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

[Jay112]

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.

That's all really interesting! I had no idea the crackling noise could be avoided! I noticed that only 1 of my heaters didn't make noises, but I didn't know how.

Could you please explain to me a little more about the emittance? I was reading a datasheet on a zero voltage switch today, and it was describing how the RFI was eliminated due to the zero crossing switch, but I didn't know what that means. What's the problem with emittance? How would it affect the circuit or other things/people nearby?
Edit: Also, how does switching at zero volts stop the emittance, and how would starting at a different point cause more?

[Photon939]

The zero crossing switching means there is no current flowing when it changes state. When triacs suddenly switch on during the sinewave like a typical lamp dimmer it can cause radio interference from the sudden current spike and if bad enough can actually interrupt or crash electronic devices nearby (much more of a problem for inductive loads though).

Turning off my soldering station used to sometimes crash my 3d printer. I had to install a MOV across the transformer primary.

[Jay112]

The zero crossing switching means there is no current flowing when it changes state. When triacs suddenly switch on during the sinewave like a typical lamp dimmer it can cause radio interference from the sudden current spike and if bad enough can actually interrupt or crash electronic devices nearby (much more of a problem for inductive loads though).

Turning off my soldering station used to sometimes crash my 3d printer. I had to install a MOV across the transformer primary.

That's really interesting! Thanks for the info!

Is it not a problem when power is cut in the middle of the wave? It only happens when power is starting, not when it's stopping?

[radar_macgyver]

Triacs will, once triggered, continue to conduct as long as there is current flowing between the two main terminals. When the AC cycle reaches zero, current should drop to zero and turn off the triac. This automatically takes care of the turn-off happening only at the zero crossing. The detector circuit is still needed for turn-on, though.

[IconicPCB]

Diode only method  will play havoc with your local power distribution transformer...

not to mention the LED lights and ceiling fans perhaps in the hood.

[RGB255_0_0]

And your Wi-Fi   

[danadak]

You can get power triacs up to ~ 100 A, just use one a control circuit.

See attached selector guide done for 30A.


Regards, Dana.

[Jay112]

You can get power triacs up to ~ 100 A, just use one a control circuit.

See attached selector guide done for 30A.


Regards, Dana.

Thanks Dana! I actually already have some CQ3P-25M triacs that can take 30A, and that's what the original pcb for the heater was using with a zero voltage switch. The problem was that the triac burned out, but it wasn't an easy replacement because some of the traces on the board got burned too, and the entire thing was covered with potting so it was difficult for me to rebuild the circuit without understanding it fully. I'm about 90% through being able to rebuild the circuit from scratch, but it's not easy for me and I had to do a lot of studying to get there, and I feel like I need another month to fully understand how the circuit works. That's why I was looking for a temporary alternative solution.

[Jay112]

Could you guys please explain why it would wreak havoc on the power distribution lines, and on other things like wifi? My guess was that maybe it weakens the top half of the AC sine wave, causing an imbalance, which causes other devices on the same line to get imbalanced power? And maybe the imbalance also creates a large electromagnetic field around the device, messing with some wireless stuff and inductive loads?

The heater originally comes with 2 different kinds of controllers (the first 2 in the list below), and I was wondering if you guys could please share your opinions about which of the following methods you think is safest in terms of interference with other devices:
1) Zero-voltage switch triggers a triac. The timing is dependent on an analog pot and a thermistor.
2) A thyristor adjusts the firing angle of the wave, control is by a single analog pot. The heaters with this method seem loud, making buzzing noises when not on full power. Also I've had interference problems with this method, with nearby lights flickering or dimming depending on the heater's setting.
3) Half-wave rectifier diodes to dim the power to about half.

Also, is there another common dimming method for high-power resistive loads, or is it that the first 2 are by far the most popular?
------
Edit: Also I guess another method was already mentioned above by radar_mcgyver: To use a pwm signal to control an SSR that has an in-built zero crossing detector. But technically that would affect the power in exactly the same way as method #1 above, right?

[IconicPCB]

A triac with adjustable conduction angle is likely to introduce harmonic distortion and potentially power factor issues into the distribution network.

A better option is to use zero crossing detection and drive the heater in block control mode.. block as in a block of integer periods of power supply .This will produce switching disturbances on the line but the resultant distortion will be negligible compared to phase control method.

