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Power supply conundrum: A wild ghost chase

Posted: 20 Jan 2016     Print Version  Bookmark and Share

Keywords:oscilloscope  trigger  motors  commutators  ammeter 

Emitter current through 1Ω; 1V = 1A (Iavg on bench supply meter was 220mA)
Q2 collector voltage
Scope ground –ve side of 1Ω

With no way to obtain another T1 of the same type, I pulled a common mode choke off a junkboxed scrapped PCB with about the same inductance rating for a temporary test. A common mode choke has two identical windings and would make a good test substitute for the DUT T1 centre-tapped unloaded primary to verify my test setup. I removed the original T1 from the circuit and jerry-rigged this choke to the DUT using the same alligator clip wires.

The results were an eye-opener. See figures 7 and 8.

Figure 7: With the common mode choke in place of T1 there is no change when Q2 is turned on, but the current is nicely ramped when Q1 is turned on.

Emitter current through 1Ω; 1V = 1A (Iavg on bench supply meter was 220mA)
Q2 collector voltage
Scope ground –ve side of 1Ω

Figure 8: Cannot explain above. Why does only Q1 have the desired current ramp?

Emitter current through 1Ω; 1V = 1A (Iavg on bench supply meter was 220mA)
Q1 collector voltage. Spike hits 200V offscale. Scope ground –ve side of 1Ω
Scope ground –ve side of 1Ω

The common mode choke load showed very different switching characteristics between the two push-pull drive transistors, unlike the original transformer. Swapping the choke windings made no difference, the problem stayed with Q2. Very obviously something was wrong with the Q2 portion of the circuit.

Finally with a solid clue to work with, I poked around again with an ohmeter. Both Q1 and Q2 collectors measured 21.5Ω to the chassis. I now had the culprit – must be that 1µF capacitor between T1 centre tap and chassis that had previously checked good. But nope – pulling that cap off the pcb made no difference. And there was no other connection to the chassis in that circuit, in spite of what the ohmeter was telling me.

Then a nasty suspicion hit, and with the ohmeter still connected I backed off the mounting screw of Q2 about a quarter turn. The meter jumped to about 8K ohms.

I backed off the mounting screw of Q1 – the meter jumped to infinity – open circuit.

Somehow the collector mounting flanges of the transistors were leaking through the mica insulators to the chassis. I had known there was a small crease where the insulators had been bent, but that should not have caused any problem even if the cracks did penetrate completely through the insulators because the thermal grease is non-conductive – the data sheet said so.

Took another look at the data sheet. The thermal grease is non-conductive if smeared on a pcb. It is intended for heat sinks attached to CPUs; our staff uses this grease when mounting heatsinks onto BGAs. In this use, even if it was conductive it would not matter.

But the data sheet says nothing about what happens when the grease is compressed under the much higher pressure of a screw holding a transistor flange to a heatsink. Remember the grease contains microscopic silver particles immersed in an oil base – were these silver particles coming in contact with each other when squeezed tightly? And if so, when squeezed into a crack that fully penetrates a mica insulator this grease could cause exactly the problem I was seeing.

I emailed the Arctic Silver company and received a phone call a couple hours later. Yes, they confirmed that their product would become electrically conductive when under pressure.

I then obtained two more intact mica insulators and some real thermal grease that did not contain silver particles – you know, that white stuff everyone refers to as bird-poop. No more problems after that.

Note: Glen Chenier, the author, passed away on January 5, 2016. This article originally appeared on Scope Junction.

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