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AC vs DC on heat element

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arhi

Member
did some googling and then gave up .. (last time I did that I found solution just after posting but ..) anyhow .. here it is ..

Is there any difference if I use AC or DC for heater element (inside a soldering iron for example)....

Why do I ask .. I have some rework station that I use and I'm satisfied with ... I managed to get a good supplier of the new tips etc, but the best thing of all is that I managed to find a supplier for soldering iron part (I call it pencil .. donno how it is called).. and now I have few spare (the heater in new ones is ceramic compared to metal one in original but everything else is same)...

Now, I want to make additional soldering iron using these pencils using simple uC to monitor (and display) temperature and to control the heater inside the iron. The original uses AC to heater (24V) and from what I see it just turn it on/off with some hysteresis ... Mine idea is to use PWM to keep the temperature as close to "selected" as possible... but, I cannot PWM AC signal (or can I???) so I'd use 24V DC to heat the iron - will this have any side effect that you know of? I do not see any but .. better safe then sorry
 

Nigel Goodwin

Super Moderator
Most Helpful Member
No point whatsoever using PWM, thermal inertia of the element means it doesn't respond that fast. Just run the power ON until it reaches temperature, then turn it OFF, turn it back on when it drops your desired distance below temperature.

For a light bulb you use phase control (similar to PWM) for heating you use 'burst fire' control.

AC or DC makes no difference, 24V RMS gives the same heating effect as 24V DC - that's what RMS actually means.
 

arhi

Member
No point whatsoever using PWM, thermal inertia of the element means it doesn't respond that fast.
makes sense .... I was thinking about that but the idea was to have
1. "on"
2, "off"
3, 50%
4 "off" or "on"
5. goto 2 or 3

Just run the power ON until it reaches temperature, then turn it OFF, turn it back on when it drops your desired distance below temperature.
that's what original is doing

For a light bulb you use phase control (similar to PWM)
did that for AC motor, light ... with triac and zero cross detection .. this is clear but not sure how would it work with heat..

for heating you use 'burst fire' control.
this I will have to google :) ..

AC or DC makes no difference, 24V RMS gives the same heating effect as 24V DC - that's what RMS actually means.
that I know - but ... magnetic something ... other something .. just asking if there's anything I need to think when working with heaters - never did that before
 

dknguyen

Well-Known Member
Most Helpful Member
did that for AC motor, light ... with triac and zero cross detection .. this is clear but not sure how would it work with heat..
It would work the same way. Just the dimming effect would be a lot less noticable since heat reacts very slowly compared to light.

Mine idea is to use PWM to keep the temperature as close to "selected" as possible... but, I cannot PWM AC signal (or can I???) so I'd use 24V DC to heat the iron - will this have any side effect that you know of? I do not see any but .. better safe then sorry
Triac zero-cross detection phase control for AC is the equivelant of PWM for DC.
 
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arhi

Member
dknguyen, I kinda hoped to use that but I believe Nigel is right, I made few experiments and there's really not much to gain from changing the simple hysteresis on/off approach .. so I'll just go with some pic with adc for scanning the sensor data (2R when cold going up to few hundred when hot) and turning the heater on/off via simple relay ... will probably use 2x16 display to show "target" and "current" temperature ... not really needed but fun :)
 

dknguyen

Well-Known Member
Most Helpful Member
Remember that the triac will drop about 1V across it and so it will probably require a heatsink while the relay will not.
 

arhi

Member
1V will not mean much ... not that the output from the transformer will be "exactly 24V" (not that power grid is exactly 230V either) .... as for the heatsink, I kinda oversize them anyway so I do not worry about it ...

have not measured the current trough heater ... (I think it changes resistance with heat so I will have to measure - cannot just simply calculate) ... I had problem with "poor" relay contacts too many times earlier ..
 

dknguyen

Well-Known Member
Most Helpful Member
Example:

5A through a switch that drops 1V means that 5W are being dissipated. At the optimistic 40C/W for a unheatsinked plastic power IC package, that means the package will heat up to 200C above ambient. THat's why it matters- not because of efficiency but heating.
 
