Controlling a 1KW 230v Heater from an AVR Pin (help)

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Bingo600
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Posted by Bingo600: Mon. Oct 4, 2004 - 07:36 PM
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Guyzz

Im going to build yet another Avr SMS controller.

It needs to control some 230v 800W..1200W heating panels in my dads sommercottage (also used in wintertime).

I think i have the SMS part in place , and the DS18S20 part.

But how do i control 4 pcs. of heaters at that Current.

I was thinking about either SolidState or Relays , but dont know if relays like the temps. We could be talking about -20 deg celsius here.

I would like to separate the boards so my M32 board , and the "Power" board was on 2 different PCB's.

Any hints (read schematics / Appnotes) , would be most welcome.
Im an electronics beginner in EMC & Analog. :oops:

/Bingo

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AVR Microcontrollers, megaAVR and tinyAVR
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ka7ehk
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Posted by ka7ehk: Mon. Oct 4, 2004 - 07:47 PM
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At that power level, I would use relays to control the heaters.

Such relays probably take too much coil current to be actuated directly by the AVR. Here, FETs make a lot of sense. This would call for DC (coil) relays. The coils need to be rated for the low voltage power supply you have and the contacts need to be rated for the heater load (both voltage and current).

The only thing to really watch for while driving relays is that you need to transient-protect the driver. Either and RC "snubber" or a diode. The diode is connected so that it is reverse biased when the relay is activated. Then, when the driver shuts off and the voltage makes a transient above the power supply level, the diode will "catch" it. This diode does not need to be terribly fast but may need to handle a fair amount of current. A 1N400x ought to do it.

The FET you use for driving needs to be able to handle the coil current AND it needs to be rated for the relay coil supply voltage. That is the voltage present across the FET when the relay is off. You ALSO need to make certain that the gate threshold is low enough so that the AVR can be guaranteed to turn it on at ITS lowest supply voltage.

There ARE some issues about constructing it, especially in EU with higher mains voltages. You will need to use good electrical practice in all places with mains voltages.

Thats about it.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Bingo600
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Posted by Bingo600: Mon. Oct 4, 2004 - 07:56 PM
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Hmm wonder why everybody is talking about fets :(
I have never used one of those , but have used some bc547 or 2N2222

I read a suggestion where someone said a diode on the fetgate would "lift" the voltage so one could be sure the AVR could turn it off , is that correct understanding ??

I would like to be 99.99% sure that the circuit would be off if something went "haywire" or my dad would kille me (The electricity bill :? :? )

I guess a n.o relay would fall back to "open" if AVR was off or died.

/Bingo

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glitch
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Posted by glitch: Mon. Oct 4, 2004 - 08:07 PM
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I suggest using the standard thermostat controlled relays for switching the power to the heater itself. And then using a simple low voltage relay to switch the relay main relay. Typically the thermostat relays, include a 24v transformer for powering the relay and thermostat, in this case you won't be using a thermostat, but a custom controller.

Provided there is no frost/condensation on the relay, it should work fine well below -20C. Triacs may be another good option for switching the main relay.

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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c_hirst
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Posted by c_hirst: Tue. Oct 5, 2004 - 04:03 AM
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Opto-isolated triacs are about the safest way. Have a search for stage lighting circuits (schematics will probably get you 110V). Most of them have triacs in that will handle 1 or 2 Kw at 240V. Opto isolating keeps the higher voltages away from your AVR and will fail in the "off" position. You don't need the dimming, only the gate driving bits.
C.H.

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It's only waste if you don't use it!

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Wang Guohui
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Posted by Wang Guohui: Tue. Oct 5, 2004 - 05:02 AM
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You may use a MOC3041(Zero-Cross Optoisolators Triac Driver) and a TLC336A(SCR).

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Bingo600
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Posted by Bingo600: Tue. Oct 5, 2004 - 09:11 AM
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Could this be used ??

http://www.hoelscher-hi.de/hendr...

/bingo

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Erik_L
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Posted by Erik_L: Tue. Oct 5, 2004 - 10:10 AM
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I used a simple and quite cheep lightcontrolled relay(230v 16A) from claesohlsson(Swedish company)
Here you have all the safty encapsulaion ready and a simple 230v (Swedish) standard wall connector.

You simple put an LED infront of the light-sensor on the unit and encapsulate it good so you dont get any sorrunding light.

This way you controll the relay with a single LED.

If this sounds like a suitable solution i can give you some more information.

Regards
Erik.

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Wang Guohui
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Posted by Wang Guohui: Tue. Oct 5, 2004 - 11:09 AM
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Quote:

Could this be used ??

http://www.hoelscher-hi.de/hendr...

Yes, just like this. easy and safety.
You even can control the output power if you wish[quote

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