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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 

To eliminate the possibility of excessive loading I removed the bridge rectifiers. The circuit still squegged.

Magnetics?
With all defective components already replaced, could the problem be in the magnetics? No spare for the main transformer T1 (potted in a metal case, custom made and now obsolete), but T2 was several turns of magnet wire on a toroid core. Was there a shorted turn? Was the core somehow magnetized and saturating from the previous faults? Took it off the PCB and measured the inductance with the following circuit of figure 3:

Figure 3: Measuring transformer inductance with a scope and square wave function generator.

Figure 4: The response of the DUT transformer With C = 1nF the inductance calculates to 21.3µH using the formula L = 1/[(2πf)2C].

Figure 4 looks not too bad but is quite damped even with all windings unloaded. Shorting the other half of the primary winding caused the inductance to decrease drastically, proving that there were no shorted turns.

Because of the low voltage involved it was unlikely that the transformer was breaking down under actual operating conditions, and the toroid core was not electrically conductive so had not been zapped by a high voltage (lightening strike) surge. But there was still the question of possible core magnetisation or an invisible crack, and since I had a few toroid cores of about the same size in the junkbox I wound a few with the same turns count. The original core material type was unknown, but one of my jerry-rigged attempts came close, see Figure 5.

Figure 5: Still 1µs/div, the Amidon FT-50A-77 toroid material has higher inductance (85.9µH) but is otherwise suitable.

Unfortunately, the squegging continued, so I put the original T2 back onto the PCB and threw the others back into the junkbox for future use.

Pulled the main transformer T1 off the PCB and did similar tests. Again shorting another winding caused the inductance to decrease drastically, proving that there were no shorted turns inside the transformer. But what was it doing under the higher voltage of normal operation?

Note (in figure 2) the T1 winding in series with the 82R5 resistor and 0.1µF capacitor. I opened this branch up and wedged in (series) a small transformer (Midcom 50398, another junkbox artifact) and drove it with a square wave from a function generator. This forced the circuit into continuous operation to get a better look at what was happening to the squelching of the squegging. The series 0.1µF capacitor differentiated the square wave to limit the duty cycle of the transistors' on time, hopefully to a safe value that would not blow up the transistors with the continuous operation.

I reconnected T1 back into the circuit with some alligator clip wires. To measure transistor and transformer current I inserted a 1Ω resistor into the emitter branch of the circuit and shorted the second 129µH choke (the one in parallel with the diode and 27Ω resistor) to remove its effects from the measurement. Current could then be read on the scope as the voltage dropped across the 1Ω resistor. I should have seen a rising edge on each current pulse due to the unloaded transformer inductance. Instead I saw this:

Figure 6: It does appear that T1 primary current is not ramping upwards as expected. Is something wrong with T1?


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