Heating Elements (Resistors)

[drkidd22]

Hello,

I have a circuit that I need to modify, the operating principles of the modified circuit have to stay at same as original circuit, or as close as possible. Basically I have a heating circuit with 4, 5Ohm 50W resistors as shown in the schematic below. (the original circuit)



The problem is that the source voltage is going to change to 120VAC instead of 28VAC. In the original circuit each resistor generates about 39W of heating power for a total of about 156W.

I can't change the original resistors.
I can add resistors, I have many different type.
I can use a transformer to drop the 120VAC down to the 28VAC. (Trying to avoid this).

Any Ideas will be appreciated.

 

[jpanhalt]

Your drawing seems accurate, your description has a typo. They are 5Ω, not 50Ω.

Put all your resistors in series to give 20 Ω, then add a new resistor to reduce current to 2.8 (2.79) A. The new resistor will need to be about 23 Ω and 200W or more. That is a pretty big resistor and a lot of additional heat/power loss.

I would go with a transformer.

John

 

[alec_t]

Those heating elements plus their surroundings presumably have a fair heat capacity. If you use a triac to supply current to the resistors for only 1 cycle in every 18 you will get ~ the same average heating effect and very little switching noise, albeit with a slight ripple of the temperature. Or you could use phase-angle control of a triac to get the required power, with greater switching noise but less temperature ripple.

 

[drkidd22]

I can't rearrange the resistors. They must stay the way they are. I'll see if I can find a triac or I will have to use a transformer.

 

[MikeMl]

I can't rearrange the resistors. They must stay the way they are. I'll see if I can find a triac or I will have to use a transformer.

If you are stuck with 5Ω, you could do the following: The capacitor must be rated for the AC voltage and ripple current. Try a motor starting capacitor **broken link removed**. Since the values of these are approximate, you may have to parallel two to trim the current...

 

[drkidd22]

Pretty nice Mile, thanks. I didn't even know what a motor starting capacitor was. I might give that a try.

 

[ronv]

I think I would use a dimmer like you use for your house lighting.