240Vrms Output amplifier (low power)

[Jester]

I have a need for a programmable voltage source 0-240Vrms (60Hz). The load will be > 500kΩ resistive

I have a HP33120A signal generator, that I will use as the PC controllable sinewave generator.

I'm wondering if I can use an amplifier like this:

**broken link removed**

If I run it with +/-35V rails and use it to drive a transformer something like this (backwards) :

https://www.digikey.com/product-sea...=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

Do you think I will get a decent sinewave at the output?

Should I simply drive the 12V transformer winding, or try to incorporate the transformer into the feedback loop?

I will ac couple the amplifier input via a capacitor just in case the amplifier fails, I don't want to kill my signal generator.

If anyone has a better approach or suggestions, please describe.

 

[smijesh]

Your output load is 500KΩ, so the maximum output current is 0.5mA !!!!!!! It is cool

Output impedance of the audio amplifier is 4-8Ω, this is the problem You should get a transformer with maximum 8Ω input impedance. It is too hard to get such one. If you are connecting output of your transformer to output of the amplifier it will work some extent (Transformer output impedance is 40Ω)

For low output current (.5mA) you can try this. Don’t worry it do not fry your signal generator
Try with zero signal voltage and slowly increase the signal voltage , check any output clipping.

Alternate option is class B push pull amplifier with transformer.

Hope it is helpful to you

 

[KlausST]

Hi,

No need to use a transformer with 4..8 ohms impedance. Almost any low power transformer will do.

*****
You need less than 0.2W at your load. A 100W amplifier is overkill in my eyes. But you need to calculate with a lot inductive current.

Regarding the transformer:
Especially low power transformers have an increased prim: sec turns ratio to compensate for losses.
If you drive them in reverse then you need a lot more input voltage than expected. If you want 240V output voltage then there is a good chance that the core saturates and the waveform is distorted.
Therefore I recommend to use two equal transformers.
Connected in parallel at the low voltage side and connectedin series at the high voltage side.
Then each transformer needs only 120V output voltage. This prevents from saturation.

If you want it to be low power, then I recommend to use a class D amplifier with a low pass filter at the output.

Klaus

 

[FvM]

Transformer in feedback loop is of course preferred, gives better voltage accuracy and lower distortion.

 

[crutschow]

..............
Output impedance of the audio amplifier is 4-8Ω, ..................

Not true for sold-state amps.
Yes they are designed to drive 4-8Ω loads, but their output impedance is a fraction of an ohm due to feedback.

I agree with the recommendation to use two 220V:6V/12V transformers, and driving the 6V/12V windings in parallel with the 220V outputs in series to avoid transformer saturation at 240V output.
For a 6V winding you only need about 3.2Vrms to generate 240V with the two outputs in series and, for that, a 2W amplifier should be adequate to provide the required voltage and power (115mW).
Actually, for that low power you could drive the transformers directly since the signal generator will generate 10Vpp (3.5Vrms) into a 50Ω load or about 4.5Vrms into the transformer with a 500k load on the output (89Ω reflected to the input).

 

[KlausST]

Hi,

I doubt it:

A small 240:6 maybe has a 240:9 ratio (no load voltage 9V, and those 9V reduce to 6V with full load.)
Therefore with loss you need to input more than 9V (maybe 11V) to get 240V output, but this may make the core to saturate.
In detail it depends on the transformer. Usually the lower the power the weaker the voltage the worse the winding ratio.

Short answer: Try it.

Klaus

 

[Jester]

I found a 240V: 8V in my transformer bin, driving it directly with HP 33120A @10Vpp I get a nice clean sinewave @ 140Vrms, so +/-20V rails might be enough. I have ordered the amplifier in my original post. Thanks for your advice.

 

[Jester]

Follow up: I picked up an amplifier similar to the one in the original post (smaller based on LM1875), with one transformer the waveform is clean to about 200V. To get a clean 250Vrms I tried two transformers and the waveform is quite acceptable at 250Vrms.