How to control a power mosfet ?

[3eprabu]

Guys need some help here . Im using a irf830 power mosfet to control a dc light , but for some reason i can't control the mosfet via the gate . The light turns on automatically without even supplying signal to the gate . I connected the source terminal to the negative of the battery , one side of the light is connected to the battery and the other is connected to the drain of the mosfet . I tried the same connection using a transistor and it worked fine . totally confused . need some help PLZ . I have tested the mosfet using a multimeter and it seems to be fine . thanks in advance

 

[jpanhalt]

Do you have a resistor (1K to 10K) connected from the gate to the source? The gate can accumulate enough static charge to turn on. If you ground gate to source, it should turn off.

Are you certain of the Drain/Source identifications? Mosfets will conduct in the reverse direction due to a body diode.

Search on testing Mosfets. You can test their operation with a simple VOM/multimeter.

John

Edit: Thanks Les. I meant gate and don't know how I did that. Post has been corrected.

 

[Les Jones]

Have you biased the gate to the negative rail. The input resistance of a mosfet is so high it can float to any voltage with respect to the source if left floating. If it happens to be above the threshold voltage the the mosfet will conduct.

John, I have just seen your post of a few minutes ago. I think you are mixing up the drain with the gate in your first sentence.

Les.

 

[dr pepper]

Maybe you blew the fet, they can go short drain source, temporarily wire the gate to the source, th ebul bshould go out.
The gate resistor as mentioned is essential.

 

[3eprabu]

Thank you for the help guys . I think I understand what u guys meant . I added a pull down resistor from the gate to ground and I'm able to controll the gate now .

My question now is , will this pull down resistor effect the PWM signal I'm going to supply to the gate ? I'm planning to build a light dimmer with the mosfet .

Thanks in advance .

 

[tomizett]

Well... that depends on what circuit you are going to use to control the gate, but the answer is probably "no, not very much".

Most gate control circuits (and all ICs designed for the purpose) will actively pull the gate down to turn the FET off nice and fast, so you wouldn't need a pull down resistor in that case. If you're intending to design your own circuit to drive the gate, you *could* just have an active circuit to pull up, and use the resistor to turn the FET off again *but only if* you're intending to use very slow PWM (like no more that a few kHz). At the switching frequencies usually used these days, you need to short the gate and source to discharge the gate capacitance in as little time as possible. A quicker turn off means less switching loss.

Generally, you might as well leave the gate-source pull down resistor in place, as it will give some protection of the driver circuit fails or becomes disconnected.

The only case where you might not want to do that is if you where using a "high side" switch (load connected to source) with a bootstrap capacitor - in that case a G-S resistor is simply wasting charge and limiting your maximum on time.

 

[audioguru]

A circuit that produces PWM pulses to control a Mosfet should already have something (an active transistor, not a simple and slow acting resistor) to drive its output low and another transistor to drive its output high.
You did not post your schematic so we do not know anything about your PWM circuit and we do not know if its output high voltage and current are enough to properly turn on your Mosfet.

 

[dr pepper]

The only thing I'd add in laymans terms the fet will run cooler if you use a driver ic, or circuit, for leds you probably wont need it, or make do with a heatsink.

If your driving the fet from a transistor, logic gate, or 555 or the like the gate pull down wont have any effect worth considering, so long as there isnt a highh value resistor in series with the gate, high being more than 1/100 the pull down.

 

[kinarfi]

In my experiences with the killing of many FETs, I have learned the pull down resistor is very important and if there is any possible kick back, inductive, diode is extremely important. I have settled on driving my FETs with a 555 and putting a diode across the load. If I remember to follow my own advice, I don't lose my FETs so often.

 

[3eprabu]

Thanks for the advice guys . appreciate it very much . Any idea how I can step down 250V DC to 10 to 15 V DC

 

[MikeMl]

Thanks for the advice guys . appreciate it very much . Any idea how I can step down 250V DC to 10 to 15 V DC

Dont.

Get a transformer and convert 240Vac to ~12Vac, followed by rectifier, filter, etc.
Or just use a ready-made plug-pack power-supply, either transformer-based or SMPS-based.

 

[Les Jones]

Hi Mike,
You did not notice that the 250 volts was DC so it rules out a transformer (50/60 hz) based supply. I see no reason why a normal switched mode power supply should not work as they convert the AC to DC before chopping it at some high frequency. The only thing that may cause a safety issue is the breaking ability of the fusible resistor that they normally have on the input with DC.

Les.

 

[dr pepper]

250vdc is very risky, unless your experienced with this kind of power start off with something safer and lower voltage.

 

[MikeMl]

Hi Mike,
You did not notice that the 250 volts was DC so it rules out a transformer (50/60 hz) based supply. .

Yes!

Where did the OP get the 250Vdc in the first place? I'll bet it is from Line voltage. If yes, then just get a second, low-voltage DC supply!