Power transformer with a thermostat

[salomonander]

This is NOT for an audio project. Maybe someone can still be kind enough to help me out?

I want to use a 110v heating coil with a temperature controller, that turns the ac going to the coil on/off constantly. Since im in a 230v country, id like to use a transformer to convert to 110v. Can a power transformer deal with this constant on/off voltage, or would i damage it over time?

Thanks!

 

[CJ]

Should not be a problem, maybe put a movister across the 110 side to suppress any spikes caused by the inductive kick although with a 2 to 1 transformer any spikes would probably be small compared to a 120 to 600 volt xfmr,

 

[cyrano]

What's the on/off frequency?

 

[salomonander]

Thanks guys. I do not know the on/off frequency yet. The temperature controller will measure every 0,5 seconds… im powering a heating coil, so i guess that if i keep my hysteresis within a moderate setting, it wont be too often. The hysteresis setting determines, how much the measured temperature at the coil can differ from the set temperature on the controller, before it adjusts.

 

[salomonander]

Oh, and since i have CJ here, may i ask what wattage the transformer should have, if the coil draws 100w and i want this to run as efficient as possible? Thanks

 

[CJ]

I would think that a 100 VA transformer would be ok because

1) you are not using rectifiers on the secondary side, (distorted waveforms occur with bridge or full wave circuits because of the interruption the current, thus changing the RMS value)

and

2) since the heating element will be cycling on and off, your duty cycle might only be 50%, thus the windings will have time to cool while the heating element is off

Power transformers are usually rated in VA, so in our case, 100 watts = 100 VA.

Interesting to note that power transformer engineers are concerned with Pt, or apparent power. The transformer has to handle the power in plus the power out plus losses.

So for instance if you want 100 watts out then the primary will also have to handle 100 watts.
So Pt will be 200 watts plus maybe a 5 percent loss factor so figure 205 watts. So when you buy a power transformer rated at 100 VA, hopefully the engineer knew about apparent power and designed it for 205 watts. So you want to buy a pwr xfmr based on VA, not watts. But don't worry about it because I have never see a 100 watt power transformer. I guess you could multiply the secondary current and voltage and call it that but that's not the way they sell them.

Now those heating elements are going be drawing more current when they are cold so you want to buy from someone who does not puff up their ratings. Or if you do choose an inexpensive vendor, make if a 200 VA transformer.

 

[salomonander]

Thanks a lot CJ!
One last question: you are talking about a movister across the 110v side… what is a movister ?

 

[Matt Syson]

A Movistor is a Metal Oxide varistor, a specialist component that limits voltage 'spikes' and could be considered similar to a zener diode BUT they work with AC voltages and can handle high currents for a short period of time (milliseconds). You select the voltage rating to suit the expected supply voltage at that point so for 120 Volt mains it might be 150 Volts. I think there should normally be a fuse on the incoming feed so that if a large sustained overvoltage occurs the fuse should blow, but not under normal transient '(spike' occasions. There will be details available online from the manyfacturers of such devices (such as Bourns in the USA) but it may be a trademarked name so other devices that are the same could be named differently. Transzorb is, or may be a similar device.
they protect switching contacts or semiconductors against voltage spikes that may be a killovolt or so in some siituations.

 

[CJ]

Did I say movister? Sheesh, my mind was on bike racing and Milan San Remo , way to go Jasper!

I meant varistor.
Movistar is a bike team but they did make varistors, or is it it movister? I picked a bad week to quit sniffing glue. sorry about that,

 

[salomonander]

Thanks guys!
One more question regarding efficiency:
The higher the VA rating, the better the efficiency of the toroidal transformer… so would i benefit from a higher rating? Or would it actually be less efficient than choosing one close to the 100VA?

 

[Matt Syson]

I suppose one rated at a higher power (current) would have lower copper resistance so likely a bit more efficient but in reality not a significant difference to the overall excercise.

 

[cyrano]

Doesn't an oversized transformer run cooler?

Just a thought...

 

[nsalis]

Should not be a problem, maybe put a movister across the 110 side to suppress any spikes caused by the inductive kick although with a 2 to 1 transformer any spikes would probably be small compared to a 120 to 600 volt xfmr,

I used a resistor (100 Ohm) / capacitor (47 nF) combination for this purpose.

Kemet PMR209MB5470M100 (see encl.)

 

[CJ]

I like the rc network better as failure mode is not an explosion with molten metal being launched inside the chassis, and those MOV's do have a lifespan, they are usually rated for 10,000 hits or something like that.