Pulsed DC Supply for magnetizing and demagnetizing of electromagnet

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#1 21677078 04 Mar 2017 05:12

Priya dharshini Priya dharshini

Anonymous

Post #1
21677078 04 Mar 2017 05:12

hello all,

I am developing a pulsed DC power supply for magntezing and demagnetizing the electromagnet.

1) I used an 555 timer IC which generates 15V PWM signal with 0-100 % duty cycle.and T of 6.5ms,

2) an Bridge rectifier circuit which converts

a)240V, 5A AC supply to 200V DC which is suppled for MOSFET.

b) also same 240V AC supply is converted to 15V to power up 555 timer IC.

3) switching circuit using MOSFET for magnetizing and demagnetizing the electromagnet.

I have succeeded with part 1 and part 2. but stuck with part 3 in choosing proper MOSFET. frequently burnt MOSFET when connected in the circuit... Please help me out to sort out the problem..

Do I require a MOSFET driver to drive MOSFET?

Thanks & Regards,

Priya
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#2 21677079 05 Mar 2017 17:26

richard gabric richard gabric

Anonymous

Post #2
21677079 05 Mar 2017 17:26

What are the properties of the electromagnet.
What sort of specifications have you used in deciding on the mosfet to use.
How are you dealing with the inductor generated back emf when the mosfet turns off, are you relying on the body diode of the mosfet, and having the mosfet go into avalanche mode, diode clamping across the inductor, snubbing, or what?
What voltage is the 555 running on, it should be high enough to fully enhance the mosfet chosen at your ratings, but not excessively high.
The 555 output stage is not really designed for driving fets, which are highly capacitive, you should use a buffer. A push pull transistor arrangement running off a suitable voltage will often work, and there are plenty of IC drivers available. In either case, you have to be sure that the input drive voltage is adequate to drive the output, so level translation will probably be needed.
There is a lot to know in getting a mosfet to switch and run reliably.
Equally, there is a lot of information available from the manufacturers, so I think you need to do some searching and reading.
Cheers,
Richard
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#3 21677080 06 Mar 2017 22:38

Priya dharshini Priya dharshini

Anonymous

Post #3
21677080 06 Mar 2017 22:38

these are the circuits so far developed for circuit... in the attached circuit the 240v to 15V conversion is not done,,, becoz 555 timer is powered 15V DC supply.
#4 21677081 10 Mar 2017 02:06

richard gabric richard gabric

Anonymous

Post #4
21677081 10 Mar 2017 02:06

You need to go back to basics.
The inductor is the only thing you have to concern yourself with at this stage. What is it's inductance and resistance, there are other issues as well, but lets not get side tracked at this stage.
The inductance determines it's switching behaviour and the resistance determines how much power it dissipates. Knowing the inductor electrical and physical properties, and how it behaves when switched, you can then find what sort of field you are generating. Be aware that the inductive properties will change if something with magnetic properties is inserted into the field.
From there, you can work out what properties the active switching device has to have.
Get the fundamentals correct first, or you are going to destroy a lot more FETs.
Magnetization is commonly done by applying a very large current pulse to an inductor(watch out for the back emf), demagnetization is commonly done using a decreasing AC field.
All the best,
Richard
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#5 21677082 16 Mar 2017 04:21

Priya dharshini Priya dharshini

Anonymous

Post #5
21677082 16 Mar 2017 04:21

circuits developed..
I used multilayer coil inductance formula to find out inductance and resistance of the electromagnet. is it the correct way to find inductance??
#6 21677083 16 Mar 2017 12:19

richard gabric richard gabric

Anonymous

Post #6
21677083 16 Mar 2017 12:19

Best way to do it is to measure them, but the formulae will give an approximate answer.
What values for L and R do you have?
Cheers,
Richard
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#7 21677084 17 Mar 2017 21:04

Priya dharshini Priya dharshini

Anonymous

Post #7
21677084 17 Mar 2017 21:04

L value is about 5 to 6 Henry. and DC resistance of the coil is 285 to 300 ohms... This value I got it by website...
#8 21677085 18 Mar 2017 14:17

richard gabric richard gabric

Anonymous

Post #8
21677085 18 Mar 2017 14:17

OK, now you have those figures, you can do some maths to figure out what's going on.
You must know that when you apply a voltage to an inductor, the current cannot change instantly to the maximum current determined by the applied voltage and the inductor's resistance, but rather to a rate determined by the inductance.
If you are not sure about his, then you need to do some reading, good tutorials here.
http://www.electronics-tutorials.ws/inductor/inductor.html
Also look at this,
http://electronics.stackexchange.com/question...rough-inductor-in-dc-circuit-with-pwm-control
You must also watch the back emf when the current is turned off, and understand the safe operating area curves for the mosfet.
The drive circuit of the mosfet will need to be adequate enough that the turn on and turn off times of the mosfet do not keep it in a linear region for long enough that the power dissipation is exceeded.
Although spice has many limitations that you need to understand, you may be able to simulate the basic system, LTSpice is free,
cheers,
Richard
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Topic summary

A pulsed DC power supply is being developed for magnetizing and demagnetizing an electromagnet using a 555 timer IC generating a 15V PWM signal with a 0-100% duty cycle and a 6.5 ms period. The supply includes a bridge rectifier converting 240V AC to 200V DC for the MOSFET and a 15V DC supply for the 555 timer. The main challenge is selecting a suitable MOSFET for switching the electromagnet without frequent device failure. Key issues discussed include the need for a proper MOSFET driver since the 555 timer output cannot adequately drive the MOSFET gate due to its capacitive load, and the importance of managing the inductor's back EMF to prevent MOSFET damage. The electromagnet has an inductance of approximately 5-6 Henry and a DC resistance of 285-300 ohms, values initially estimated by formula but recommended to be measured for accuracy. Proper understanding of the inductor's switching behavior, safe operating area of the MOSFET, and adequate gate drive voltage and timing are critical. Suggestions include using a buffer or dedicated MOSFET driver IC, implementing snubber circuits or diode clamping to handle back EMF, and ensuring the MOSFET is fully enhanced without excessive voltage. The discussion emphasizes revisiting fundamental inductor and MOSFET characteristics to prevent repeated component failures and achieve reliable switching performance.
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Pulsed DC Supply for magnetizing and demagnetizing of electromagnet

Pulsed DC Supply for magnetizing and demagnetizing of electromagnet

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