Vic,
Run the 4 wire circuit and call it a day. I ran this very issue by a
licensed master electrician on Friday.
Direct quote: "If any portion of the load is 120 volt, you need a
neutral. If you are hell-bent on a 2 wire + ground supply, get a 208
volt fan and appropriately tapped transformer"
Martin
w2rwj
Vic K2VCO wrote:
> My amp project power supply has several 120v loads as well as the 240v plate
> transformer.
> They include a fan that draws about 100 mA, a screen supply that will take
> about 350 mA,
> and a filament transformer that will probably draw about 1A.
>
> The problem is how to connect the 120v loads. One solution is to just use a
> 4-wire circuit
> which will provide a neutral in addition to the two hot wires and ground.
> Then the 120v
> loads can be connected between the hot wires and the neutral.
>
> Another option is bridging the 120V loads across the two halves of the 240V
> plate
> transformer's primary. I've seen this done with fans, etc. The primary acts
> as an
> autotransformer to provide half voltage. Then I could use a simple 3-wire
> hookup.
>
> But I wonder how far I can go with this. If I put the filament transformer
> across one half
> and the screen supply across the other, would the unbalanced currents cause
> the voltage on
> the lightly loaded side to be excessive? This is a smallish amplifier -- the
> plate
> transformer is about 1 kVA.
>
> Has anybody tried this?
>
> No, I am NOT thinking about using the ground as a neutral! If the bridging
> idea is
> impractical, I'll bite the bullet and put in a 4-wire circuit.
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