transformer ratio of an ac adapter

[John Parker]

How can I predict the transformer ratio of a regular ac adapter?
I would like to disassemble and use up just the transformer to battery operated high voltage project. I need high ratio but I know these adapters transform voltage just near to output voltage.

 

[dr pepper]

If its a low frequency iron cored transformer as a general rule the turns ratio is the input voltage divided by the output voltage, so a 240 in 12 out trans would be 20.
If however the adaptor is a switch mode device the turns ratio is nearly entirely down to the designer.

What output voltage do you want?

 

[audioguru]

A battery is DC. A transformer uses AC.
You gave no example of a transformer so I am giving one: 120VAC to 12VDC at 500mA.
The bridge rectifier has a 2V drop so the peak of the secondary AC is 14V. Then it is 14V x 0.707= 9.9V RMS.

The transformer power rating is 14V x 0.5A= 7W. If you fed its low voltage winding with 9.9V RMS then you would expect 120VAC at 7W/120V= 58mA from its high voltage winding.
The output voltage will be higher when the load current is less.

Do camera stores still sell "use one time" cameras then theydevelop the film and throw away the cheap camera? Ask them to give you the camera and use its high voltage circuit that was for its flash tube. It produces a few thousand volts.

 

[John Parker]

If its a low frequency iron cored transformer as a general rule the turns ratio is the input voltage divided by the output voltage, so a 240 in 12 out trans would be 20.
If however the adaptor is a switch mode device the turns ratio is nearly entirely down to the designer.

What output voltage do you want?

I would like to use it backward and make about 180V from a 9V battery and a capacitor-MOSFET LC circuit. A spark gap doesn't need high current but need high voltage.
How I know it's a switch mode device or a regular one?
And the frequency will be high. Is it problem for a regular core? What type would be good to kHz freq? OK, I checked: I need air core to higher freq. but I'm affraid I have to do it myself. Manually it won't be easy.

 

[dr pepper]

Just the job then.

If you went for a 12-0-12, or a 9-0-9 centre tapped transformer you'd be able to knock up a simple blocking oscillator circuit from just a few components.
Only you' dont find centre tapped trannys in wall warts, you'd have to go and buy one.

 

[John Parker]

Just the job then.

If you went for a 12-0-12, or a 9-0-9 centre tapped transformer you'd be able to knock up a simple blocking oscillator circuit from just a few components.
Only you' dont find centre tapped trannys in wall warts, you'd have to go and buy one.

I don't need flyback transformer but it would be good two windings on one side because I have to drive the gate of MOSFET with min. 4V and max. 20V. A 0-9-18 or 0-6-12 and 0-120 or 0-240 can be enough I think.

 

[dr pepper]

Not quite what I meant, a blocking osc doesnt have to be a flyback, one side of a 12-0-12 would be the primary and the other side feedback to get the circuit oscillating.
However sounds like you allready have is sussed.
Your right about the mosfet gate, its a good idea to protect the gate with a zener or a mov in circuits like these, a few times I've had fets just go short due to the gates being overdriven, kind of annoying when they short gate to source and blow something else too.

 

[John Parker]

A battery is DC. A transformer uses AC.
You gave no example of a transformer so I am giving one: 120VAC to 12VDC at 500mA.
The bridge rectifier has a 2V drop so the peak of the secondary AC is 14V. Then it is 14V x 0.707= 9.9V RMS.

The transformer power rating is 14V x 0.5A= 7W. If you fed its low voltage winding with 9.9V RMS then you would expect 120VAC at 7W/120V= 58mA from its high voltage winding.
The output voltage will be higher when the load current is less.

Do camera stores still sell "use one time" cameras then theydevelop the film and throw away the cheap camera? Ask them to give you the camera and use its high voltage circuit that was for its flash tube. It produces a few thousand volts.

I know many hobbyist do this camera trick but I would like to build my own for fun. And I can tune my own circuit easily.

 

[John Parker]

Not quite what I meant, a blocking osc doesnt have to be a flyback, one side of a 12-0-12 would be the primary and the other side feedback to get the circuit oscillating.
However sounds like you allready have is sussed.
Your right about the mosfet gate, its a good idea to protect the gate with a zener or a mov in circuits like these, a few times I've had fets just go short due to the gates being overdriven, kind of annoying when they short gate to source and blow something else too.

Not quite what I meant, a blocking osc doesnt have to be a flyback, one side of a 12-0-12 would be the primary and the other side feedback to get the circuit oscillating.
However sounds like you allready have is sussed.
Your right about the mosfet gate, its a good idea to protect the gate with a zener or a mov in circuits like these, a few times I've had fets just go short due to the gates being overdriven, kind of annoying when they short gate to source and blow something else too.

Oh, I understand you! Your idea need a regular BJT NPN power transistor and from secondary I give some voltage/current back through the base of tran? Good idea but also good if I drive back from a second winding of primary doesn't it? However I need a plus winding if I don't want to loose voltage due to keep up oscillating. I will do it according to your idea, I have a good power NPN to do this. That's a very robust one, not easy to kill.