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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.repair
Subject: Re: Voltage halver.
Date: Wed, 3 Apr 2024 22:14:32 +0100
Organization: Poppy Records
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Roger Hayter <roger@hayter.org> wrote:

> On 3 Apr 2024 at 21:29:19 BST, "Liz Tuddenham" <Liz Tuddenham> wrote:
> 
> > <peter@easthope.ca> wrote:
> > 
> >> A variable power adapter is required to drive a 6 V, 30 W incandescent
> >> bulb in a microscope.
> >> 
> >> Similar to the 3-12 V 5 A adapter here.
> >> https://www.ebay.ca/itm/305207182573
> >> An adapter providing 1.5-6 V output would be better but I haven't
> >> found one.
> >> 
> >> Rather than limit the adjustment to 6 V, I think of halving the
> >> output.  An integrated circuit in a 4 port package is conceivable.
> >> https://www.youtube.com/watch?v=VJfqBQ2ybpk
> >> 
> >> Does the marketplace offer a solution?
> >> Another idea?
> > 
> > A 6v transformer and an adjustable resistor made from an old electric
> > fire bar and two jubilee clips?
> 
> I thought of that, but it is not very useful to adjust while looking down the
> microscope and I wondered if AC might affect microphotography.  Have you seen
> the price of 30W WW pots or "rheostats"?

I hadn't, but I expect they would be expensive..  By comparison, making
big power resistors is surprisingly cheap -- I had to make some for a
5kW organ blower starter, so I had some slate bars cut for me by a
monumental mason and wound them by hand (with tappings).   The total
cost was surprisingly low.

I don't see how AC wold be a problem with microphotography unless you
were worried about ripple in the light output.  Low voltage lamps like
that heve a short thick filament with high thermal inertia, so they were
often used as the exciter lamp for for sound film reproduction.  Any
ripple would have generated a 100c/s hum, but there has been very little
sign of that on the vintage projectors I have worked on.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk