# AC Voltage Step-up

Discussion in 'Beginner Electronics' started by Gordon, Nov 1, 2003.

1. ### GordonGuest

Hi,
My outboard boat engine has an AC auxillary output for onboard 12 volt
use (so the manual states), however I have noticed the voltage varies
between 8 and 19 volts AC depending on idle to full revs. As most of
my boating is with low engine revs I am concerned that I'll never
reach 14.7 volts DC (after AC to DC)for battery charging.
So I am considering stepping-up the AC using a transformer and then
using a voltage regulator to limit the DC to 14.7V accepting that the
available current will be reduced.
Two questions:
1) Is there a smarter way of doing this?
if not, then
2)How well would a standard 50Hz transformer perform over a frequency
range of 25 to 250Hz? I'm considering a 115V to 240V converting
transformer for this.

Any help would be gratefully received!

Regards,

Gordon Jago.

The AC output may be adequate after it is converted to DC. The AC you are
measuring would be "RMS", to calculate the DC equivilant you would use: RMS
* 1.414 therefore at 10.5 VAC the full wave rectified DC would be 14.87V. If
you can get your Rev's up to give that voltage you should be OK. If not try
this:

You could take the AC Voltage and put it through a "DC Voltage doubler
circuit" then regulate the output to 14.7 VDC. Or rectify the AC to DC and
use a DC to DC converter. A 50 HZ transformer can not be used for 25HZ, it
will burn up.

3. ### GordonGuest

Thank you for the prompt response and also for the reminder on RMS
from my old school physics theory!. I was wondering if I was trying to
fix a problem that didn't exist!

GJ.

4. ### Guest

I don't agree. A transformer will work fine on half the frequency but
there will be a decrease in performance. The power output will drop.
If you still need the same power you need a bigger transformer to
compensate for the losses. At 250hz the performance will increase but
depending on the core material there will be a limit.

Regards Simon