[HomeBrew] Dual output power supply?

[N2EY@aol.com]

In a message dated 2/14/04 5:37:20 PM Eastern Standard Time, 
n6kyr@netzero.net writes:


> Glad you answered this one! HV choke is a 6-12 Henry "swinger" while the 
> low voltage item is a 6 Henry 150 mA unit. 

OK, let's do the math.

If the HV choke is a 6-12 Hy swing choke, you need for the load resistance to 
never be less than 12,000 ohms. This means your load must never draw less 
than 45 mA at 540 volts in order to keep the output voltage from soaring. Also, 
the choke must have 12 Hy with 45 mA flowing through it. 

The 45 mA can be bleeder current, load current that is present all the time, 
or some combination. Dissipated power is 24.3 W, and if you are using 
resistors they should be derated in power by a factor of 2 for safety. 

On the low volt side, with 6 Hy you need a 6,000 ohm load, and it works out 
to 45 mA as well. But the power dissipation is only 12 W.

The big question is how the load behaves. If it's a steady load like a Class 
A amp, you may only need a safety bleeder (high value, just to make cure the C 
discharges.

But if it's a load that varies a lot, like a keyed Class C amp, the bleeder 
may have to do the whole job. 

Everything will be solid state rectification. I will probably use some fairly 
large 
> caps for filtration. I do have a number of 520 uFd/400 VDC caps in stock 
> and I also have access to the 100 uFd/350 or 100uFd/450 vdc caps if needed. 

oh mama

A pair of 100s in series should be plenty for the high side, and a single 
100/450 OK for the low side with choke input.  


> 
> On the filament side of the house: I don't have to use the onboard filment 
> windings on the plate. In fact, the 6.3 VAC c.t./20 Amp Triad I have can power 
> both the 812A's, the two 6146's, the 807, the 6L6, and a pair of minis with 
> lots of headroom to spare. 

If you do that, remember that the 812As are directly heated! The center tap 
must be ground-returned for them. 

What's the exact rig lineup?

Or, if it is going to be critical, I can always get another smaller filament 
xfmr 
> dedicated for just the Millen modulator and transformer. I will probably 
> use, however, the 5.0VAC tap with a reversed 6.3VAC to 120 VAC filament 
> transformer for biasing voltage. I need -45 VDC so I'll probably just half wave 
> rectify and filter the output of that and then use a pot to tap off the -45 VDC 
> spot to bias the transmitter.

That works if the bias doesn't have variable load. 

Another trick is to series up all the filament windings and use a voltage 
multiplier. If you have, say, two 6.3 windings and a 5.0, all of them in series 
will give 17 V. A voltage doubler will give 2.8 x 17 = 47.6 V, which is pretty 
close! 

You need lots of PIV in the rectifiers, btw. With a 600 V trans, each diode 
in the bridge will have to withstand at least 840 volts peak. But in real life 
I'd have at least 2000 PIV in each leg. (actually, in real life I'd use tube 
rectifiers, but that's another story...)

> 
> Other than that, that's it! Let me know your thoughts. Cal. N6KYR. 
> 
> 

That trans is more than adequate to supply the loads mentioned. When you use 
a bridge rectifier with a transformer designed for fullwave centertap, it's 
good practice to calculate the total VA that will be pulled from the 
transformer, because in the original design they expected each half of the HV secondary 
to produce current only half the time, and with bridge the whole secondary 
produces current all the time. The limiting factor is how much primary copper and 
core iron you have.

The Ancient Ones used to push transformers pretty hard in the economy 
circuit, but  that was when you could get replacements much more easily and 
inexpensively than today. They also relied on things like the low duty cycle of CW and 
SSB, and underloading of the heater windings.

What sort of rig are you building with all those bottles?

73 de Jim, N2EY


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