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Electronics => Projects, Designs, and Technical Stuff => Topic started by: Jester on November 24, 2016, 02:45:31 pm

Title: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Jester on November 24, 2016, 02:45:31 pm
I have a need for an adjustable source 0-240Vrms (60Hz). The load will be > 500k \$\Omega\$ resistive

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

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

http://www.ebay.com/itm/TDA7293-100W-Audio-Amplifier-Board-AMP-Mono-Audio-Single-Channel-AC-12-32V-/162184383852?hash=item25c2f14d6c:g:hZ8AAOSwgZ1XvQGd (http://www.ebay.com/itm/TDA7293-100W-Audio-Amplifier-Board-AMP-Mono-Audio-Single-Channel-AC-12-32V-/162184383852?hash=item25c2f14d6c:g:hZ8AAOSwgZ1XvQGd)

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

http://www.digikey.com/product-search/en/transformers/power-transformers/786735?k=transformer&k=&pkeyword=transformer&pv588=1261&FV=15c4003e%2C15c40042%2C15c4004d%2C15c4004e%2Cfff4000c%2Cfff8012f%2C19440060&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25 (http://www.digikey.com/product-search/en/transformers/power-transformers/786735?k=transformer&k=&pkeyword=transformer&pv588=1261&FV=15c4003e%2C15c40042%2C15c4004d%2C15c4004e%2Cfff4000c%2Cfff8012f%2C19440060&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25)

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

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, please describe.


Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Yansi on November 24, 2016, 03:43:20 pm
That depends on what is your definition of decent.

Also a simple variac (plus optional isolation or stepup transformer) should do.

What do you need it for? I've no idea.
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Jester on November 24, 2016, 03:52:17 pm
Quote from: Yansi on November 24, 2016, 03:43:20 pm
That depends on what is your definition of decent.

Also a simple variac (plus optional isolation or stepup transformer) should do.

What do you need it for? I've no idea.

I have a Variac, however I need computer control of the signal, so the Variac is useless.

I need it to test analog input circuits that measure MAINS voltage 0-250Vrms
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Yansi on November 24, 2016, 03:56:09 pm
So that little distortion should be no issue.  Mains THD can be as high as about 20%. I'd use the linear audio amplifier boosted with a transformer in this case - I think it is the most adequate way to do it here.

Still, you would need a reference TrueRMS meter hooked to the secondary to calibrate/test against.
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Seekonk on November 24, 2016, 04:30:11 pm
Times have changed.  During the bad economy I was buying California Instruments power amplifiers without oscillator for under $50 shipped.  Good thing I stocked up.  They run about $250 now.  That is all they are, an amplifier and transformer.
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Jester on November 24, 2016, 04:34:02 pm
Quote from: Seekonk on November 24, 2016, 04:30:11 pm
Times have changed.  During the bad economy I was buying California Instruments power amplifiers without oscillator for under $50 shipped.  Good thing I stocked up.  They run about $250 now.  That is all they are, an amplifier and transformer.

Is the transformer in the feedback loop, or do they simply drive the transformer with the amplifier?
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: David Hess on November 25, 2016, 01:12:16 am
A 60Hz transformer driven in reverse is how I would do it.  If the high voltage output is used for feedback, then distortion can be insignificant.

I would probably use a composite amplifier for greater current output but there are many integrated parts which could drive the transformer directly.
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Audioguru on November 25, 2016, 02:08:12 am
240V into 500k is a whopping power of 0.12W. Then why use a 100 amplifier when a 1W amplifier is plenty.
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Jester on November 25, 2016, 02:11:13 am
Quote from: Audioguru on November 25, 2016, 02:08:12 am
240V into 500k is a whopping power of 0.12W. Then why use a 100 amplifier when a 1W amplifier is plenty.


Most of the low power amplifiers are also low voltage. The $3.65 delivered price in a ready to use format motivates me, I cant buy the IC from Digi-Key for that
Title: Re: 250Vrms 60Hz (low power) adjustable source - what do you think?
Post by: Zero999 on November 25, 2016, 09:16:53 am
I don't see why the voltage is important, as a transformer can be used to increase it. The amplifier just needs to be powerful enough to provide the magnetising current for the transformer. There's no reason why a lower voltage amplifier and a 3V transformer can't be used.

One thing to note is that the putting 12V on the secondary winding of a mains transformer designed for 240V to 12V operation, will not give 240V. It will be significantly lower than that. This is because the 12V secondary will have extra turns, to compensate for the voltage drop under load. In the case of the transformer linked to by the original poster, it has a regulation factor of 15%, so to get 240V out, the input will need to be 15% more than 12V = 13.8V. Fortunately, this won't be a problem with the amplifier driven off a dual 35V power supply.

http://catalog.triadmagnetics.com/Asset/TCT3-04E07AE.pdf (http://catalog.triadmagnetics.com/Asset/TCT3-04E07AE.pdf)