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!
*Strictly speaking, this heater already has a half power setting, but it achieves it by turning off one of the two elements, letting the other run full blast. This results in:
1) A likely shorter element life -- bummer, as this heater is ceiling mounted, facing down, and a pain to take down if need be. Plus, elements are sometimes hard to find and disassembly is "significant".
Best yet, this setup makes it possible for me to run at 1/4 power - great in cool but not cold conditions, and easier on the thermostat contacts for sure.
2) The switch to change power is literally a pull switch -- who knows how long that thing is going to last.
3) Illumination via the elements is NOT particularly desired -- operating both elements at half power is rather than one at full power is less obtrusive.***
**I may add a SMALL 600v capacitor to kill any RF that might be generated.
***I've built a small version of this before: In that case I'm operating a 750 watt 230/240 v quartz heater off 120v, and then doing the rectification bit for the "low" setting. That one heats a chicken house and is so much better than brooder lamps, it isn't funny! The visible light emitted is nil, basically infrared only, so the chickens sleep. Plus, the element will probably outlive me. :-)
Your beat estimate of potential demand plus 50%.
I've also modified to switched series vs. (original) parallel element wiring to run elements "easy" but that takes a bit more of a switch, and I've definitely never seen a pull switch like that...
PFFFTT.......!!!
If you have a few in your parts box, then build it, run it and see if it blows up. It probably wont, but hey, this is the excitement of tinkering of electronics.
It’s a custom job for me. Tho’ I suppose it MIGHT be marketable to have a 4 heat setting heater? In any event, such diode control is used in some (maybe 20% of the market?)
floor standing space heaters.
Amen to that
Well, yeah, but I’m really curious how diode “overhead” is generally “figured” / estimated. The empirical data should go back to the origins of the p/n junction, I’m sure!
When i found out you could pop capacitors plugged into a switched outlet boy we had fun
Temperature coefficient of nichrome wire is pretty low - not a problem for current changes here. The 6.5 amp theoretical would go to 7 amps - maybe.
Parallel a couple of rectifiers. Put a .2 or .5 ohm resistor in series with each to keep the current division reasonably equal. Watch the wattage on the resistors.
I suppose I could start with 2 diodes in parallel, if it runs cool, take one out for the next project. :-)
Isn’t the element/elements accessible by removing a back plate and allowing access to remove them?
We charged capacitors up to 350V and then played hot potato.
That (current coefficient) lines up with what I observed in my smaller project, granted that I was starting off at half voltage to begin with, so I never got close to the element being “bright”.
Noted on the resistors - good point.
Oh, heck, hook up a woofer with, say, cone damage, and with maybe a 2” voice coil to 60 Hz 120 VAC!
done that also
No, one has to go in from the front; some significant disassembly involved. Consumer unit — I don’t think the mfgr. really intended repairs...
But, that said, the ceiling mount is the 1st deterrent, and I’m not getting younger.
Roger that. Any two diodes, even from the same manufacturing batch, will have somewhat different forward voltages for a given current. So while they will tend to share the current, it won't be 50/50.
Placing a very low resistance in series with each diode means there will be a small V=I*R voltage drop across the resistors, which added to the forward voltage of each diode, will tend to equalize the current in each parallel leg.
You can do that with two, three, or more diodes if needed.
I use as a rule of thumb to drop 0.2V across the resistor (assuming silicon diodes with 0.6 to 0.7 forward voltage). And definitely watch the wattage, it can be surprising at high currents.
YMMV.
;-)
Silicone makes lousy semiconductors. Pretty good to seal gutters. Look to Mr. Schottky.
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