Posted on 02/03/2024 5:48:43 PM PST by Paul R.
I've built power supplies using full wave rectifiers in the past, and a couple circuits with single diode half wave rectification for non-critical supplies, but in all cases the power requirements were low and the diodes I had on hand several times overrated for the current and voltage / power involved.
This time though, I want to add a half power function to a 1500 watt 120 volt quartz heater. (Over-simplification - see below comments. But ~half of the 1500 watts is the design target.)*
So... I know a diode in series with the quartz element and no power supply capacitor** will essentially cut the power in half, as current is flowing ~1/2 the time. (Area under the curve.) So, is a silicone diode rated at 10 amps & 1000 reverse volts (I have a few in my parts box) going to have enough current headroom?
1500 watts at 120 volts means the quartz element draws ~ 12.5 amps. Half that is 6.25 amps, but since the element will be operating at a lower temperature, current drawn and power emitted will be a bit higher. That will be slightly mitigated by the diode's forward voltage drop. I'm pretty sure the current won't be over 8 amps, but, it also seems to me that 8 amps actual is pushing a 10 amp diode pretty hard. Safest bet might be to parallel 2x 10a diodes, or, order some 20 or 25 amp diodes? (I'd prefer to use the parts on hands.)
Note that the switch & diodes will be in a steel "handy box", most likely, so I can give the diodes significant but not "great" heat sinking.
From experience I know the 1000v reverse voltage rating is more than enough. I've never even blown a 600 volt diode in a 120 volt AC rectification application. (Yes, I know, one has to be careful with DC ratings vs. where the peak of a dirty sine wave may go.)
Comments by those with some experience are welcome / requested!
My brother’s 750,000 volt Tesla coil got me one time. Knocked me on my butt. Luckily, there was not a lot of current behind it...
You could always adapt or build a PWM controller driving an H-bridge, to give you an infinitely variable temperature control.
*Yes, sir!* "Do you feel the urge to get up and send me a thousand dollars?" Close! I thought he was talking about me there for a second.
Maybe I should have called this the “Fireworks Night” thread. And I was off by a day - geez!
feel the lightning...
Seems pretty reasonable.
Sending a PM shortly... (Just a "funny")
BTW you basically will have no CEMF from a heating element. Therefore your reverse voltage only has to be at the peak of the AC line voltage. So for 120 volts RMS you have a Peak of around 170 volts. So a Diode with a 200 volt reverse voltage would handle it.
Even if you plan to use this only when you are running with one element, you still have to make sure you can handle both, unless you build this inside the heater after the switch.
Sounds like the electronics are a second level challenge.
The first level challenge will be the ladder ballet.
I would tell you, but then I’d want a cut of whatever market you manage to corner....😁
The only rectifier I ever built was a 4-diode full wave to charge my battery when the voltage regulator in my Fiat went out - and I would have had to buy a while new alternator to replace it - just used the biggest diodes I could find in our bench stock....
You’re Grounded !!!
lol
I have some large oil filled caps. You don’t play hot potato with them. They are heavy. They don’t leak voltage.
They are good for solar power applications, because of the low voltage drop.
I keep quite a few of them on hand.
Correct!
Heh, 2nd comment to that effect.
In this case the prospective market is just me.* This diode technique to provide “half power” is, as I mentioned above, used in maybe 20% of the quart” heaters on the market.
Basically, if you see all the elements dim on “low power”, vs. “high”, a series diode has probably been switched into the circuit. More commonly, the heat setting is determined solely by the number of elements running full blast. I have a different floor standing quartz heater with 3 elements, so the user can turn on one, two, or all 3. Each element is ~500 watts.
*That said, I just now ordered my 20 pcs. of 20 amp diodes. So, I can modify a couple other heaters too, maybe even that 3-element unit. I don’t “need” 6 heat settings, but, as a preventive measure to extend the lifetime of both the elements and (using the controls wisely) the thermostat, it might well be worth it.
You punny! :-)
I never thought to wonder, are there “Schottky” transistors? In some uses I can think of, a low voltage drop at the p-n junction in a transistor could be useful.
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