Dear All, i have a coil of large size and resistance of 20 OHMs. the coil is drived by about 6 Amps to do its function..
Now how to drive this coil electronically.. By the way using a variac is not good where in my country the variac is expnsive and short life time..

i think it may be made by chopper circuit and PWM controlling the average DC voltage of about 120vdc, but i don't know exactly how to design such circuit..

is there any one can help me in such design?

thanx a lot .... M.Shendy.

 

What voltage runs from your outlet?

 

What is the device, does it have a part number, what is it's coil voltage?

 

the voltage to be PWM modulated is 120-140Vdc, and is pwm modulated from 10% to 90% to support variable current.. But how to design such switcher? using power BJT / power FET or what?
thanx again and wait for reply...bye

 

I do not know what your ultimate purpose is going to be, and I don't know how much time and effort you wish to spend on this design. For something robust and uncomplicated I would start with a class AB push-pull amplifier. The output stages will need to have transistors that can handle the voltage. The predrivers for the output stage can also be BJTs with less impressive characteristics.

A FET design will complicated by the need to switch high voltage on and off the gate rapidly. You also did not mention the frequency of the PWM scheme you intend to use.

Probably not much help, but you really didn't give us much to work with.

 

Well, if the coil is 20 Ohms and rated for 6 Amperes, then the voltage is 120.

Seems like this would be a good application for a MOSFET, perhaps an IRF630 or two with a decent heat sink.

Is your power really 120 to 140 VDC? That would be a surprise, because DC doesn't work well over long distances.

You're going to need low voltage DC for the PWM control circuit. If your incoming power is DC, you won't be able to use a transformer to step down the line voltage, so this will have to be known before a design could be worked up.

 

Well the N_channel IRF630 will work for the pull side of the class AB output stage, but on the high side the OP will need a gate voltage of 140 to 160 V depending on his DC source. As you have already pointed out using transformers does not seem to be an available option.

I do agree that a circuit can be designed around such a part.

 

thanx SgtWookie and Papabravo for your help, i will explain each point you wonder:

1- the supply in our country is 110VAC.
2- i used a 35A bridge to rectify this voltage.
3- i use a capacitor bank of about 20000 uF to smooth the rectified voltage.
4- then i have peak dc voltage of equal 110 * sqrt(2) = 155 volts.
5- from the original 110VAC i use a small transformer (110vac to 9vac) to supply the PWM circuit (it is very light w.r.t the using of variacs)
6- my large coil is fed by the smoothed +ve terminal and the other terminal of the coil is connected to power NPN collector that is base-switched by 555 circuit.
7- the emitter of the NPN is connected to ground.
8- is this good design or what.. a lot of power npn are destroyed from me why? its ratings is good (400v / 12A )..
i use free whiling diode parallel with the coil.
why destruction happens? what about driving the coil with MOSFET n-channel?
i attach a file, how to modify this design to meet my requirement?

thanx a lot... m.shendy

 

Your diode must be anti parallel to your coil.

 

Well I see several problems.
The circuit shown is for 12 volts not 120 to 140
The circuit shown uses a generic MOSFET, your're talking about a BJT
Besides popping the transistors you're probably overworking the diodes as well.

I'm guessing we need to see the actual schematic and parts list not something pulled from a magazine that you started with.

 

If the 555 has a 12V supply then it can provide only about 90mA through the 100 ohm resistor as base current to a BJT. For a power BJT to saturate well with only 90mA base current then its load current must be only 0.5A to 2A, not 8A.

 

Have you tried connecting it straight to 110VAC?

 

I'm admittedly lazy at the moment, and don't feel like calculating the frequency of your 555 driver circuit.

What frequency did you calculate it to be, or vary around?

If you're using a FET device, the gate has quite a bit of capacitance; it takes a good bit of current to change the state of the device. If you don't give it enough current to overcome the capacitance, it will spend time in a transitional state (ie: moderate resistance; partial conductivity) and that translates to dissipating power by generating heat. Running a PWM circuit at too high a frequency will result in a FET device spending porportionately longer amounts of time in a transitional state, generating more heat. If you're running at higher than 10KHz, you probably need to lower it.

You could also drive the gate using a Darlington pair.