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N-MOSFET as AC switch, triggered by DC (Hypothesis).

AnwarM.E.

New Member
Read about N-CHANNEL MOSFET’s. Gate and Base act as a capacitor; When gate reaches Threshold Voltage it allows current to flow from Drain to Source terminals by attracting negative charged electrons in between, constructing a conductive bridge.

It is easy to select two oposite charges on DC without major variations, however rectified AC waves, both have to variate in synchrony to keep a stable potential difference.

This shouldn’t go further with independent Base terminal MOSFETS, which could (theoretically) preserve its own potential difference with the Gate, from Drain and Source current. Nevertheless, common commercial ones comes mostly with Source and Base merged terminals.

Because capacitance variations transforms into D-S ohmic variations, Vds it is supposed to change quadratically(or so), being Vs = Vb, affecting capacitance bridge and creating acute waveforms.

From other perspective, DC Gate, DC Drain-Source MOSFETS circuit configurations also should be affected from smaller capacitance issues in the ohmic region, but they are used commonly that way anyway.

Explanatory attachment from a scheme where a DC current is applied to the Gate, while AC is passed through the D-S terminals.

¿What do you think will happen if this circuit is implemented?
 

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gophert

Well-Known Member
Most Helpful Member
Read about N-CHANNEL MOSFET’s. Gate and Base act as a capacitor; When gate reaches Threshold Voltage it allows current to flow from Drain to Source terminals by attracting negative charged electrons in between, constructing a conductive bridge.

It is easy to select two oposite charges on DC without major variations, however rectified AC waves, both have to variate in synchrony to keep a stable potential difference.

This shouldn’t go further with independent Base terminal MOSFETS, which could (theoretically) preserve its own potential difference with the Gate, from Drain and Source current. Nevertheless, common commercial ones comes mostly with Source and Base merged terminals.

Because capacitance variations transforms into D-S ohmic variations, Vds it is supposed to change quadratically(or so), being Vs = Vb, affecting capacitance bridge and creating acute waveforms.

From other perspective, DC Gate, DC Drain-Source MOSFETS circuit configurations also should be affected from smaller capacitance issues in the ohmic region, but they are used commonly that way anyway.

Explanatory attachment from a scheme where a DC current is applied to the Gate, while AC is passed through the D-S terminals.

¿What do you think will happen if this circuit is implemented?

It depends what the gate is connected to.
 

Pommie

Well-Known Member
Most Helpful Member
When I was about 10 (about 1970) I opened/examined a drill speed controller. It had the same arrangement but with a thyristor shorting the bridge. I eventually realised that the bridge was between the mains and the drill. The drill still saw AC but the thyristor saw DC.

Mike.
 

AnwarM.E.

New Member
When I was about 10 (about 1970) I opened/examined a drill speed controller. It had the same arrangement but with a thyristor shorting the bridge. I eventually realised that the bridge was between the mains and the drill. The drill still saw AC but the thyristor saw DC.

Mike.
Yes. Hear about using thyristors for power operations, but since they only switch off when reach 0 or negative voltage, are a bit dangerous, same hear about IGBT.

I would test it also with a high current bipolar transistor.
 

AnwarM.E.

New Member
And AC is at 0V 100 or 120 times every second. By triggering part way through each (rectified) half cycle you can vary the power to the device.

Mike.
Thanks. Also want to add a potentiometer or a PWM function after solving the switching, and it may even overwrite it.
 

Pommie

Well-Known Member
Most Helpful Member
The circuit I (vaguely) remember had an RC circuit that turned on the thyristor after a time set by a potentiometer. I assume it turned on from after 1mS for the fastest speed and 19mS (50Hz) for the slowest. I was amazed at how simple the circuit was.

Mike.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The circuit I (vaguely) remember had an RC circuit that turned on the thyristor after a time set by a potentiometer. I assume it turned on from after 1mS for the fastest speed and 19mS (50Hz) for the slowest. I was amazed at how simple the circuit was.
That's a standard light dimmer/power controller, there's very little in them, basically a thyristor and a diac, and placing it inside a bridge means it works on both cycles. Back in the day there were a number of CRT TV's that used such circuits as regulated PSU's.

There seems little point in attempting to do something similar with an FET?, when the thyristor is so simple, reliable and effective in this role. If you're wanting to control DC fair enough, but for AC no real reason.
 

olly_k

Member
That's a standard light dimmer/power controller, there's very little in them, basically a thyristor and a diac, and placing it inside a bridge means it works on both cycles. Back in the day there were a number of CRT TV's that used such circuits as regulated PSU's.

There seems little point in attempting to do something similar with an FET?, when the thyristor is so simple, reliable and effective in this role. If you're wanting to control DC fair enough, but for AC no real reason.
What if you want to turn on at zero phase and turn off part way through Nigel? I have never actually considered how this might be achieved but thyristers
 

Nigel Goodwin

Super Moderator
Most Helpful Member
What if you want to turn on at zero phase and turn off part way through Nigel? I have never actually considered how this might be achieved but thyristers
Same way you would do it for DC, you use another thyristor to switch a discharged capacitor across the first one - bit messy though.

However, it's not generally something you would need to do.

Funnily enough last Sunday I called at an Exhibition at Lincoln Cathedral, basically promoting technology in the area. One of the exhibitors was a semiconductor manufacturer, who makes fairly large control modules - they had one sliced open, showing the multiple silicon wafers inside. Unfortunately, as 'large' isn't really what we do (and I wasn't anticipating this thread :D), I didn't pay much attention to the specifications, or even if it was MOSFET or Thyristor? etc.

A quick google reminded me of the company:

https://www.dynexsemi.com/
 

AnwarM.E.

New Member
That's a standard light dimmer/power controller, there's very little in them, basically a thyristor and a diac, and placing it inside a bridge means it works on both cycles. Back in the day there were a number of CRT TV's that used such circuits as regulated PSU's.

There seems little point in attempting to do something similar with an FET?, when the thyristor is so simple, reliable and effective in this role. If you're wanting to control DC fair enough, but for AC no real reason.
Well, make it finally work at simulation. These are common circuit arrangements; IC555 Multivibrator astable and Dimmer Control Circuit.

Tried merge both for a functional high power AC PWM, but not yet getting the double Opto connections to drive the TRIAC load-switcher in double-cycle. Also TRIAC's R-C filter won't let down pulses reach 0V.

I may construct Dimmer TRIAC Control Circuit with an extra Thyristor for on/off switching, but idk correct component's magnitude yet. Any advice is well received.

Captura de Pantalla 2019-05-09 a la(s) 19.25.44.png


Captura de Pantalla 2019-05-09 a la(s) 18.56.55.png
 

alec_t

Well-Known Member
Most Helpful Member
Don't try building the post #12 first circuit for real. The PC817 s will let the magic smoke out.
 

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