Both above methods will have a symetrical loading on the grid during positive and negative periods of AC voltage.( Ok may be half a wave might slip through here and there ).

A diode method will ALWAYS present asymetric load to the grid. The resulting current will be asymmetric.

This asymmetric current will flow through the distribution transformers ( ugly things sitting on powerpoles ).
Asymetric current == DC current. Transformers are intended to operate in alternating current environment.
A DC current will cause the magnetic flux in the transformer to be higher in one half of the waveform. This means the transformer inductance will be different between the two halves of the wave. If inductance is different the resulting current will be asymetricbetween the two half waveforms.. creating harmonics.. in fact as DC current goes up.. the magnetic core becomes biased allowing higher magnetising current peaks to flow through the transformer ( leakage reactance ). this means transformer gets hottr.. surrounding equipment gets exposed to gigher levels of harmonics.

Led lighting starts to flicker.. ceiling fans start to hum audibly..stuff gets stressed out... Your power meter does not show correct power usage... Power distribution authority starts to hunt for source of distrbance.. You get stuck with a bill .

Well not necesarily .. depends on the transformer and the burden it was designed to supply and burden it is actually handling.

[Jay112]

A triac with adjustable conduction angle is likely to introduce harmonic distortion and potentially power factor issues into the distribution network.

A better option is to use zero crossing detection and drive the heater in block control mode.. block as in a block of integer periods of power supply .This will produce switching disturbances on the line but the resultant distortion will be negligible compared to phase control method.

Both above methods will have a symetrical loading on the grid during positive and negative periods of AC voltage.( Ok may be half a wave might slip through here and there ).

A diode method will ALWAYS present asymetric load to the grid. The resulting current will be asymmetric.

This asymmetric current will flow through the distribution transformers ( ugly things sitting on powerpoles ).
Asymetric current == DC current. Transformers are intended to operate in alternating current environment.
A DC current will cause the magnetic flux in the transformer to be higher in one half of the waveform. This means the transformer inductance will be different between the two halves of the wave. If inductance is different the resulting current will be asymetricbetween the two half waveforms.. creating harmonics.. in fact as DC current goes up.. the magnetic core becomes biased allowing higher magnetising current peaks to flow through the transformer ( leakage reactance ). this means transformer gets hottr.. surrounding equipment gets exposed to gigher levels of harmonics.

Led lighting starts to flicker.. ceiling fans start to hum audibly..stuff gets stressed out... Your power meter does not show correct power usage... Power distribution authority starts to hunt for source of distrbance.. You get stuck with a bill .

Well not necesarily .. depends on the transformer and the burden it was designed to supply and burden it is actually handling.

@IconicPCB: Thank you so much for the detailed information!! It's interesting to me that the effect can even be seen by the power company. Do you think a single 120V heater is able to cause that much trouble, or would it usually require more power? After I installed the diodes, I measured the amperage at 6A. I incorrectly stated earlier that the unmodified heater draws 15A, but after double-checking I learned that it says 12.5A.

I have a few simple questions (for anyone), if you don't mind:
1) Hypothetically, if I put 2 of those heaters side by side, and had 1 using only the positive half of the wave, and the other using only the negative half of the wave, would that balance everything out?

2) Is there an easy way to use only the negative half of the wave? Like maybe some kind of special rectifier diodes that only allow negative current to pass through?

3) Are there any regional regulations that cause manufacturers to have to be careful with the amount of power distortion their devices are producing?

[MarkF]

Sorry if I've overlooked the heater description some where.

Question:  Are the heaters just a resistive coil with a separate controller?   Or, is the temperature controller built into the unit?

If the former, wire two units in parallel for full power and wire two units in series for half power.  It would only need a simple switch with no worries with interference with WIFI, cordless phones, etc.  I would need to do a little doodling to see if a DPDT switch would do the trick.

Edit.  You would need the verify that the controller could handle two units in parallel !!!

[Jay112]

Sorry if I've overlooked the heater description some where.

Question:  Are the heaters just a resistive coil with a separate controller?   Or, is the temperature controller built into the unit?

If the former, wire two units in parallel for full power and wire two units in series for half power.  It would only need a simple switch with no worries with interference with WIFI, cordless phones, etc.  I would need to do a little doodling to see if a DPDT switch would do the trick.

