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CRT Oscilloscope - controlling deflector coil with MOSFET

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sleeper1987

New Member
Hi.
I'm trying to control an old CRT using a microcontroller, and I need to drive the deflector coils with reasonably large voltages.

I've got a few questions I'd really appreciate some help with:

- To deflect the beam both ways, I need to be able to apply a positive and negative voltages to the coil. Simplest way, I figure, is to bias one side at a mid voltage, and swing the other beween 0 and the maximum. Does this sound reasonable?

- I'm not sure how to switch the larger voltage using the microprocessor's 5v output. I could use PWM to control a mosfet in saturation, or get an analogue voltage via a resistor ladder, to operate it in linear mode. How would I go about doing either of these, and as I understand I need Vgs to be greater than Vds in either case?

- The coil has a resistance of something like 60ohms. How should I isolate it from the power supply, so as to prevent it effectively shorting the supply when connected by the mosfet?

Any help would be greatly appreciated!
 
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MikeMl

Well-Known Member
Most Helpful Member
How about an H-Bridge?
Deflection amplifiers are usually a voltage-controlled current source.
 

mneary

New Member
I would use an audio power amplifier chip. Just make sure it's stable into an inductive load.
 

sleeper1987

New Member
Thanks for the replies!
As for the audio power amp: How can I tell if it's capable of handling inductive loads? Would any amp designed to drive speakers directly be ok for that?

Unfortunately, I'm restricted to buying from here, so there's little choice:

Power Amps : Maplin Electronics

Do any of those seem suitable?

Thanks again.
 
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ronsimpson

Well-Known Member
Most Helpful Member
Typically the vertical coils require little current and are slow to respond. They are built to run at 60hz. The horizontal coils take much more current and run faster. TV=15khz. VGA=32khz. The coils deflect the beam with current so the amplifier must have current feedback.

Is the CRT from a TV or monitor?
 

sleeper1987

New Member
Typically the vertical coils require little current and are slow to respond. They are built to run at 60hz. The horizontal coils take much more current and run faster. TV=15khz. VGA=32khz. The coils deflect the beam with current so the amplifier must have current feedback.

Is the CRT from a TV or monitor?
Interesting! In that case, I'll probably rotate the coils so that the horizontal one is slower. The CRT's from a TV.
 
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