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dknguyen

Well-Known Member
Most Helpful Member
For plastic power package that is about 40-60C above ambient and since ambient is 25C and many electronics stop working around 85C (though power electronics tend to be higher), it may still be cutting it close. So you should look up more on it.
 

spitso

Member
and plus using DC to heat is better than using AC. AC rises to a certain voltage which heats element then drops down back to zero which cause the element to cool abit more than voltage rises again. Whereas DC is always at its maximum so heating will occur mush faster and steadier.
 

Sceadwian

Banned
spitso, the 'thermal constant' of almost all heaters makes AC vs DC heating irrelevant.
Look at at a nearby light bulb, does it flicker? No? Odd... It's running on 50 or 60hz Ac... That's because on average it's heating evenly.
 

spitso

Member
thats because were using it too produce light...not heat. Our eyes have problems seeing things faster than 30hz.
I have learnt and agree that DC is better for use in heating.
 

Sceadwian

Banned
I can see abrupt changes in light over 120hz easily. The flicker is still there. The reason it's not apparent in incandescent bulbs is because the time it would take the filament to cool down bellow it's ability to produce light is so long that the median fluctuation of current going through it never goes low enough. Some CDS cell based meters can detect AC line fluctuation in incandescent bulbs. Tut not the human eye, it's not the cycles per second that's the issue, it's the relative dimming. The modulation factor.

I have plugged in in anti paralel enough LED's to do full wave rectifiaction of AC. I can easily see the flicker while I can't in a standard incadescent bulb. 30hz is the generically accepted (aside from 25hz film) limit of frame refresh for the appearance of full motion video, it has nothing to do with the ultimate sensativity of our eyesight.
 

Hero999

Banned
thats because were using it too produce light...not heat. Our eyes have problems seeing things faster than 30hz.
I have learnt and agree that DC is better for use in heating.
Sorry but that's nonsense, it makes no difference.

The thermal time constant of a heating element will be very long compared to an AC cycle so it won't have enough time to even cool measurably.

When you turn your electric fire off how long does it take to turn off?

It doesn't go off instantly, it takes minutes just to stop glowing.

Even a light bulb filament doesn't cool down enough to stop glowing between AC cycles. You can see flicker at 50/60Hz, incidentally lights don't flicker at 50/60Hz but at 100/120Hz since the current lights the lamp in both directions. I can see 100Hz flicker at the edge of fluorescent tubes where there is no phosphor.
 

Sceadwian

Banned
That's exactly what I just said hero =)
 

tcmtech

Banned
Most Helpful Member
:rolleyes:
How many unesisarily over complicated and just pointless "construct a device to use a complex circuit to do a simple process" threads or "theoreticaly nearly impossible to actualy happen" threads are there on this site now? I dont think that this one would be much different.

Just pick a genralized catigory name from below and file it under that.

1: How can I tell if my lights are out without getting out and looking.
2: How can I use way too many components to charge a simple battery.
3: How can I use a microprocessor to blink a LED.
4: How can I get the last nanowatt of energy out of a AA battery for under a $50 investment.
5: How can I build a common device without actauly using common parts.
6: How can I build a store bought device using salvaged parts from several of the same store bought devices I already have that still do work.
7: How do I run some device off a power source not designed to run it.
8: I am cheap and have no common sense can some one help me out.
9: Do you have my home work done yet.
10: How do I make something work when I dont even know how to describe what it is.
11: How do I solve an unlikely problem that is realisticaly never going to happen.
12: How do I convince others that what I think is right without evidence or proof to support my opinions.
13: If I took my OCD meds would this senario I am so worried about still seem important to me.
:rolleyes:

This just fits in so many places! :D
 
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Sceadwian

Banned
tcmtech, half or better.
 
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