Battery charging question

Using a Variac to convert voltage down to 12 volts is a really dangerous idea for a couple of reasons.

First, since the output voltage is adjustable over a huge range, you need to size the bridge and caps for the maximum voltage that you can get out of the Variac or you have a bomb waiting for the neighbor kid to turn the knob for you when you aren't looking. Or maybe you bump the knob... Anyway you can get a lot more capacitance rated at 26 Volts or

50 Volts than you can for 250 Volts for the same dollar figure. You can probably get 180 VDC out of a bridge rectified Variac, that would be really tough on a 12 volt input charger.

The second reason is more subtle. Because the variac does not provide line isolation, the (-) side of the bridge will sit below earth ground and the (+) side will sit above earth ground. You can't ground a standard scope to the negative side without letting the smoke out of something. You can get around this by using the the (+) output of the bridge and the neutral wire instead of the (+) and (-) outputs on the bridge.

Fixed output transformers are not that expensive, or just get one of the horrible freight adapters.

Good Luck, Bob

Thinking a little more about this, the line isolation is a really important safety issue. With a non-isolated transformer like a Variac, you are very exposed to the problems caused by an outlet with the hot and neutral conductors swapped.

12 Volt equipment often has the negative (ground) circuit brought out and exposed or even tied to the case of the equipment. If the hot and neutral wires were swapped in your outlet, the exposed negative circuit is now sitting at 120 VAC off ground.

An isolated power transformer is a required item in all consumer electronic equipment sold in the US.

Good Luck, Bob

--OK here's the problem: I've added a bridge to convert the output of my variac to DC and then I used a cap to smooth it a bit, but according to the oscilloscope the output is anything but "smooth". Anyway I'm just trying to use this to juice my battery charger, which is designed to run off of a car battery, but it turns out that the variac trick ain't working because the charger keeps tripping out; i.e. if I try charging a 12v battery it takes *days* to get it up to snuff; I talked to the guy at the local hobby shop and he said this is a symptom of insufficient power being sent to the charger. I get better results using a 15v wallwart. --What I need is to be able to charge a couple of different batteries, including motorcycle batteries and my 3 remaining 24-v packs of NiMH cells. What would be nice is a benchtop power supply that puts out 10 to 20 amps of smooth DC, but I haven't been able to find one. --Any suggestions appreciated..

A good lab supply works well for this. You can charge in constant current mode for nicads and constant voltage mode for lead acid.

A possible source for 12 V or so is an old PC power supply. I looked at a 300 Watt one that I have in the house and it shows 15 Amps availible from the 12 V outputs. If you use a PC power supply, you will probably need to put an amp or so load on the 5V outputs to get the supply to regulate properly. A 12V brake light bulb is probably enough. The AT supplies will be less hassle to hook up than the ATX ones because the ATX ones are electronically switched, where the AT ones have a mechanical switch in the 120V wiring. The 12 V out of this will work fine for feeding a 12V powered NiMH charger, but it won't charge a motorcycle battery (too low output voltage).

The little cheapo trickle chargers work OK for charging motorcycle batteries and should be availible for under $20US.

Bob