2 rectifier diodes in parallel to dim a 120v 15A heater?

[Jay112]

I have a bunch of 10A10 rectifier diodes (datasheet: http://www.rectron.com/data_sheets/10a05-10a10.pdf). They're rated for 10A current.

Since the IR heater element is rated at 15A, that's obviously more current than the diodes can handle. But could I wire 2 of the diodes in parallel, so that only 7.5A is passing through each one?

I've read that some people say it's fine most of the time, and that it's rare for problems to arise from using diodes in parallel. But others seem to say that since the diodes are never the same rating, that one will get hotter than the other, causing it to absorb more current, and then get hotter, etc.

What do you guys think? Is it too risky to use the diodes in parallel like that? Or is it wrong for my particular application? Am I overlooking anything with this idea?

I'm not sure if I'm ever actually going to do this. I'm just interested in learning about the possibilities.

Thanks!

[BobsURuncle]

Show how they would be used in the circuit

[madires]

The problem with paralleling diodes is the thermal runaway of Vf. With increasing temperature Vf decreases, i.e. the diode will pass more current. So the current won't be distributed equally across the paralleled diodes. If you use diodes of the same batch (assuming their Vf is about the same) add some additional diodes for a better current distribution allowing some variation without exceeding the limit. Maybe use 3 or 4 diodes in your case.

[Jay112]

Show how they would be used in the circuit

This is probably the first one I've ever made, so go easy on me!

[Jay112]

The problem with paralleling diodes is the thermal runaway of Vf. With increasing temperature Vf decreases, i.e. the diode will pass more current. So the current won't be distributed equally across the paralleled diodes. If you use diodes of the same batch (assuming their Vf is about the same) add some additional diodes for a better current distribution allowing some variation without exceeding the limit. Maybe use 3 or 4 diodes in your case.

Thanks for the info! It's nice to know that using 3 or 4 together makes it much safer.

[edpalmer42]

As you stated, it's fine most of the time.  Is it good practice?  Certainly not!

The worst case of diodes in parallel that I've seen is in a 13.8V 50A power supply that I picked up some years ago.  It's a commercial product rather than a DIY special.  I didn't believe it when I opened it up and found a full wave rectifier arrangement where each of the two rectifiers is made from twenty-five 3 amp diodes in parallel!  Yes, a total of 50 1n5402 diodes!  

It seems to work but........  Do I like it?  Would I ever do something like that?  No flipping way!!

Ed

[BobsURuncle]

Or mount them on the same heat sink and put a 0.01 ohm 1% 2W resistor in series with each.  Mounting on the same heat sink will help to limit the temperature differential and the resistors would provide some negative feedback to prevent runaway.

[Jay112]

Or mount them on the same heat sink and put a 0.015 ohm 1% 2W resistor in series with each.  Mounting on the same heat sink will help to limit the temperature differential and the resistors would provide some negative feedback to prevent runaway.

Are resistors with values <1 ohm common? The smallest resistor I have is 1 ohm at 3W. Would I be able to do anything with that? Is it useful to have resistors with such small values on hand?

I have some old pcbs from some broken stuff, like old coffee makers, etc. Is there a chance there might be some <1 ohm resistors there?

[danadak]

If you want to play real safe place ballast resistor in series with each diode.
Note you have to get one rated for the power. You can sim this in spice easily
and see what the variation in currents would be for a different Vf.


Regards, Dana.

[langwadt]

Or mount them on the same heat sink and put a 0.015 ohm 1% 2W resistor in series with each.  Mounting on the same heat sink will help to limit the temperature differential and the resistors would provide some negative feedback to prevent runaway.

Are resistors with values <1 ohm common? The smallest resistor I have is 1 ohm at 3W. Would I be able to do anything with that? Is it useful to have resistors with such small values on hand?

I have some old pcbs from some broken stuff, like old coffee makers, etc. Is there a chance there might be some <1 ohm resistors there?

just use separate with wire to and from each diode and make them as long as practical

a meter of 0.5mm^2 copper wire is roughly 0.033 Ohm and has positive temperature coefficient

[BobsURuncle]

Or mount them on the same heat sink and put a 0.015 ohm 1% 2W resistor in series with each.  Mounting on the same heat sink will help to limit the temperature differential and the resistors would provide some negative feedback to prevent runaway.

Are resistors with values <1 ohm common? The smallest resistor I have is 1 ohm at 3W. Would I be able to do anything with that? Is it useful to have resistors with such small values on hand?

I have some old pcbs from some broken stuff, like old coffee makers, etc. Is there a chance there might be some <1 ohm resistors there?

Yes readily available..
I don't know what your board layout plan is but here is a 29 cent SMD at Newark
MCS3264R010FER
or a more expensive axial leaded
12FR010E

It needs to be low resistance for low power dissipation and because the current through a diode is exponentially related to the voltage across it.  A quick back of the envelope calculation and I got 0.01 or so ohms.  Maybe someone could run a simulation to get a more reliable number or since this is just a one-off you could experiment a bit with the diodes you have.





 

 
 

[danadak]

Here is a diode ballast sim as a trial, attached. Monte Carlo
on one R, 20% variation.


Regards, Dana.

[danadak]

5% R tolerance Monte Carlo. attached.


Regards, Dana.

[Jay112]

Thanks everyone for the help and the tips!!

