24 to 120

[ptonsparky]

A radio control system we have asks for a 24v to 120v step up transformer in one application. My supply house rep suggested back feeding a 120x24 control transformer. I can order one from the RC maker but don't see a reason if the back feed will work. Thoughts? Or a source for one? I suspect that I would get a standard control transformer from the Mfg anyway.

I may have one on the shelf to play with. Haven't checked yet.

 

[Little Bill]

Isn't that what most buck/boost transformers are rated at?

 

[hillbilly1]

How much power do you need? A bell transformer might be sufficient. Assuming the 24 volts is a/c. If it’s dc, you will need to take a different route.

 

[ptonsparky]

Isn't that what most buck/boost transformers are rated at?

Yes, but they may be over kill.

How much power do you need? A bell transformer might be sufficient. Assuming the 24 volts is a/c. If it’s dc, you will need to take a different route.

I rigged up a couple DB transformers to check the output on the second Worked fine.

 

[ptonsparky]

101 ma is the max measured load.

 

[Jraef]

Control transformers often have what are called "compensating windings" to give you stable output (LV) voltage under varying line and load conditions. When you use them in reverse that can work against you and you might end up with higher than 120V on the high side. So check into the tolerance range of the 120V device.

A simple doorbell transformer would not have that problem and for 101mA of load may be a better choice for several reasons.

 

[ptonsparky]

Control transformers often have what are called "compensating windings" to give you stable output (LV) voltage under varying line and load conditions. When you use them in reverse that can work against you and you might end up with higher than 120V on the high side. So check into the tolerance range of the 120V device.

A simple doorbell transformer would not have that problem and for 101mA of load may be a better choice for several reasons.

Thanks, the recent thread with a control transformer with a +130 volt secondary reminded me of that. I only had the low voltage transformers to play with. They were HVAC units vs DB as I stated earlier.

 

[gar]

200307-1100 EST

A normal ferro-magnetic (steel core) transformer for normal operation is considered a linear device. To make a voltage regulating transformer you have to have something non-linear.

The usual simplified equivalent circuit for a power transformer consists of an ideal transformer of N turns ratio, and a series output impedance of a resistor and inductor.

Many power transformers will be wound so the secondary (output side) voltage will of a desired value at full load rating. Control transformers typically are made this way. For a stepdown application this means a smaller ratio than the input-output voltage ratio at full load. The reverse is the case for stepup where the turns ratio is greater,

On a Sq-D 500 VA transformer connected 1 to 1 with no load I read ---
Direction 1 --- 123.5 Vin 118.1 Vout, ratio = 1.046
Direction 2 --- 123.4 Vin 129.3 Vout. ratio = 1.048
Very good correlation. With full load on the transformer you can expect the 129 V to be closer to 123, and in the other direction you will be worse off with the 118 being even lower.

ptonsparky:

Pick a transformer with adequate power capability. Power with measured 24 V in, and read output open circuit voltage. If this is adequate, then put your load on it. Is voltage still adequate?

.



. .

 

[ptonsparky]

Two different brand of transformers from my ‘good stuff‘ shelves. In 120 down to 24 to 24 up to 120 Out. IDR the exact voltage but it was only off about .2 volt from in to out.

 

[gar]

200307-1451 EST

ptonsparky:

Your experiment relative to the question in your original post does not prove anything.

What the experiment shows is that the two transformers are quite similar. Both have the same turns ratio, have high shunt impedance, low series impedance, are not saturated, and have good magnetic coupling. The 24 V windings could be 12, or 24, or 48 V or anything else, and your result would be the same.

,

 

[ptonsparky]

It proved I could get 120 volts with it as it was connected. Open circuit.
I reversed the connections today and got 113.

Alas, I checked voltage today as it cycled and discovered it dropped to about 101 from the 120. Not satisfactory, even if it still worked.