12vDC to 24vac module needs modification

[dsmith]

dsmith
Today
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Hi guys,

It's been decades since I've done this so helping a little help to jog my brain cells. I have this module where I put in 12VDC (from solar panel) and trying to get 24VAC out 60Hz. I get 12VAC at 60Hz output. (yes pic says 50Hz but adjusted to 60Hz) I put 150ohm 5W resistors at the output terminals (+DC center output is common to both 150ohm resistors) to see the waveform. What do I need to do to get the 24VAC out? Note low power eg 10W usage expected. I think I'm not understanding MOSFET correctly or something.

Thanks

 

[Harald Kapp]

Use a transformer with a 1:2 winding ratio?

 

[duke37]

Or take the output from across the whole primary.

 

[dsmith]

I think the output should already be 24vac but I only get 12vac. Sounds like I should not be adding any transformers. What might be wrong with existing design?

 

[Ylli]

Output is 24 V p-p differential (across both outputs). If you look at either output in reference to ground, you will only see 12 V p-p. The two outputs are out of phase so differentially, you have 24 V p-p.

 

[dsmith]

I get 12V p-p. If I look at output to ground it is 6V either output.

 

[Alec_t]

I get 12VAC at 60Hz output.

What did you use to measure the voltage?

 

[BobK]

Or take the output from across the whole primary.

I doubt that would work. The outputs most likely each switches from high impedance to +12V. That is why a center tapped transformer is used.

Bob

 

[dsmith]

Measured with both a Fluke multimeter as well as PC based o-scope. Same result

 

[Alec_t]

Can you post a link to the DC-AC module?

 

[dsmith]

it was on ebay, no docs just pics, I drew the schematic by tracing the solder path

 

[Alec_t]

Done a bit of googling. The spec for that module can be found here. This shows the same pic as your second pic.
Note that the output shown as "0V-" is in fact the +12V rail, if you follow the pcb trace !
That begs the question as to where you placed the probes when you made your measurements.
I don't see why you are expecting 24V AC from the module.

 

[dsmith]

Thanks for googling. The measurement is from the one output to other output terminal that's where I get 12VAC. If I measure from output terminal to the 0V- (center) terminal I get 6VAC square wave. Something still seems wrong or I'm really confused about those MOSFET. Can I modify this module to get 24VAC?

 

[duke37]

I doubt that would work. The outputs most likely each switches from high impedance to +12V. That is why a center tapped transformer is used.

Bob

The two halves of the primary are inductively coupled so when one side is pulled to ground, then the other side rises an equal amount above the supply voltage. The transformer needs low leakage inductance between the two halves.

I have used this system to run higher resistance speakers, Not a great success due to the diddy transformer having high resistance windings.

There will be voltage drop across the switches and any rectifying diodes so you will not achieve quite double the input voltage.

 

[Alec_t]

The measurement is from the one output to other output terminal that's where I get 12VAC. If I measure from output terminal to the 0V- (center) terminal I get 6VAC square wave.

That's what you should get. Here's a sim, where S1 and S2 represent the FETs :


Edit: If you replace the resistors with the two halves of a centre-tapped transformer winding then, for the reasons Duke gives, you should get close to 24VAC between A and B.

 

[dsmith]

Thanks Alec_t for running that sim. That waveform picture is what I expected but that's not what I get. Where you have V(0V-, A) showing a 12V swing I only get 6V swing. Same for V(0V-,B), Your resultant V(A,B) gives 24V swing whereas I only get 12V swing.

 

[duke37]

Thanks Alec_t for running that sim. That waveform picture is what I expected but that's not what I get. Where you have V(0V-, A) showing a 12V swing I only get 6V swing. Same for V(0V-,B), Your resultant V(A,B) gives 24V swing whereas I only get 12V swing.

A 12V swing will show as 6V AC i.e. +/- 6V on an analog meter.

What is the load? Is it a rectifier which selects the peak?

 

[Alec_t]

As Duke says, 6VAC = 12V peak-to-peak for a square-wave.