Practically making a very low power 60 Hz transformer

There must be a optocoupler that would do the job. How about

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

void _-void-_ in the obvious place

I once used a small transformer for interstage audio tube operation. Quite small actually, fit in my hand (not like the output transformers...). The impedance was few kilo-ohms, DC resistance still quite high. Didn't complain at all when connected to the main because of the so high impedance, it run totally cool as if no power was connected to it. I used it for sensing voltage with good galvanic isolation. Just my idea...

Jack

I am trying to find the easiest way of making a very low power transformer. Its job will be to convert 240V to some voltage such as

So... With that in mind... What is the easiest design for a transformer. What is important is that

How about:

Anything else that is quick and dirty?

Sure.

I built this phase converter:

it works fine. Actually quite nice, it is easy starting and quiet and balanced.

I want to add some bells and whistles to it. So, I bought a K3NX-AD1C meter on ebay.

I want to make it measure various things such as voltage between all 3 legs of 3 phase power, AC currents, etc.

One way suggested was with a differential amplifier like AD629. I have some doubts about using them, perhaps out of ignorance, as I am trying to measure with reference points of 240 VAC and that could be too much for it. If I am wrong, I will be happy to use them.

So, I was wondering if I can convert AC to AC with a different reference point (with a xfmr), rectify it, and use a little divider to get a small DC potential to go into the meter.

With such big equipment why on earth does the transformer have to be so tiny? Just get a small 240V to 120V transformer plus a fuse, drill four holes into your chassis and mount it.

Regards, Joerg

Just get any small wall-wart transformer, but make sure its AC output, ie just a transformer and not a switch-mode type with DC out. You can usually tell by the weight - transformers are heavier. Even the older DC types can be used if you can rip them apart just for the transformer. There must be heaps of these lying around in junk piles. At least with a wall-wart the input side is mains rated and relatively safe, much better than you can do by winding your own.

You can resistively divide your 240 VAC down to something that you can safely apply to the differential amplifiers, then you can do any signal processing you want on the outputs of the diff. amps. You will want to reference your differential amplifiers to ground, which means that you won't be quite isolated, but you're able to use really high resistance sense resistors so it's probably not a big deal.

Note that if I were doing this for $$$ I'd be paying _close_ attention to electrical codes _and_ I'd probably want to subcontract some consulting to an expert. In a production environment I'd probably want to make something that's totally isolated, so I'd do something like power a board off of the AC, measure the AC, convert the measurements to pulse trains with V/F converters, optoisolate the results, then convert the (isolated) pulse trains with F/V converters (or measure the pulse trains with a counter built into a microcontroller). This would be cheaper than transformers if you were making 1000 boards a year, but for a 1-off it'd be a waste of engineering effort.

Yes, that should work. Do a web search for "precision rectifier" -- just a diode bridge will introduce nasty errors. Keep in mind that a power-grade transformer will be optimized for cost and power transfer, _not_ for accuracy. You can expect that the output of such a beast will be phase shifted and probably show saturation effects; I wouldn't expect more than 10% accuracy out of a power transformer and if I were under contract I wouldn't guarantee _anything_.

In this case, however, 10% accuracy is probably all you really need, and it's certainly all I'd look for in an economy sensing unit.

Well, if you want it small, maybe use a little audio transformer from a pocket radio and add a 0.1uF cap in series. You will get a few milliamps from the secondary, but I don't know what the voltage would be.

-Bill

He needs three, but Mouser has little transformers that are quite reasonable, albeit I don't immediately see a 240/120 unit.

Good Luck! Rich

Sure, reducing the difference is not a problem. However, the difference would still referenced to phase voltage.

Think about trying to measure voltage between leg 1 and leg 3.

That makes good sense. I will think some more about it. I think that I can get away with using a differential amplifier if I reference voltages to legs 1 and 2 (two hots of the 220V electrical system), which are only 1220V away from ground.

Hello Rich,

You can buy little transformers that operate EU gear on US power and vice versa. Usually cheap,

I am eyeing some transformers that may cost me 60 cents apiece... But I need to buy 50.

So what if you'd sell the remaining 47 of them for 99c on ebay? Then each of your three transformers would have cost you -$5.50 ... 8-D

Regards, Joerg

Yes... You are right... I bought 27 Semikron 240A diodes for $4 apiece on ebay and m selling them now in sets of 4 at $15 apiece. With these transformers though, it is too much hassle.

selecting a DC current process meter for measuring AC voltages and currents is a rather poor choice. generally for that task the selection is an AC voltage / current process meter. then the switching and scaling is relatively easy.