Sounds like you need an "autotransformer".

Many Variacs are actually autotransformers, in that they accept 110V input,
and output from 0 to 130 volts AC. The principle works as such:

=======================
mmmmmmmmmmmmmmmm
| | ^
A B C

If you supply the inputs to wires A and B, and outputs on A and C, and C is
the adjustable portion of the Vairac. If the voltage from A - B is 110
Volts, when C is farther out on the transformer, you will see a higher
voltage from A - C (above 110 Volts). When it is inside of B you will
reduce the voltage (below 110 Volts).

My bench Variac actually inputs 110V, and outputs from 0 to 220V.

You can also do this with a multi-voltage transformer:

Do not care about Secondary
==========================
mmmmmmmmmmmmmmmmmm
| | | | |
A B C D E
100 110 120 130
This is a common multi-input voltage transformer used in international
equipment, having inputs of 100, 110, 120, 130 Volts. This will typically
be a low voltage transformer, but you really don't care about the secondary
side.

If you typically see from 100 to 130 volts and you want 110 volts, you would
hook up the output to A-C. and the input to:

100V A - B
110V A - C
120V A - D
130V A - E

With this setup, you will boost or decrease the voltage depending on where
you have the input.

You will need to find a transformer large enough to supply your peak
wattage, and remember that even if you normally draw 9A at 110V, in the 100
V boost configuration, your input current will be 9.9A.

Also autotransformers DO NOT isolate the secondary from the line, so
electrical shock is still possible.


"Panama" wrote in message
...
I have a 40 amp battery charger that has a 5 position switch to select
the battery charging voltage. Works great. Allows steps from about
13.2 to 17 volts. Prob done by a multi-tap transformer?

Trouble is - AC voltages in Mexico can be as high as 128-130 VAC and
other places can be as low as 105 VAC. The charger reacts by changing
the battery charging voltage up or down - a little but enough to vary
the DC amps that get to the batteries. I'm not going to replace the
charger.

I'm looking for a cheap variac/scr/triac something gizmo that I can
wire into the AC line to the charger and make the effective voltage
(RMS value?) go up or down by +/- 10% or so. The AC in is about max 9
amps. It could even be a big wired wound variable resistor I suppose.
Then I can set the AC line voltage to what my genset puts out and use
the charger at it's max 40 amps all the time.

Any ideas or products. Preferably cheap and never needs replacement
like the 27 years old charger.

(No I'm really not going to replace the charger.)