Edit.  You would need the verify that the controller could handle two units in parallel !!!

That's a really neat idea about wiring them in series! I never thought of that! I'll have to keep that in mind.

If I did wire 2 of them in series with no controller board, and with 120v mains (USA), then technically would each be pulling half of its usual (12.5A) power, thereby making the total power consumption the same as if I was using just 1 heater on full power?

The heating element looks like a 4-foot long resistor, similar to how an oven bake element looks but straightened. BTW, since it's classified as an "infrared" heater, does anyone know if this makes it have different emissions than something like an oven bake element?

As for the controllers, they all basically broke in different ways, so that's why I'm either trying to rebuild the original controllers, or make my own. On one the triac burned out, and on another the zero voltage switch got fried. But replacing the triac or the switch didn't fix these, so there seems to be other problems too. And a 3rd heater that broke recently has a different type of controller, and on that one the thyristor definitely blew out (it was like an explosion!), but replacing the thyristor didn't end up fixing it, so I have to search for other problems on that board too.

[MarkF]

If I did wire 2 of them in series with no controller board, and with 120v mains (USA), then technically would each be pulling half of its usual (12.5A) power, thereby making the total power consumption the same as if I was using just 1 heater on full power?

Yes.  You would be doubling the resistance with two resistive strips in series, thereby halving the current by applying the same voltage.

Keep in mind that the current will be DOUBLE when you put them is parallel.

[Circlotron]

That's a really neat idea about wiring them in series! I never thought of that! I'll have to keep that in mind.

If I did wire 2 of them in series with no controller board, and with 120v mains (USA), then technically would each be pulling half of its usual (12.5A) power, thereby making the total power consumption the same as if I was using just 1 heater on full power?

Not quite. Double resistance of two in series = half current. Also each heater now has half voltage.  Half voltage x half current means each heater is 1/4 normal output. Total of two heaters = half normal output of one heater at 120V.

[Raj]

try bta20

Triacs  could be really rugged, My dad built one in 1980s, its still working

http://uk.farnell.com/stmicroelectronics/bta20-600cwrg/triac-20a-600v-to-220ab/dp/1057283
https://www.google.co.in/search?q=20a+triac&oq=20a+triac&aqs=chrome..69i57j0l5.3907j0j7&sourceid=chrome&ie=UTF-8
https://www.google.co.in/search?q=triac+based+regulator&newwindow=1&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjIkbqi_efMAhWHe6YKHbmoBm8Q_AUIBygB&biw=1366&bih=643#imgrc=cS-F8Ax-JizfjM%3A

[Jay112]

That's a really neat idea about wiring them in series! I never thought of that! I'll have to keep that in mind.

If I did wire 2 of them in series with no controller board, and with 120v mains (USA), then technically would each be pulling half of its usual (12.5A) power, thereby making the total power consumption the same as if I was using just 1 heater on full power?

Not quite. Double resistance of two in series = half current. Also each heater now has half voltage.  Half voltage x half current means each heater is 1/4 normal output. Total of two heaters = half normal output of one heater at 120V.

Thanks for the info! Now it's seeming pretty messy to me to hook up 2 in series. I just wanted to brainstorm the idea a bit.

Those zero crossing SSRs seem on the pricey side! Lots were in the $40-$60 range, but I found one with correct specs for $28 with free shipping. It's an Omron that can handle 20A at up to 240V, and it takes 5VDC for the input, so I think I could easily hook it up to an Arduino's PWM. I might buy it at least for experimentation.

[IconicPCB]

Dont use it in the PWM mode.

Devise a block firing controller.

say...

if heater on and sensor reading below setpoint  fire
else switch off

kind of an approach.

You might even want to try to model delay between heater going off and sensor registering it to provide some rate control.

[ali6x944]

See this:

Dont use it in the PWM mode.

Devise a block firing controller.

say...

if heater on and sensor reading below setpoint  fire
else switch off

kind of an approach.

You might even want to try to model delay between heater going off and sensor registering it to provide some rate control.

Why not pwm?
I think if used pwm with high power mosfet -if powered from a DC source- ,or a phase fired controller with a regular triac it will be just fine

[Kleinstein]

Parallel diodes should be thermally coupled to reduce the tendency for thermal runaway. Also a little resistance, like separate longer wired help.

