Powerstat and transformer for variable voltage output

You can do it, but you can not do it safely. The problem is that Powerstats are autotransformers. They do not provide isolation from the mains. In order to reduce the effective voltage to the primary of the transformer, you would need to connect the normally "Neutral" side of the Powerstat to the "Hot" side of the mains, in order to provide a voltage that is out of phase with the mains voltage. The normally "Hot" side of the Powerstat would connect to neutral. The "Low" side of the existing transformer transformer would connect to the tap of the Powerstat. I would not do this without an additional isolation transformer.

Build yourself a variable charger with the variac.

Add a bridge rectifier to the output of the variac, input and output fuses, an ammeter, a power switch. You could also add a timer.

Then you have yourself a 0 to 120 volt dc unregulated charger. Charge 8x

For lower voltages you could use your 12volt chargers transformer but if the charger has some kind of electronic output control you would have to bypass it because it certainly is designed for a specific input/output voltage. Won't hurt the charger to try though.

----------------------------------- AcidBURN

Nonsense

If the powerstat is connected normally then the Charger transformer provides the isolation just the same as when it was connected directly to the mains.

Powerstat top end to Mains Active. Powersat bottom end to Mains neutral and to bottom end of charger input transformer. Powerstat slider to top end of charger input transformer.

This will only allow slightley more than 12 volts charging if the slider passes the Powerstat top end that the Active is connected to, but it will allow any voltage lower than 12.

The efficiency is hardly a consideration when at full load the thing only uses 120watts plus losses.

Don't listen to AcidBURN, unless you want to die. :( That half assed design is a death waiting to happen.

The OP had the right idea in the first place. I've used a 3 amp variac with a 24 V C.T. "Motor home Power Converter" transformer for over 30 years with no problem, other than the original rubber power cord rotting away. I have charged single 2 volt cells, up to 12 volt car batteries with it. I like that I can set the charging current to what I need, from a few mA, up to 30 Amps.

You may have a point about safety but what I described is exactly what I have here that was bought commercially, a variac with a bridge rectifier in a plastic case, and mine is a 240vac. Been using it daily for years. It was even manufactured in Sebring, Florida. If he wants to be safe then he shouldn't modify any equipment that was designed for a specific purpose. So I guess all hackers and experimenters are suicidal.

Probably not. Ordinary household battery chargers are designed to put out a specific voltage. There's a possibility that your charger uses a "ferroresonant" transformer, that's got regulation built in - what this does is, when you decrease the input voltage, the input current actually _increases_, so that it can still put out the rated output. BTDT - it's weird watching the voltmeter and ammeter on the primary of a new ferro xformer, and watching it draw, like, 7 amps when the input is at, like,

If your battery charger is just an ordinary linear transformer/rectifier, then maybe, if you're very careful, have at least a voltmeter and ammeter, and know what you're doing.

How hard is it to open the charger and see exactly what kind of circuit we're dealing with?

Thanks, Rich

Its just a simple high current 100AMP shop charger, full wave rectifier/ct transformer configuration. The main thing i was worried about was efficiency at 1/2 the input voltage but i should have figured that out because the Powerstat says its 120 or 240 volt input, so i guess it was designed for 240 but will also work on 1/2 the input (120v) just as well.

Powerstats designed for 240 VAC input can be used on 120 VAC in boost mode, where you get 0-240/280 VAC out, but at only about 1/2 the nominal current rating. If you connect 120 VAC to the high end of the coil, you can get

Simple battery chargers generally are current limited by impedance of the transformer, and voltage limited by a zener diode and maybe a series pass transistor, with some voltage adjustment to set the float charge voltage. It is risky to use such a charger for batteries of different voltage, because the float charge limiter won't work. Also, unless it has settings for different currents, you could overcharge smaller batteries.

It should be possible to add an external current limiter and float voltage limiter to match the battery you need to charge.

Paul E. Schoen

Is this made to charge an enclosed, connectorized high voltage battery pack, or does it have exposed connectors where the user can be exposed to 170 volts pulsating DC?

It is made to charge from 1 to 20 x 12V or 40 x 6 volt lead acid batteries at up to 6 amps. The connections are made with standard copper clamps. Obviously not 100% safe but it works very well for charging.

I also have a couple of more sophisticated chargers that are 600vac 3ph input and 350vdc 50A output for charging industrial and auto motive batteries. Sometimes charging 300 batteries in series/parallel. These I have been using for 20 years. Connections are made with copper clamps.

This is a work environment so maybe it would not be a good idea in a home environment unless the proper precautions are taken.

AcidBURN

The variac/rectifier arrangement may be totally safe.... Remember that there ISOLATED variacs... those that have a secondary that is isolated from the primary, making it a plain old transformer with variable secondary voltage. Cheers!!!!

It wouldn't make any difference that it has isolation if you accidentally touched both leads with over 100 VDC at a couple amps.

Well... duhhh Obviously there is the risk of injury or damage if someone touched something of that voltage. What I was referring to was the risk of getting the hot/neutral mixed up and making the output live WRT the power line. The isolated variac indeed removes that risk unless the internal wiring has been altered.

-- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address)

Never take a laxative and a sleeping pill at the same time!!