Chasing wild ghosts: Page 4 of 7

January 10, 2016 //By Glen Chenier
Chasing wild ghosts
Glen Chenier describes how he sets out to track down some wild ghosts in a system.
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 1 ohm resistor into the emitter branch of the circuit and shorted the second 129µH choke (the one in parallel with the diode and 27 ohm resistor) to remove its effects from the measurement. Current could then be read on the scope as the voltage dropped across the 1 ohm resistor. I should have seen a rising edge on each current pulse due to the unloaded transformer inductance. Instead I saw this:

Emitter current through 1Ω; 1V = 1A (Iavg on bench supply meter was 220mA)

(Iavg on bench supply meter was 220mA)

Q2 collector voltage

Scope ground –ve side of 1 ohm

Figure 6  It does appear that T1 primary current is not ramping upwards as expected.

Is something wrong with T1?

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

Design category: 

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