not very elaborate) may offer further insight.

RL