wrote in message
oups.com...
On Apr 12, 8:42 pm, "Homer J Simpson" wrote:
"Arfa Daily" wrote in message

...

Does the circuit basically require a 12.0A 200V device?

Possibly 400V 6A instead.

Yeah, I was thinking that too, but couldn't quite get my head around
it.
I'm not sure how they rate bridges. When it's being used normally, two
diodes are in conduction at any one time, in what amounts to a series
circuit, so the quoted current rating must only be the rating of any
one
diode. However, as there are two diodes series'd into the circuit, the
voltage rating should be twice that of any one diode, so when a bridge
is
quoted at 200v, is each individual diode 100v ? That would make the
arrangement that's being enquired about, still 200v, but at twice the
current, as all four diodes will be in conduction. Either way, should
be
easy enough to pick up either a direct replacement bridge, or 200v 3A
diodes, which should be plenty enough.

I imagine it's all about dissipation so the amps should be the same.

The application is an Elmo school overhead projector. The unit uses an
82v 360W bulb. It looks like the bridge, that is, two sets of diodes
in series with those two sets then in parallel, is in series with the
lamp. When these bridges short, (usually just one diode in the bridge
shorts) the bulbs burn out. In this application it is used as a half
wave rectifier. I thought that perhaps by using a bridge in this
manner they were attempting to get a higher voltage drop across the
unit and less voltage to the bulb as there would be two junction
drops instead of one. However with those junctions then paralleled
with the other diodes maybe it would be a wash. Lenny.

So is this arrangement hooked directly to the incoming line voltage ( I
guess 110v in your neck of the woods ? ) The drops across the diodes will be
insignificant in this application, I feel, and it will be all about 'area
under the curve' from the half wave rectification that the diode is
performing. A bulb rated at 82v ( RMS ?? ) 360W will pull about four and a
half amps RMS when hot, more when it's cold. I guess that this is what this
is all about. The designer figured that it was a good way to get a high
voltage high current diode in a compact package with enough surface area to
self-cool. As it's a schools unit and probably subject to all sorts of
health and safety regulations, I think that I would feel inclined to stick
with the way it was designed originally, possibly uprating the specs of the
bridge, if they are a regular failing point.

Arfa