[Boatanchors] (no subject)

[Bob Macklin]

I have not seen anywhere that says how much the voltage is off.

Why not just buy a good 5V filament transformer. Allied and Mouser have 
them.

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- 
From: "Drew P." <drewrailleur807 at yahoo.com>
To: <boatanchors at mailman.qth.net>
Sent: Sunday, April 18, 2010 7:42 PM
Subject: [Boatanchors] (no subject)



Barrie Smith wrote:

> I've been juggling 5 volt transformers all day, trying to
> find one that would actually produce close to 5 volts for a
> pair of tubes.
>
> They were all either too high in voltage, or too low.
> One did give me almost exactly 5 volts, but it ran way too
> hot.
>
> So I've selected one with plenty of current capacity and am
> playing with resistance in the primary to get the secondary
> voltage down to 5 volts.
>
> Using a 25 ohm, 150 watt rheostat, I find that 11 ohms gets
> me real close. The rheostat runs quite warm, which
> leads me to my question.
>
> The voltage drop across the rheostat is a measured 13
> volts. Knowing that and the resistance, Ohm's law
> tells me that I have 1.18 amps running through, which equals
> 15 watts.
>
> I don't want to install the rheostat in the transmitter
> because it is quite large, so I thought I'd substitute a
> fixed resistor.
>
> I tried a 15 ohm, 30 watt fixed resistor. It got
> very, very hot real quick!
>
> I must be making a mistake. I thought I should use
> the "dropped voltage" in Ohm's law. Perhaps I must use
> the full 115 volts, instead.

Another method of reducing the secondary voltage would be to remove turns 
from the secondary winding, assuming that the transformer construction is 
amenable to this type of modification.

I've done this to low voltage transformers on several occasions with good 
success.

Drew





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