Posted on Feb 5, 2014

Malcolm Moore`s schematic is good, but it`s not enough for a dangerous man with a soldering iron to do anything useful with. Namely, what is T1 A coil(s) of some type, but the configuration Also Q1, Q2 values ditto SCRs. I was intending to respond to your query on Tuesday under the ``69 450 desmo regulator` thread so I`ll do both here. If anyones eyes glaze

Click here to download the full size of the above Circuit.

over, go to the next message :-) Firstly regarding my schematic on Willy`s site: T1 is a transformer with four separate windings (coils), A B C & D. The dashed line is intended to indicate the windings are all on the same core. Where isolation is required, triggering SCRs via a trigger transformer was (is) a common industrial control technique in the days before optocouplers were readily available. To understand how the circuit works it is best to start with Q2. If you imagine Q1 to be erased from the drawing, Q2 is a simple oscillator. The coil in it`s collector circuit (T1-B) is magnetically coupled to the coil in it`s base circuit (T1-A). When power is first applied Q2 will start to conduct. This causes an increasing current to start flowing in coil T1-B and so a changing magnetic field is produced in the transformer core. The coil T1-A acts like a good little transformer secondary and a voltage is produced at it`s terminals. The coil is connected such that this voltage acts to turn Q2 off. The now reducing current in T1-B then produces a voltage in T1-A of opposite polarity to before and this turns Q2 on again. The end result is Q2 rapidly turns on off on off etc. The other two coils on T1 are also secondaries and produce pulses of current that are applied to the gates of the SCRs. An SCR will start rectifying when it has BOTH the gate triggered AND the anode is +ve with respect to (wrt) the cathode. For each SCR this...

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