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How much reserve needed in power supply diodes? Sunday Night Electronics...
Me | 2/3/2024 | Paul R.

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!


TOPICS: Hobbies; Miscellaneous; Reference
KEYWORDS: diode; electronics; itssaturday; power; supply; vanity
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To: mabarker1

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...


41 posted on 02/03/2024 7:36:01 PM PST by Paul R. (Bin Laden wanted Obama killed so the incompetent VP, Biden, would become President!)
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To: Paul R.

You could always adapt or build a PWM controller driving an H-bridge, to give you an infinitely variable temperature control.


42 posted on 02/03/2024 7:38:35 PM PST by JustaTech (My mind is the weapon. Everything else is tools.)
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To: mabarker1
"Are you sitting in a bean bag chair, naked, eating Cheetos!"

*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.

43 posted on 02/03/2024 7:40:32 PM PST by Lockbox (politicians, they all seemed like game show hosts to me.... Sting…)
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To: Paul R.

Maybe I should have called this the “Fireworks Night” thread. And I was off by a day - geez!


44 posted on 02/03/2024 7:41:07 PM PST by Paul R. (Bin Laden wanted Obama killed so the incompetent VP, Biden, would become President!)
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To: mabarker1

feel the lightning...


45 posted on 02/03/2024 7:45:26 PM PST by Chode (there is no fall back position, there's no rally point, there is no LZ... we're on our own. #FJB)
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To: samiam5
I usually double the expected rating.

Seems pretty reasonable.

Sending a PM shortly... (Just a "funny")

46 posted on 02/03/2024 7:45:50 PM PST by Paul R. (Bin Laden wanted Obama killed so the incompetent VP, Biden, would become President!)
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To: Paul R.

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.


47 posted on 02/03/2024 8:35:09 PM PST by Revel
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To: Paul R.
During the time when the diode is on, the full half wave current will be drawn. The average voltage and current (RMS actually) are your quoted 120 volts and 12.5 amps. However at the peak of the wave will be 170 volts and 17.7 amps. Even with the diode in the circuit, you will still get those values in the active half of the cycle.

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.

48 posted on 02/03/2024 10:11:41 PM PST by KarlInOhio (Democrats' version of MAGA: Making America the Gulag Archipelago. Now with "Formal Deprogramming")
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To: Paul R.

Sounds like the electronics are a second level challenge.

The first level challenge will be the ladder ballet.


49 posted on 02/04/2024 5:49:50 AM PST by KC Burke
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To: Paul R.

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....


50 posted on 02/04/2024 5:59:19 AM PST by trebb (So many fools - so little time...)
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To: Paul R.

You’re Grounded !!!


51 posted on 02/04/2024 10:10:02 AM PST by mabarker1 ( (Congress- the opposite of PROGRESS!!! A fraud, a hypocrite, a liar. I'm a member of Congress!!!)
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To: Lockbox
Why
52 posted on 02/04/2024 10:20:23 AM PST by mabarker1 ( (Congress- the opposite of PROGRESS!!! A fraud, a hypocrite, a liar. I'm a member of Congress!!!)
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To: Chode

53 posted on 02/04/2024 10:25:33 AM PST by mabarker1 ( (Congress- the opposite of PROGRESS!!! A fraud, a hypocrite, a liar. I'm a member of Congress!!!)
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To: mabarker1

lol


54 posted on 02/04/2024 11:06:45 AM PST by Chode (there is no fall back position, there's no rally point, there is no LZ... we're on our own. #FJB)
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To: fretzer

I have some large oil filled caps. You don’t play hot potato with them. They are heavy. They don’t leak voltage.


55 posted on 02/04/2024 6:13:29 PM PST by Texas Fossil (Texas is not about where you were born, but a Free State of Heart, Mind and Attitude.)
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To: sasquatch

They are good for solar power applications, because of the low voltage drop.

I keep quite a few of them on hand.


56 posted on 02/04/2024 6:16:24 PM PST by Texas Fossil (Texas is not about where you were born, but a Free State of Heart, Mind and Attitude.)
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To: KC Burke

Correct!


57 posted on 02/05/2024 4:47:50 AM PST by Paul R. (Bin Laden wanted Obama killed so the incompetent VP, Biden, would become President!)
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To: trebb

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.


58 posted on 02/05/2024 5:00:18 AM PST by Paul R. (Bin Laden wanted Obama killed so the incompetent VP, Biden, would become President!)
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To: mabarker1

You punny! :-)


59 posted on 02/05/2024 5:00:50 AM PST by Paul R. (Bin Laden wanted Obama killed so the incompetent VP, Biden, would become President!)
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To: Texas Fossil

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.


60 posted on 02/05/2024 5:04:10 AM PST by Paul R. (Bin Laden wanted Obama killed so the incompetent VP, Biden, would become President!)
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