I think I'm just going to stick with using 4 of the diodes in parallel, without any resistors or anything else. Recently I decided to build this circuit now, because it'll probably only take 5 minutes, and I'm curious to see how it works. I'm working on fixing the regular PCBs for these heaters (I have 3 broken heaters now, each worth $350!), but that's going to take me a long time.

Do any of you think that there's any chance of harming a heating element by giving it a rectified AC signal? Technically wouldn't that cause the element to only be receiving half the power, so that there's a greater chance of extending the element's life rather than shortening it?

[BobsURuncle]

The variation to worry about is the Vd vs If of the diode, presumably the Is and n tolerances.   A .01 or .02 ohm ballast resistor will only work if he can find two closely matched diodes.   0.1 ohms would almost certainly work but would dissipate as much power as the diodes.  The diode data sheet doesn't offer any guidance, but assuming he bought these all together off one reel they would be a lot closer than what a datasheet tolerance would indicate anyway. 

[Jay112]

The variation to worry about is the Vd vs If of the diode, presumably the Is and n tolerances.   A .01 or .02 ohm ballast resistor will only work if he can find two closely matched diodes.   0.1 ohms would almost certainly work but would dissipate as much power as the diodes.  The diode data sheet doesn't offer any guidance, but assuming he bought these all together off one reel they would be a lot closer than what a datasheet tolerance would indicate anyway.

Thanks for the info. I bought a pack of 50 of these all at the same time. Also I could test the 4 I'm using, to make sure they're similar, right?

[Jay112]

It took me a while, and I had to go through the entire bag, but I finally found 4 diodes that tested exactly the same with my multimeter! I was surprised how different they all were! It seemed the values ranged from 5.2 to 5.6. I got 4 that were all 5.37. (I didn't have time to look it up, but I assume it was measuring the voltage drop? I just put the meter on the diode test mode).

[Jay112]

I hooked the diodes up to a 60w bulb, and it successfully dimmed the light. What could possibly go wrong when I hook it up to a big heater, right?

If this is my last post ever, y'all know what happened. 

[Jay112]

Yay, it works! Thanks everyone!!! 

 

[Jay112]

Do you guys happen to know any neat/simple ways that I could use to dim the half-wave rectified power even further? Is there an easy way to cut the rectified power down by another half?

If I'm starting with 120Vrms AC, with a peak voltage of 170, then after the diodes I should be getting 60Hz pulses of 170v DC, correct? Is there an easy way to halve those pulses?

The reason I'm asking is because if I can find a way to do that, then I could implement a switch that allows me to choose between full power, half power, and quarter power. Then I wouldn't even have to rebuild the original circuit boards, as described in this tedious thread here: https://www.eevblog.com/forum/beginners/possible-to-reverse-engineer-this-pcb-heater-controller/

[Photon939]

Do you guys happen to know any neat/simple ways that I could use to dim the half-wave rectified power even further? Is there an easy way to cut the rectified power down by another half?

If I'm starting with 120Vrms AC, with a peak voltage of 170, then after the diodes I should be getting 60Hz pulses of 170v DC, correct? Is there an easy way to halve those pulses?

The reason I'm asking is because if I can find a way to do that, then I could implement a switch that allows me to choose between full power, half power, and quarter power. Then I wouldn't even have to rebuild the original circuit boards, as described in this tedious thread here: https://www.eevblog.com/forum/beginners/possible-to-reverse-engineer-this-pcb-heater-controller/

The way I have seen this done in large electric duct heaters is a solid state relay with a little PIC mcu doing some low speed PWM. Frequency around 0.25Hz or so. Heater on, heater off. Prevents crappy power factor and buzz caused by dicing up each sine wave. Then you can have nice analog control of the heat output while also allowing you to do whatever control scheme you please with the mcu.

Going a bit simpler than that you could buy a china 555 timer relay board and set a low duty cycle on that and use it as a lower power option with an extra switch or something.

[Jay112]

The way I have seen this done in large electric duct heaters is a solid state relay with a little PIC mcu doing some low speed PWM. Frequency around 0.25Hz or so. Heater on, heater off. Prevents crappy power factor and buzz caused by dicing up each sine wave. Then you can have nice analog control of the heat output while also allowing you to do whatever control scheme you please with the mcu.

Going a bit simpler than that you could buy a china 555 timer relay board and set a low duty cycle on that and use it as a lower power option with an extra switch or something.

Thanks for sharing! Those are neat ideas!

I've never heard of flipping an SSR on and off frequently, but it sounds nice and easy. Technically I could just use an Arduino Nano, hook the SSR up to a PWM output, and use an analog pot to adjust the frequency of the PWM, right? I wonder how good that would work across the full power range. Do you see any problems with this method? Am I overlooking anything?

[Jay112]

Question I had right after I posted: Don't SSRs only work with AC? So that would only work if I removed the rectifier diodes first, right?

[radar_macgyver]

Yep, lose the diodes.

Also, try to locate an SSR that has an in-built zero-crossing detector. The SSR will go from off to on only when the AC cycle crosses zero. This will significantly reduce the stray emissions.

The period of the PWM would depend on the thermal mass of your heater. I found that electric baseboard heaters work well with a period of ~5 seconds. Any longer than that and you can hear them "crackle" when they heat up/cool down.

[edpalmer42]

Really all you're describing is a big light dimmer.  Then you could set it to any level.  No SSR, no Arduino or PIC.  Bad news is I don't know if they make dimmers that big.

Ed