Electronic – Need Clarification on Choke Placement (Vintage PS)

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I came into possession of an old Electro D-612T variable DC power supply. Based on what I've read here and other places it was manufactured in the 1950s, and sold for a whopping $39.95 US at Radio Shack, which today would run around $390 US. These were apparently used quite often at car radio repair shops. I wanted to clean it up a bit so that it could be used 'safely' in the garage for odds and ends where I don't need super clean filtered DC, and might need higher currents. This meant removing the selenium rectifiers and cloth insulated wiring.

I started tearing it apart last year, and as these things sometimes go I did not document the original wiring. After sorting out the windings for the variac portion, the rest is fairly simple to put back together. This brings me to my question. There is a reasonably large iron core coil, which I am fairly certain is meant as an AC line choke, but wanted to get some clarification in case someone else on here has come across one of theses supplies or similar. I have seen chokes in configurations both before and after the variac on various power supply circuits, but in this particular case I am leaning towards it being on the input due to the fact that the variac steps down the input mains voltages to a range of 0 to 12VAC. So I am thinking it would be more effective to filter harmonics / noise before the step down occurs. Would love to get feedback on this to make sure I do this correctly.

Best Answer

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This site says this about the circuit (clues): -

It has two ranges, 0-6 VDC and 0-16 VDC, and meters output voltage and current. It can deliver 10 amps at 12 volts continuously and 20 amps intermittently. It consists of a variable transformer, four large selenium rectifiers in a bridge, and an L-C filter consisting of an 8 millihenry choke and a 6,000 microfarad capacitor. Those of you used to dealing with chokes on the order of 10 henries in the high voltage supplies of radios and other tubed equipment may think that 8 millihenries is mighty small, but this choke is physically large to handle more than 20 amps of intermittent current, and the large filter capacitor makes up for its small inductance.

The clue from above is that it talks about the choke (8 mH) being on the 20 amp side and that has to mean on the low voltage side of the variac.

So I am thinking it would be more effective to filter harmonics / noise before the step down occurs.

Not according to the website above if we're talking about the 8 mH choke (no other is mentioned). It should go on the AC feed to the rectifier and not on the DC output because it would more readily saturate the inductor core on the DC output and that is to be avoided. If in doubt, use a simulator to check the basic functionality.

One more thing; it now being on the low voltage side means there is less danger of breakdown causing a fire.

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