Using just the diode for larger loads is not nice to the grid. Depending on the country it is tolerated for portable / handheld tools - many hair-dryers use this method to get the lower power setting. Having two such circuit in parallel with opposite direction will cancel out the DC current (at least most of it) - so this would be no problem. With the diode the power per heater will be about half. With two heaters in series the power per heater will be 1/4 the nomonal power.
Also remember that IR emissions are a nonlinear function of the temperature and power. So if you give only half the elctrical power to the IR element, the output IR power can be way lower than half.

If it is just to get a longer lifetime, one might consider a transformer to reduce the power a little, like 20 %. So a 110/12 V transformer might be enough. The transformer only needs to be rated for the current so something like a 140 VA would be enough for the 12 A current.

In the US the transmission transformers are often relatively small, so that a smaller DC current might upset them - especially the small pole mounted ones might have a problem with this. With a larger transformer like in city areas or in much of Europe (due to higher voltage and thus longer permissible lines) the transfromers can stand more DC current. 

[IconicPCB]

Ali6x944,

There is a difference between PWM drive and phase locked PW drive required to effect zero ross over variable conduction angle drive.

Just stating PWM is insufficient and may result in undefined  drive conditions.

[Jay112]

Parallel diodes should be thermally coupled to reduce the tendency for thermal runaway. Also a little resistance, like separate longer wired help.

Using just the diode for larger loads is not nice to the grid. Depending on the country it is tolerated for portable / handheld tools - many hair-dryers use this method to get the lower power setting. Having two such circuit in parallel with opposite direction will cancel out the DC current (at least most of it) - so this would be no problem. With the diode the power per heater will be about half. With two heaters in series the power per heater will be 1/4 the nomonal power.
Also remember that IR emissions are a nonlinear function of the temperature and power. So if you give only half the elctrical power to the IR element, the output IR power can be way lower than half.

If it is just to get a longer lifetime, one might consider a transformer to reduce the power a little, like 20 %. So a 110/12 V transformer might be enough. The transformer only needs to be rated for the current so something like a 140 VA would be enough for the 12 A current.

In the US the transmission transformers are often relatively small, so that a smaller DC current might upset them - especially the small pole mounted ones might have a problem with this. With a larger transformer like in city areas or in much of Europe (due to higher voltage and thus longer permissible lines) the transfromers can stand more DC current.

Wow, Kleinstein, I think you just answered every question in the thread! Thanks!!

This forum has so many knowledgeable users! It's unbelievable!

There's a green metal transformer box just outside my house. I'm not sure why they put it there, but I'm in a rural area, the previous owners were farmers, and I have a long driveway (maybe 500') so it's far from the road.

Does having the transformer on the ground just outside the house provide any different effects than if the transformer was on a pole? Or has everything you guys said about mains transformers taking the hit still apply?

[Circlotron]

You probably have a high voltage line on the road and along the driveway leading up to this transformer, which drops the voltage down to 120VAC for your house. Drawing half wave dc from the mains will affect only this transformer and consequently only your house. If you are running the transformer at a reasonable percentage of it's rating and then also make dc flow though it you may trip a circuit breaker or fuse somewhere on the high voltage side and you will be in the dark until the electricity company arrives to fix it. 

[Jay112]

You probably have a high voltage line on the road and along the driveway leading up to this transformer, which drops the voltage down to 120VAC for your house. Drawing half wave dc from the mains will affect only this transformer and consequently only your house. If you are running the transformer at a reasonable percentage of it's rating and then also make dc flow though it you may trip a circuit breaker or fuse somewhere on the high voltage side and you will be in the dark until the electricity company arrives to fix it. 

Thanks for the info, Circlotron! I'm glad to know that I'm not affecting any of the neighbors!

I've been running these kinds of IR heaters for years (some of them are for my livestock), sometimes all at the same time, without noticing any problems. So for example in the winter I might have 3 IR heaters running at the same time, 2 with a zero crossing switch and the 3rd with a phase controller, along with other high power stuff, and so far I haven't noticed any big problems.

[IconicPCB]

Winters must be cold.. helping keep your transformer cool.

How many wires do you have coming off the transformer?

[Raj]

i just remember reading an article about using non contact ir paired up with triac and microcontroler (microcontroller isolated via opto coupler triac) as thermostat

could easy write a program if you want one.

