[Boatanchors] Ohms Law

[Barrie Smith]

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.

The amount of heat I'm producing indicates that is true.

Thoughts?

73, Barrie, W7ALW