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?

In article .com,
wrote:

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?

Yes, you should be able to do this with a bridge rectifier and a 1000uF
capacitor. Don't know whether there's any off the shelf units that would
convert 12vac to 12vdc.

Smitty Two wrote:
In article .com,
wrote:

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?

Yes, you should be able to do this with a bridge rectifier and a 1000uF
capacitor. Don't know whether there's any off the shelf units that would
convert 12vac to 12vdc.

I did some Googling and could only find a couple of industrial-type
12Vac-12Vdc converters; problem is, they're meant for 1-2 kW loads and
cost about $400 - kind of overkill.

However, if you feel like having a whack at building your own rectifier,
here's a schematic I found:

http://www.ee.washington.edu/circuit...its/Supply.GIF

Now this one is set up for a 110Vac input, but since you've already got
the 12Vac step-down transfomer, just work from the secondary on that
going into the bridge rectifier per the diagram. This should be able to
supply 5A ouptut, which is more than you'll need, but better safe than
sorry. Should all fit nicely on a small chunk of perfboard in a project
case from Radio Shack. I'd also recommend getting a bridge rectifier as
a unit rather than building it from discrete diodes; the unit will be
marked with the AC inputs and the DC output polarities, which eliminates
one possible source of trouble in building it - getting a diode backward
(voice of bitter personal experience here :-D) Radio Shack has one, p/n
276-1185, which will be ample; it's rated for up to 50 V at 25 A, and
the cost is about three bucks. You can probably get the capacitors there
as well.

The LED and resistor across the output can be ignored; they're there
just to indicate that the output is good. Useful, but not required.

Yours aye,
W. Underhill

--
"Take sides! Always take sides! You may sometimes be wrong - but the man
who refuses to take sides must *always* be wrong! Heaven save us from
poltroons who fear to make a choice!" R.A. Heinlein, "Double Star"

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