Component identification

[TheMG]

I hate troubleshooting "blind" but Lambda unfortunately don't release much if anything in terms of schematics or other service information for their power supplies. The unit in question is a Lambda LUS-11-12 switching power supply.

I'm trying to determine what this particular component is (see attached picture). On the silkscreen it is identified as "SR1".

Out of circuit multimeter readings (diode test mode +/-, diode test -/+, and resistance respectively):

Pin 1 to 2: O/L, O/L, O/L
Pin 2 to 3: O/L, O/L, O/L
Pin 1 to 3: 0.04V, 0.04V, 79.5 ohms

My guess, considering what part of the circuit it's in and the "SR1" on the silkscreen, I'd guess it's some type of SCR. If so, then it would be faulty considering those measurements.

If anyone can positively identify this part it would be a great help!

[wraper]

Pin 1 to 2: O/L, O/L, O/L
Pin 2 to 3: O/L, O/L, O/L
Pin 1 to 3: 0.04V, 0.04V, 79.5 ohms

My guess, considering what part of the circuit it's in and the "SR1" on the silkscreen, I'd guess it's some type of SCR. If so, then it would be faulty considering those measurements.

For SCR this is completely fine measurement, nothing wrong.

[TheMG]

Hmmm yeah then I've got more digging to do.

Also now that I look closer at what it's purpose is in the circuit, even if it were failed it would not cause the symptoms the power supply is experiencing. It is simply there to bypass the 5 ohm inrush limiting resistor once the capacitors charge up to a certain point. If it were bad the result would be either more inrush or a slight loss of efficiency.

BTW, the symptom is that the power supply outputs correct voltage when unloaded, but as load is applied the output voltage decreases rapidly as more load is applied.

Maybe I need to start looking at the feedback circuitry. Sucks not having schematics.

[EPTech]

Hi There,

The current and voltage feed-back are often wire-orred with some diodes. This should be easy enough to spot. There will be some diodes (most likely with common kathode) in series with some opamp output. You can disconnect the current feedback circuitry here by desoldering (lifting) the diode. Be careful not to overload the supply output because the current feedback will be disabled. If the supply stil does not deliver enough current, there may be a secondary short circuit (fold-back type) protection. This fold back circuit can take many forms. It may be an extra comparator in the CC circuit, an extra  opamp amplifier + comparator or the classical series resistor + transistor.

Try disabling the current feed-back first, that will rule out a lot. DO NOT short circuit the output.

Good hunting.
 

[TheMG]

I've been poking around at it quite a bit and I this point I'm fairly confident the feedback circuits are actually ok. I've measured the voltage across the collector-emitter junction of the feedback photocoupler and get about 3.5V with no load. If I lower the pot for the current limit to where it triggers and goes into current limiting, the voltage across the photocoupler output goes up, as one would normally expect.

When I add a moderate load to the power supply, the voltage across the output of the photocoupler goes to about 0.1V, in other words the maximum amount of feedback "give me more juice!!!" signal. If it were the current limit circuitry kicking in, the voltage would go the other way, up.

Also, I've observed the switching frequency goes from about 40kHz with no load up to 125kHz with load applied, with a small increase in duty cycle but not much. Not sure if that is normal as I don't know the topology of this power supply without reverse engineering it.

[EPTech]

Hi There,

My apologies. I assumed you were talking about a linear lab power supply. I did a quick search for your supply bit also did not find schematics. So the things I explained before do not apply.

If the frequency increases that much, you should get more output power, unless the transformer is shrinking with it .

Anyway, do not rule out the simplest of components, like the fast rectifier diode. Sometimes they are dual, maybe this one is not dual anymore.

Send us  a photo of the board please.

Good hunting.

[TheMG]

Well, I had shelved the power supply for a while and just yesterday decided to dig into it some more.

On a hunch, I decided to replace a 100uF 25V electrolytic capacitor in the primary side of the power supply which is located above some power resistors which do get quite hot.

BINGO! That did it!

Although the cap still measured 101uF on my multimeter, it must have gone high ESR. I should have clued in on this earlier as this is definitely not the first time I've encountered bad capacitors in proximity to heat dissipating components.

It's probably time I spend the money on an ESR meter...