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Posted by Dave Martindale on July 6, 2007, 1:02 pm
rakes@rakefamily.fsnet.co.uk writes:
>I have a 12v AC transformer supply to lights in my garden. I want to
>add a 12v low energy lamp in a summer house. This will only work on DC
>supply. I understand that the answer may be a Bridge Rectifier.
>Assuming a 11W 12v low energy bulb please can you advise on what I
>require and how to use?
That *might* work depending on what the "low energy lamp" actually needs
in the way of a power supply.
To begin with, you should realize that 12 VAC is actually a sine wave
with peak voltage of about +- 17 V. It's called "12 V" because that is
its equivalent-to-DC voltage with resistive loads like a light bulb.
So an incandescent bulb connected to 12 VAC will glow as brightly as it
does when connected to 12 VDC, even though the sources are quite
different.
If your summer house lamp needs 12 VDC, it is probably electronic. If
you use a full-wave rectifier with your 12 VAC supply, you'll actually
get pulsating DC that goes from 0 to 17 V and back down to zero again
120 times per second. Your lamp might be happy working on this, or it
might be damaged (unlikely), or it may buzz or flicker. You can add a
filter capacitor to smooth out the voltage from the rectifier, which
will give you about 16 VDC with less ripple, and *that* might be OK for
your light.
But the only way to get true 12 VDC without ripple is to use a regulated
DC power supply of some sort. You can often find power supplies from
old laptop computers rated at 12 V and several amps at thrift stores, so
that's one possibility.
(Also, note that a "12 V" lamp which is intended to be powered from a
car electrical system must be designed to operate over a range of about
10-14.5 V at least. 10V is a nearly discharged battery, 14.5 V is with
the engine running and the battery fully charged).
Dave
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