PWm control a 7200watt water heater

Jonathan

Hi All,

I'm helping a friend with his radiant floor heating setup. The heating element he is using is 220VAC, 7200watts, drawing about 32 amps. The heater has two settings, and one is too high and the other too low. I want to implement PWM control of the heater to allow·for more precise adjustment.

So, Anyone have a MOSFET circuit to do the job? I really only need the MOSFET power part, I can deal with the PWM part.

Or, I have some SS relays that are good for 40-65kHz. Will these survive being used for PWM reliably if I keep within the specs?

Or, any other options/pointers?

Jonathan


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Phil Pilgrim (PhiPi)

For something like radiant heating, high frequency PWM is unnecessary. You can switch it on and off on the order of seconds, or even minutes, due to the long time constant between turning on the heat and feeling the room get warm. Instead of a MOSFET, use an SSR designed for AC. It will have a zero-crossing detector built-in to minimize turn-on transients.

-Phil

Jonathan

Phil,

The system uses a small flash hot water heater. It is designed for a higher flow rate than the system can achieve, so the output temp. exceeds the maximum rating for the heater. We can't really pump more water, so controlling the element seems the best option. We aren't trying to control the room temp directly per se. just the output temp.

I've been looking into SS relays for this purpose. They aren't cheap, but then if it lasts thats OK. My hope was to kludge something together, but it looks like I'll have to order something.

Thanks!

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Mike Green

Jonathan,
Rather than PWM or even a dimmer-like partial half-cycle control, how about something like 4 half-cycles on, 2 half-cycles off or something similar. The SSR would only switch on and off at zero-crossings thus minimizing the power dissipation. The thermal mass of the heater would average the half-cycles. You'd need only the simplest control circuitry with the SSR and a zero-crossing detector.

Jonathan

Mike,

Sounds great. Any links or keywords to search for? Schematics? We don't need fine control, what you suggest sounds more than adequate.

Thanks!

Jonathan

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JonnyMac

Jonathan,

If you look in the November issue of N&V you'll find a lamp dimmer project I did with the SX. The circuit has a zero-cross detector and opto-coupled triacs; you'll need to bump the triac size, but otherwise -- and as Mike indicated -- it's pretty easy.

sam_sam_sam

Jonathan,

If you need to read the· November issue of N&V lamp dimmer project that was done·with the SX

Please PM me and I will give you a link so you can read it if do not get N&V your self i also get the online of N&V





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Sam

Jonathan

Sam, All,

I ended up trying a SSR I had laying around. It seems to be working great! I can control the output temp. of the heater quite nicely. The SSR is only getting slightly warm.

I'd still love to see the N&V article. I couldn't access it as I am not a e-subscriber, but maybe I'll change that, always a great read.

Jon, nice to see you, and thanks for posting.

Jonathan

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sam_sam_sam

Jonathan

I send you·a link November issue and the December issue

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Sam

MarkS

All you need is a simple thermostat. There is no need for complicated control circuitry. When the temp gets above a set point, turn the element off. Its worked in the HVAC industry for decades, so I'm pretty sure it would work for you.

Shawn Lowe

Thermostat....pah! Who wants anything that simple??? [noparse];)[/noparse]

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Shawn Lowe


Maybe I should have waited to do that......

Jonathan

It's not quite that simple. The flow rate of the system is too low for the heater to run at full power. Using one element, the heater can't achieve the needed temps. So, control of the second element is required.

Looking at all I could find, and considering that the element id NiCHrom, zero crossing switching is best. With other types of elements, phase angle control is better. Luckily for me, zero cross is easy to implement. I made a detector usig the N&V article that Sam so kindly (Thanks Sam!) sent me. It's working great, and I have single cycle control of the heater, and it was cheap and easy!

Thanks for all the help folks!

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