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Proud Mary

Thu Dec 12 2013, 11:06PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Is the "-20, -10, -5, 0, +5, +10, and +20" electrically continuous? What are the DC resistances between these lugs?

Ash Small

Fri Dec 13 2013, 12:10AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

Is the "-20, -10, -5, 0, +5, +10, and +20" electrically continuous? What are the DC resistances between these lugs?

I've cut it off when I stripped the transformer out, but I suppose I could re-solder it.

It only connects to four primaries, two are ~4 Ohms, I think (110?) and two are ~0.5 Ohms or less (my DMM isn't accurate at these values) (I assume these are 10V).

My plan is to use a fuse on the output of the 24V step down transformer (which will probably be ~28V, due to voltage drop of rectifier, and "24 batteries are nominally 25.2V, I think, so if I use a variac, I can adjust it to 24V, and then measure the outputs of the various secondaries.

I need to use a fuse in case I get the phases wrong, and the inductances cancel out and it's only the DC resistance that's regulating the current. It's a pretty beefy 24V transformer. I don't want to 'cook' the Parmeko.

Proud Mary

Fri Dec 13 2013, 12:37AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

1A slow blow.

If the transformer starts humming loudly, if you disconnect at once, not much harm will have been done.

Ash Small

Fri Dec 13 2013, 01:44AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

1A slow blow.

If the transformer starts humming loudly, if you disconnect at once, not much harm will have been done.

Thanks. Chris also suggested 1 Amp. I think I've some in the shed somewhere

EDIT: Maybe the +/-5V is for Americans? (110 Volt land)

Proud Mary

Fri Dec 13 2013, 12:27PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I've just been in the shed and poked about in a box of Parmeko Neptune transformers - the cased oil-filled type - and selected four which had roughly the same physical size as yours, being perhaps a bit bigger.

Three had the 0 - 115V 0 - 115V type of primary where you have to wire them in series for UK mains, and one had strange voltages perhaps related to supplies on board ships of long ago with 100V and 107V options.

Anyway, three of these had total primary DC resistances of about 15Î© in the UK series configuration - or 7.5Î© per 115V section - and one had a DC resistance of 6Î©, or 3Î© per section.

I hope this gives you some idea about the order of DC resistance you are likely to find on your primaries.

Ash Small

Fri Dec 13 2013, 12:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks. My reading of ~4 Ohms per section seems pretty close to your value of 3 Ohms per section.

I suppose I could check I have the phasing correct by connecting the 'four series connected primaries' in parallel with a known value capacitor, and 'sweep' to find the resonant frequency. by re-arranging the primary series connections so that the inductance is at a maximum, I'll then know I have the phasing correct?

Proud Mary

Fri Dec 13 2013, 01:21PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

Thanks. My reading of ~4 Ohms per section seems pretty close to your value of 3 Ohms per section.

I suppose I could check I have the phasing correct by connecting the 'four series connected primaries' in parallel with a known value capacitor, and 'sweep' to find the resonant frequency. by re-arranging the primary series connections so that the inductance is at a maximum, I'll then know I have the phasing correct?

I did the tests with a cheap multimeter, which can be about half an ohm out when you get below about 10Î©.

On the Parmekos I looked at, the four primary lugs are arranged in a row like this 115 - 0 115 - 0 and the two lugs in the centre have short bridging wires soldered across them to connect the primaries together in series. I think it likely that yours would be organised in the same way, (which I have seen on many other valve era transformers) so that only a short jumper is needed to connect the two primaries together in the correct way.

If you stick your 24V AC on what you think are the primaries, you can then measure the voltages across the other windings, which should be roughly 10% of the value you'd get if connected to the mains. (Measure the AC input voltage at the point it's connected to the transformer, as a battery charger transformer might go to 30V or so with no or only a light load, and use this voltage as the one to calculate the ratios of the other windings with)

A Parmeko transformer with no load will only warm very slightly after an hour.

Ash Small

Fri Dec 13 2013, 01:53PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

OK, so I leave all the secondaries 'open circuit', I assume?

Proud Mary

Fri Dec 13 2013, 02:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

OK, so I leave all the secondaries 'open circuit', I assume?

Yes, just measure the output voltages that appear across them.

If you are putting 24V AC across the series primaries, the output voltage across a 6.3V secondary is only going to be about 600 mV, which you may not be able to measure very accurately, but at least you'll have an indication that it is a low voltage winding.

Ash Small

Fri Dec 13 2013, 04:56PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

Ash Small wrote ...

OK, so I leave all the secondaries 'open circuit', I assume?

Well, I can either 'scope it, or get the AVO out. I've plenty of moving coil meters that I can use if I bias them correctly and calibrate them. It's a lot easier to just use the DMM for 'approximate' measurements, though.

I doubt I'll get round to setting it all up until Sunday, at least. I'm off to London to get a clutch for my bike tomorrow.

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