>
>Glenn McNeil wrote:
>>
>>A few questions about voltage doublers. I'm building a single tube 4CX250R
for
>
>>70cm using a voltage doubler
>>supply. This is my first attempt at FWD supplies.
>>
>>I have a transformer with about 750vac secondary, 12ohms dc resistance. I'm
>>planning on using IN5408 diodes and
>>a string of electro's.
>>
>In one word, DON'T. The secondary resistance of the transformer...
? He stated that sec. R was 12-ohms. The secondary-R in a SB-220's HV
transformer is 11.7-ohms and it delivers good regulation on SSB with a
2300w-PEP load.
>combined with the effects of primary resistance and diode on-resistance,
>which you also need to factor in - will ruin the voltage regulation. You
>will get disappointingly low voltage at zero-signal anode current, and
>it will droop severely down from there.
? Taurine feculence.
>The result will be quite hard
>limiting of RF output on speech peaks. Been there, done exactly that,
>and sorely wished I hadn't built a complete amp around that useless
>transformer!
>
? What was the secondary resistance of the schlock transformer?
>It was only many years later that I really thought about the standard
>FWD design charts, which were there in the ARRL Handbook all the time...
>and they could have told me exactly what was going to happen.
? Amen, Ian.
>
>The best practical advice is "don't ever use FWD unless you KNOW the
>transformer was specifically designed for FWD."
? Tranformer winding resistance tells one all that one needs to know.
>Otherwise it is not
>going to have a very low secondary and primary resistance, because they
>aren't needed for bridge or biphase operation.
>
? Methinks Ian would do well to look at the family of curves in the
ARRL Handbook for full-wave bridge (FWB) rectification. Secondary R for
a FWB is no less important than it is for FWDs. Also, a transformer for
FWB rectification has 2x the number of secondary turns of Thinner wire.
This increases secondary R. Anoher problem is that FWB transformers use
way more paper than an equivalent FWD transformer.
>>(1) I can either use 5 x 190 uf 400vdc caps in each side of the stack,
second
>>hand, or 5 x 1500 uf 250vdc caps
>>which are new but will give a much greater overall capacity. Is there such a
>>thing as too much capacity in the stack,
>>and will this make the supply too "stiff" if things go bang
>>
>You got it. Ideally you want just enough capacitance for adequate ripple
>rejection, and no more. Any excess is literally storing up trouble if
>things go bang.
? Hear, hear!
>Unfortunately FWD requires more capacitance than bridge
>or biphase in order to get the voltage regulation (although even
>unlimited capacitance won't rescue a transformer that has too much
>resistance).
>
>>(2) Is modern practice to still put C and R across each individual diode in
>>the rectifier stack.
>>
>This has been discussed here in times past, so I hope this is a fair
>summary:
>
>There are some valid reasons for using R and/or C,
? please name one.
>but much of what you
>see in published circuits is a holdover from the dark ages where PIV was
>very expensive and reverse breakdown was by an internal arc that was
>instantly fatal.
>
>Modern rectifier diodes have avalanche reverse breakdown characteristics
>and shouldn't need R or C (especially if they are matched in reverse
>breakdown characteristics). Editions of the ARRL Handbook since 1995
>explain this.
>
>A modern concern is that the effects of R and C on the diode turn-off
>and recovery characteristics have some EMC implications. If anyone has a
>good SPICE model of the 1N5408, for example, I'd very much like to do
>some more work on this.
>
>Some commercial designers use both R and C, some R only, some C only.
>Others use lots of diodes to get a very high PIV, with no R or C. To
>some extent these decisions seem to be guided by the availability and
>costs of components. For example, manufacturers can buy well-matched
>(and that's important) high voltage ceramic capacitors very cheaply;
>hams can't, so it may be more cost-effective for us to use the "many
>diodes" approach.
>
>>(3) Is it good practice to place a diode across each cap in the stack,
cathode
>>towards the + side, for reverse current
>>protection.
>>
>AFAIK, the only time that reverse current could occur in a capacitor is
>when a charged stack is being completely discharged through an external
>load (this is not what happens at routine switch-off, when each cap
>discharges through its own voltage-equalising resistor). Even in a 'big
>bang', a single cap could only reverse polarity when all the others were
>almost completely discharged too, so there's not much risk of severe
>reverse current. And finally, reversal could only occur if one or more
>caps is severely mismatched from the others in the stack. Unless there
>is some other scenario I haven't thought of, those diodes don't seem to
>be necessary.
? In one of my amplifiers, the rectifiers shorted - after c. 10-years -
sending AC to the electrolytic filter caps. The caps blew their pressure
vents and went kaput.
>
cheers, Ian
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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