[Jay112]

I got a 20A zero-crossing SSR, and I tested it for my first time today. Everything works correctly when I have a 60W light bulb hooked up to it: I can turn the bulb on or off, and have it flash at different frequencies with PWM.

But when I hook up the IR heater to it the heater never turns on.

When I remove the rectifier diodes (the ones we installed earlier in this thread) that were being used to dim the heater down to about 50%, then the heater works as expected with the SSR: I can have it turn on and off, and anywhere in-between. When I put the diodes back in, the heater doesn't work with the SSR. If I have the diodes in and I skip the SSR and plug the heater directly into the mains, then it works as expected with the diodes dimming it about 50%

Does anyone know why the heater won't work with the SSR when the rectifier diodes are being used?

This is the schematic I tried to draw. There wasn't an SSR symbol available and since I didn't know what to use I just used a symbol for a switch.

[Andy Watson]

If by "SCR" you mean a thyristor (as opposed to a triac), the clue is in the name; Silicon Controlled Rectifier.

[Jay112]

If by "SCR" you mean a thyristor (as opposed to a triac), the clue is in the name; Silicon Controlled Rectifier.

Thanks for responding, but there are no SCRs in my circuit. Just a zero-crossing SSR, a half wave rectifier diode, and the resistive load of the heater.

[Circlotron]

SSR = solid state relay.
Probably the SSR needs to see both half cycles for it's zero crossing detect circuitry and other internal low power stuff to operate. The diode is cutting off one half cycle so it is going strange internally. Does anything change if you reverse the diode?

Edit -> did you mean it worked okay with the diode in circuit on the globe but not the heater?

[Andy Watson]

Thanks for responding, but there are no SCRs in my circuit. Just a zero-crossing SSR,

Oops, sorry, my mistake (the afluence of inccohol is strong tonight!). But, a "zero-crossing" SSR will require a zero-crossing - which the series diode prevents.

[Jay112]

Thanks guys!

I just tried reversing the diode, and it still didn't work.

Also I hadn't tried the diode + lamp before, and I just tried that now. It had the same result as the heater: the lamp doesn't turn on at all when the diode is in the circuit, no matter which direction the diode is pointing.

It sounds like you guys got it right that the SSR needs to see the voltage crossing past zero. I wonder if there's any way to make a tiny "bounce" past zero, even with the rectifier diode in place. Any ideas?

[Andy Watson]

I wonder if there's any way to make a tiny "bounce" past zero, even with the rectifier diode in place. Any ideas?

You might be able to fool the SSR by arranging for some negligible leakage across the diode - perhaps a low wattage lamp? It doesn't need to be much - just enough to the trip the zero-crossing detection in the SSR.

[Ian.M]

You need enough of an unrectified load on the SSR for it to see a full AC waveform. It would *probably* work if you added an 18K 1W resistor from the SSR switched Live output to Neutral, unless the SSR requires a minimum load current >6.6mA.

However for the reasons other posters have listed above, diodes to reduce the power of a high current load are a dumb idea that can get you in trouble with your electricity supply company, so I wouldn't recommend leaving the diodes for half power in there.

If you are implementing cycle skipping control, you need a line frequency reference to your control circuit so you can avoid skipping half cycles, and only skip whole cycles, also to avoid DC imbalance.  One approach is to use the line frequency input to clock the PWM through a D type flipflop before feeding it to the control terminals of your zero crossing switching  SSR.

You can derive a suitable line frequency signal by tapping one of the AC outputs from the transformer to the bridge rectifier of a conventional linear PSU, and feeding it through a resistor with a Zener clamp (to 0V) to reduce it to your desired logic level.  That gives you a somewhat assymetric line frequency near squarewave, with fairly poor rise and fall times.  Clean it up with a Schmitt trigger input buffer before you do anything else with it.

[Jay112]

Thanks for the help and ideas, everyone!

Just to see if it would work, I just tested your ideas of using a small lamp/resistor across the SSR. I found a 3-watt 74 Ohm resistor, and it worked! I calculated that it was probably drawing about 1.2mA of current.

@Ian.M, I appreciate your advice about safety, and my ultimate goal is to be able to recreate the original circuits that came with these heaters but which have broken. Ultimately I'm hoping to put together a board that uses a zero-crossing switch (with adjustable pot) to fire a triac that powers the heater.