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MOSFET for high voltage and high power


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i am looking for a mosfet or any transistor able to drive about 1A

the idea is to use it powering a lamp does anyone knows any ?

and does anyone knows if there is anyware a table or somethink for transistors ,moseft,darliongton bipolar etc with specifications ? somethink like eca books ?

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Hi Plouf,
Welcome to our forum.
Is your lamp's power supply AC or DC?
Is the lamp an incandescent (with hot filiament) type or flourescent?

An incandescent bulb that draws 1A when hot will draw 10A or more when cool so a transistor rated at 1A will go

A Mosfet or bipolar transistor is used in DC circuits, AC lamp dimmers use a Triac. Many flourescent lamps cannot be dimmed.

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i was thinking about that in for lamp...
and since you confirm it i will go for "normal" ones

the idea is to use a diode bridge so no AC

the project i am working on supposed to dimm normal lamps so
an absolute maximun i guess 80 or 100 watt at 220v or less = about 0.5A if i am not wrong

i have think on connect it in series with lamp and via its gate
control the light

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Hi Plouf,
Please post your dimming circuit for us to see if it has pulse-width-modulation that is sync'd to the DC pulses of the diode bridge. Without syncronization, the light will flicker. You can avoid flickering if you filter the DC pulses with a huge capacitor, but then the DC voltage will be too high.
It might be better to use a standard AC dimmer circuit that uses a triac.
Before selecting parts ratings, how many volts is "220v or less"?

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Hi Plouf,
Thanks for posting your circuit.
1) Of course it has pulses. The output of the rectifier bridge has half-wave pulses of 309VDC peak at 100Hz. Since the pulses aren't filtered, the average voltage at full brightness will be 218V.
2) The transistor in the optocoupler and the gate of the Mosfet will receive 309V pulses too, way beyond their maximum voltage ratings.
3) The transistor in the optocoupler will turn-on the Mosfet, but your circuit doesn't have anything to turn it off.
4) If the input to the optocoupler is a DC current, then the Mosfet will heat almost as much as the light bulb, a lot!

That's why I recommend using an ordinary AC light dimmer circuit using a triac. The triac switches fully on and off so it doesn't heat much even with its maximum load of about 600W, at double the mains frequency so it doesn't flicker. Usually an ordinary AC light dimmer is controlled by a pot, but if you can find a light-dependent-resistor that can withstand 220VAC (maybe not) then you can assemble your own optocoupler.

If having an optocoupler is important to your project, an opto-coupled triac driver is available, but its input must be PWM pulses that are syncronized to the mains frequency. A very complicated circuit.

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thanks for your notes

1) yes it has pulses its not alterante i miunderstood it last time
(my english are bad)

2) that i was asking from the first time . a mosfet inside thiese specs

3) its not the optocoupler i use and there is offcourse more circuit for led . its a AVR with dac and transistor controlling led

that reason 1 i wand to avoid sine generator (complicated)

4) hmmm another idea what about two SCR reverced intead of mosfet and no diode bridge , so half of period (positive) will heat SCR 1 ,negative period of pulse SCR 2 ?

better one ;)

any SCR with load of 1-2A in its specifications

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Usge this circuit for the dimmer.

Replace the varaible resistor with an LDR.

Put the LDR in a dark box (a film case will do) with a green LED, vary the brightnes of the LED with a vairiable resistor to control the brightness of the lamp.

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Hi Plouf,
1) Good, you understand about DC pulses.
2) Mosfets are not made with a gate that can withstand 309V. Also a Mosfet will get too hot if you use it like a linear amplifier.
3) I have never seen a high-voltage photodiode, phototransistor or LDR.

Pulse-Width-Modulation does not make a sine generator. It controls power with digital duty-cycle changes.

4) A triac works the same as two reversed SCRs but a triac costs less.
5) A single SCR and a rectifier bridge has the same high voltage requirement for its opto-sensor, and its input pulses must be syncronized to the mains frequency.

Hi Alun,
I searched Google and found LDRs with voltage ratings of only 200V max. Maybe two identical LDRs can be wired in series to withstand 309V.

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Hi Alun,
I was going to say, "PERFECT!", but realised that its 320V rating is awfully close to the peak voltage of 220VAC (I have 120VAC over here).
I wonder what the LDR does during over-voltage breakdown. When your 'fridge turns off and makes a voltage spike does it get destroyed or does it just conduct for a moment and make the light blink?

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i found these

i i understand correct it can accept up to 4.5 Amps and 500Volts

and i make this quickly on proteus simulator i think it works OK

i.e max voltage on mosfet (on lamp off) = about 305V and with a lamp with a 450 ohm resistane needs 0.48 amps
( its a 100Watt lamp correct ?)

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Hi Plouf,
I don't know why you want to use a Mosfet in this AC circuit instead of a Triac. A big disadvantage of using a Mosfet is that it will get hot and will need cooling. A Triac switches and won't need much cooling.

Your sim shows the Mosfet turned on hard and dissipating 3.38V X 0.48A = 1.6W. Not much heat. But as usual I disagree with your stupid sim program:
1) Figure 3 of the Mosfet's spec's shows a typical Vgs of 4.6V with a 0.5A current. Maybe the stupid sim is thinking about the Mosfet's Vgs
threshold voltage of about 3V with almost no current flow.
2) Are you going to melt a bunch of Mosfets before you find one with a "typical" Vgs? Many of them will have a worst-case Vgs of who knows how high. Since the threshold voltage spec allows for a 2V to 4V range, maybe at 0.48A the max Vgs will be 6.1V. Then the Mosfet will dissipate 3W and need a heatsink.
3) Are you going to dim the light without using PWM? Assuming that the resistance of the lightbulb stays the same at 450 ohms, if you dim to half-voltage the lamp and Mosfet will each have 110V across them. Therefore the current is 0.24A and the lamp and Mosfet dissipate 26.9W, a lot of heat for a Mosfet.
But wait! The lightbulb is a PTC resistor so its resistance is much less when cooler. Therefore the Mosfet will dissipate maybe 40W!

A Triac would dissipate about 1/2W for any dimming amount with only a 100W load. ;D

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