Author Topic: Opto - Transistor Pulsing Circuit  (Read 7096 times)

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Re: Opto - Transistor Pulsing Circuit
« Reply #8 on: August 22, 2010, 21:12:14 pm »
when the pulse comes the coil is disconnected, is that what you want? then its ok

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Re: Opto - Transistor Pulsing Circuit
« Reply #9 on: August 22, 2010, 21:24:53 pm »
when a pulse-on comes from the microcontroller, i want this to pulse-on the power supply

would this circuit work backwards?

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Re: Opto - Transistor Pulsing Circuit
« Reply #10 on: August 22, 2010, 22:33:46 pm »
use another small npn transistor to invert the optocoupler output. then it should be fine.
at south end of the 10k and gate of the mosfet the collector, the base at the collector of the opto-transistor and the emitter to ground.
you'll need another resistor from 12V to the opto-transistors collector, the value depending on your npn-transistors base-current.

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Re: Opto - Transistor Pulsing Circuit
« Reply #11 on: August 22, 2010, 22:57:28 pm »
The way i imagine the circuit working, an on pulse from the opto gives and on pulse to the primary, can you describe how it's working so i can see what i'm missing?

i am thinking when the opto-led turns on, it completes the connection to the ground on the other side, allowing voltage to go to the mosfet and turn it on, allowing current to flow through the primary.

i'm confused about how it is inverted

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Re: Opto - Transistor Pulsing Circuit
« Reply #12 on: August 22, 2010, 23:24:25 pm »
The way i imagine the circuit working, an on pulse from the opto gives and on pulse to the primary, can you describe how it's working so i can see what i'm missing?

i am thinking when the opto-led turns on, it completes the connection to the ground on the other side, allowing voltage to go to the mosfet and turn it on, allowing current to flow through the primary.

i'm confused about how it is inverted
The opto turns on and the connection of the mosfet-gate is at ground, so voltage = 0V. if the led is off the gate is at high voltage (12V), because the 10k resistor is very small compared to the open circuit the opto-transistor has.
so if the pulse is on the gate voltage is 0V and the current on the transformer is 0A.
you need a second transistor like i described.

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Re: Opto - Transistor Pulsing Circuit
« Reply #13 on: August 23, 2010, 00:06:46 am »
Ok thanks, I see it now

This is what you described,


I'd rather not add in another transistor if I can get away with it, what do you think of either of the next two, will they work? (10k still make sense?)



this one is similar to what Stan has in one of his patents

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Re: Opto - Transistor Pulsing Circuit
« Reply #14 on: August 23, 2010, 00:22:20 am »
My advice to you  is that you use a mosfet , like the irfp250 . Transistors IMO are inferior for switching purposes .Less rise and fall time , less amps , less durable .

To drive mosfets I like the totem pole method , this is the method used in mosfet drivers .

resistors make slow rise time , remember that you wanna charge that 2000 pf as fast as you can , so resistors is bad , that circuit fom Naudin is not so good , slow rise and fast fall if a big mosfet were to be used . Remember that you need to discharge also , some of those circuits are just  non-functionnal and wont discharge . Also you need short distance between driver and mosfet because high rise + inductance + capacitance = ringing . Minimize the inductance as best you can , big wire 1/2 inch long to gate .

My advice is that you use a mosfet driver or study totem pole if you wanna make with available components . That way you can transfer the high impedance source of the opto to a high imopedance input of a mosfet driver or totem pole pair driver . That second pic right there that you claim is from stan is non-functionnal , n channel fets need 10 volts Vgs to be saturated , you would need to drive it with 22+ volts

Thats the method I always use , lazy and small . I use mosfet drivers personally because it uses less board space , max627 for example .
« Last Edit: August 23, 2010, 00:58:32 am by Dankie »

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Re: Opto - Transistor Pulsing Circuit
« Reply #15 on: August 23, 2010, 02:44:13 am »
The circuits shown are good you just need to add a capacitor between the drain and source, about 200nf works fine for irfp250 transistor (this allow the transistor to completely stop conducting the current during the of pulse and thus make it work cooler... use a mosfet driver is the best...


There are many ways you can do it... Can you tel us what you need this driver for?



In my projects page you find a good schematic of a pll circuit you can use it...


Best regards