>
>Rich Measures wrote:
>>
>>>
>>>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...
>>
>>?Ǩ 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.
>>
>That's for a significantly higher-voltage transformer (1175V?),
? the max. anode current for a pair of 3-500Zs is 800mA. For a 4cx250,
it is 250mA.
>and I seem to remember more than 10% droop at full load.
>
? not on SSB voice operation. Are you talking about AØ/NØN?
>For this application Glenn needs every last volt he can get, because
>even the off-load voltage is only just enough to operate a 4CX250
>efficiently at 432MHz. On-load with a poorly regulated supply, he's
>likely to be in trouble.
>
>>>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.
>>
>>?Ǩ Taurine feculence.
>>
>I checked the design charts before expressing my opinion. Did you?
>
? No. I have seen them a plethora of times. 12-ohms is quite
satisfactory for one 4cx250_.
>>>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!
>>>
>>?Ǩ What was the secondary resistance of the schlock transformer?
>>
>That was before I learned to measure such things first, and it's long
>gone.
>
>>>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.
>>
>>?Ǩ Amen, Ian.
>>>
>>>The best practical advice is "don't ever use FWD unless you KNOW the
>>>transformer was specifically designed for FWD."
>>
>>?Ǩ Tranformer winding resistance tells one all that one needs to know.
>>
>You need both the secondary resistance and the primary resistance. The
>primary resistance has to be multiplied by the turns ratio squared, and
>then added to the secondary resistance. This will often double the total
>effective resistance of the transformer. Then you must also add the
>effective diode resistance (see below).
>
? at 10a-peak, a 1N5408 has approx 1/10-ohm.
>And even when you have all the numbers, it's unwise to give a throwaway
>opinion without looking at the design charts to see what they mean.
>
? Surely. Who did this?
>>>Otherwise it is not
>>>going to have a very low secondary and primary resistance, because they
>>>aren't needed for bridge or biphase operation.
>>>
>>?Ǩ 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.
>
>I looked carefully this morning, and just looked again. What you say is
>not true. Secondary R for a FWB is *much* less important than for a FWD.
? And did you consider that a FWB transformer has 2x the number of turns
of smaller gauge wire?
>
>Let's take a specific example, for a supply giving 3330V off-load, and
>0.75A at full load. Use 30uF for FWB, or 2 x 60uF for FWD. What values
>of total effective series resistance RS do you need for 3000V on-load?
>
>R_load (RL) = 3000/0.75 = 4000 ohms
>
? 750mA for a 4cx250?
>Full-load / no-load voltage ratio = 3000/3330 = 0.9 = 90%
>
>FWB: omega*C*RL = 45 (@ 60Hz)
>From the chart, RS/RL for 90% regulation = 2.2%, so RS = 88 ohms
>
>FWD: omega*C*RL = 90 (@ 60Hz) - remember, twice as much C.
>From the chart, RS/RL for 90% regulation = 0.4%, so RS = 16 ohms -
>yes, only *sixteen* ohms!
>
>Also remember that those figures include the total contributions of the
>secondary, primary and diodes. RMS rectifier current is about 1.5x the
>RMS DC output current, so with say 6 diodes in series that's 6 x 0.7 /
>(0.75/1.5) = about 8 ohms. So there goes half of your 16-ohm maximum,
>before you even get to thinking about the transformer.
>
>>Also, a transformer for
>>FWB rectification has 2x the number of secondary turns of Thinner wire.
>>This increases secondary R.
>
>But not in the ratio of 88 to 16! That's my point: you won't find a
>transformer with winding resistances as low as they need to be for FWD
>unless it was specifically intended for FWD. In a transformer originally
>designed for FWB you're most unlikely to find such low winding
>resistances, because FWB simply doesn't need it.
>
Less R is always better.
>> Anoher problem is that FWB transformers use
>>way more paper than an equivalent FWD transformer.
>>
>I'd still guess that truly equivalent transformers - ones that gave
>exactly the same voltage regulation - would come out very similar in
>size, and the FWB one would be significantly lighter because it would
>have to use much more copper (half the turns but much more than twice
>the cross-section). Usually FWD transformers are smaller and cheaper
>only because the voltage regulation is poorer too.
>
? /chortle/
>
>>>>(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.
>>
>>?Ǩ 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.
>
>That's a good point, if no fuses blow and you don't hear the transformer
>groaning. Diodes directly across the capacitors would certainly take
>care of blowing the fuses, though there are probably better ways to
>tackle this problem.
>
? The electolytics failed in the blink of an eye.
cheers, Ian
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
end
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