Anti sweat heaters

[freezemo]

Hi All, I have come across anti sweat heaters in cabinets that have a diode connected in series with the 220v supply. I have also come across other cabinets that have a capacitor connected in series with the 220v power supply. I have been informed that the purpose of these are to limit the heat. If this is the case can anyone please explain how this is achieved (thechnically). Thanks

[walden]

The diode reduces the heat from the anti-sweat heaters because it reduces the current through the heaters.

The diode blocks part of the AC cycle and the average voltage across the heaters is reduced.

A capacitor does a similar thing, there is a voltage dropped across the capacitor which reduces the available voltage across the heaters and hence, less heat output.

[freezemo]

Thanks for your reply walden,. What value diode and capacitor would you use? Feeze

[nike123]

Thanks for your reply walden,. What value diode and capacitor would you use? Feeze

Diode should be of amperage rating higher than I=U/R
where U is power supply voltage, and R is resistance of heater (when on working temperature) or, I=P/U where P is power of heating element and U is power supply voltage

http://www.allaboutcircuits.com/vol_2/chpt_4/3.html

[freezemo]

Thanks nike123 for thi solid information. The unselfish sharing of information is what I love about the forum Regards, freezemo

[Brian_UK]

Say Nike, that is a very useful website, thank you.

[walden]

Freezemo, you’ve got the basics for the rating of the diode all I’d add is the “Law of Two”. If, for instance the heaters are taking 1A, use a diode rated at least 2A and if the voltage is 220V, as it is in your case, use a diode rated at least 400V. The extra cost is insignificant, diodes cost pennies, but the increased ratings will help prevent failures from voltage spikes or short circuit currents.

The size of the capacitor is more involved. You have to calculate the impedance need to drop the voltage. Use a 400V type and, for extra protection, use one labelled “X” or “X2” these are give much better protection against voltage spikes.

If I were you, I would stick with the diode! It’s easier to install and it doesn’t matter, on an AC supply, which way round you fit it!

Have a good weekend!

[freezemo]

Thanks walden. I appreciate your detailed explanation and wish you likewise. Cheers,freezemo.

[Pyr0Beast]

I'd advise you to install a resistor paralel to the capacitor to discharge it when the unit is turned on. 1MOhm should do the job great.

[freezemo]

I'd advise you to install a resistor paralel to the capacitor to discharge it when the unit is turned on. 1MOhm should do the job great.

Hi Pyr0Beast, I have come across capacitors that are used as start capacitors on single phase compressors, These capacitors also have a resistor connected in parallel.I understand its function is to discharge the capacitor when the power is switched OFF. You state that it is used to discharge it when the unit is switched ON. Is my understanding of the function of the resistor in this application incorrect or did you make a typing error? Sorry if I come across as being rude but I just want clearifacation. Thanks,freezemo

[nike123]

Hi Pyr0Beast, I have come across capacitors that are used as start capacitors on single phase compressors, These capacitors also have a resistor connected in parallel.I understand its function is to discharge the capacitor when the power is switched OFF. You state that it is used to discharge it when the unit is switched ON. Is my understanding of the function of the resistor in this application incorrect or did you make a typing error? Sorry if I come across as being rude but I just want clearifacation. Thanks,freezemo

You are right, they are used to discharge capacitor when power is switched off.

[Pyr0Beast]

Oh, yeah, such a big typo. I meant off, but was thinking of adding something and mixed things up (morning hours :P), sorry. Thanks for correcting me. Should get it right this time I'd advise you to install a resistor paralel to the capacitor to discharge capacitor to non dangerous levels, once unit is turned off and to prevent current rush when unit is turned on with capacitors facing the opposite charge. Capacitors will limit the current passing thru in both directions, this kind of limiting is sometimes unwanted because it causes bad cos fi which unnecessarily loads the network. Sometimes diodes in heaters are used to 'convert' 110V heaters to 220V ones. It depends on how powerful those heaters are (total), 2kW should be acceptable, with more, you should position diodes on the heaters to employ both, upper and lower cycle of the voltage to prevent voltage spikes in the network.