need help on Mosfet design suggestions

[vvenky88]

hi..
I have attached the schematic for n-channel mosfet..So please check this and tell me if anything wrong or any suggestions???

And can u please tell me any is there any formula for calculating the resistance values and flow of current and power calculation in the MOSFET???

 

[alexan_e]

To control a Pmosfet which has 110v connected in the drain you need a gate voltage about 10v lower ,
If your gate goes below 110v-20v(Max Vgs) the mosfet will be damaged.
There is now way to control the gate with a device connected to 12v, it that case the Vgs bias would be 110v-12=98v!!!

You can use a Nmosfet as hussain_kiet said.
If you connect it as a low side switch (providing gnd to the load) then it can easily work but the 110v would be connected to the load all the time and it would be like having a switch in the gnd side.

If you connect the Nmosfet as a high side switch (as it is now) you have to use an isolated power supply of 12v connected to the source (-) and then control the +12v side using the optocoupler to feed the gate.

Alex

---------- Post added at 09:48 ---------- Previous post was at 09:44 ----------

You can't gen 3A the way you have used the output, you have a 1K resistor in series with the relay and will not work.
You have to use the relay coin in place of the 1K drain resistor.

Alex

 

[hussain_kiet]

you can connect it like this


you can use your load directly if don't want to use relay....

 

[alexan_e]

I suppose that this is a home exercise because I can't imagine using a 110v DC power supply just to be able to control a 110v relay,
any 5 or 12v relay could be used.

Alex

 

[vvenky88]

hi hussian_kiet,
Yours circcuit is correct but in my project i have to drive 12 relays (which are available in the outside),i should provide two lines from my circuit, one is gnd from 110V supply and another one is 110V suppply through mosfet...So for this configuration how should i use it?????

 

[hussain_kiet]

so your actual load for the Mosfet is an external relay with 3A current...?
you can common one terminal of all the relays and tie it to +110V while the GND terminal will supplied to the through the Mosfets...
Don't forget to use freewheel diode across the relay coil..

 

[prmurthy]

Just put a thought on TRIAC Driver and TRIAC like BTA12 which supports AC and DC voltage and also supports upto 8A easily. easiest way to drive huge voltage and large amps through MCU unless you want give PWM output.

Thank you

 

[alexan_e]

This is why you should give an accurate description of what you want in your original question,
You keep adding requirements each time we suggest a solution.

Alex

 

[prmurthy]

This is why you should give an accurate description of what you want in your original question,
You keep adding requirements each time we suggest a solution.

Alex

I think it went in a different direction because of that "MOSFET". If vvenky88 has not mentioned about MOSFET I think probably the use of TRIAC would have been the first choice given by all members.

 

[hussain_kiet]

if the requirement is to just switch ON and OFF the 110V relays not high frequency switching or PWM which in this case i don't think it is (well up till now..)
why not use simple 5v/12v relays.....

 

[prmurthy]

if the requirement is to just switch ON and OFF the 110V relays not high frequency switching or PWM which in this case i don't think it is (well up till now..)
why not use simple 5v/12v relays.....

Yes more approtiate solution either 5V or 12V it just requires a transistor to drive the relays. 5V relay can be used for 120VAC @ 1A and 30VDC @2A where as 12V relay can be used for 240VAC @7A and 28VDC @10A or 125VAC @ 10A. So this is ideal solution and simplest.
Thank you

 

[vvenky88]

Yes more approtiate solution either 5V or 12V it just requires a transistor to drive the relays. 5V relay can be used for 120VAC @ 1A and 30VDC @2A where as 12V relay can be used for 240VAC @7A and 28VDC @10A or 125VAC @ 10A. So this is ideal solution and simplest.
Thank you

Ya...this is the simple and easiest way of designing...i knew that..anyway now i have four choices..
1. using simple 5V/12V relays
2. using P-channel mosfet
3. Using N-channel mosfet
4. Using triac
So now i have to choose it...

 

[prmurthy]

Ya...this is the simple and easiest way of designing...i knew that..anyway now i have four choices..
1. using simple 5V/12V relays
2. using P-channel mosfet
3. Using N-channel mosfet
4. Using triac
So now i have to choose it...

OK my comments corresponding to your options

1. If you have many outputs and the load are high voltage and high current. More prone to mechanical switches wear and tear due to frequent on/off.
2 & 3. you can use this if your loads are low voltage low current also resistance loads does not suitable for inductive loads.
4. Can be used if your loads more than 7A and also if you want silent working and can overcome of relay disadvantages of wear and tear.

I suggest No. 4 for you. But wait for others suggestion as well.

 

[vvenky88]

Hi prmurthy,
my requirement is 110V DC for driving relay with 3A loads..So will triac be ok for this???

 

[hussain_kiet]

well for 110V DC @ 3A load both triac and N MOSFET both are applicable but i am much comfortable with MOSFET's...

 

[prmurthy]

well for 110V DC @ 3A load both triac and N MOSFET both are applicable but i am much comfortable with MOSFET's...

Dear Friend

vvenky88 is using for motor and which can produce Back EMF so I think Triac is better than MOSFET. For resistive loads like heaters etc as you said MOSFET is good.

Thank you

 

[hussain_kiet]

I didn't say that mosfet is better... I said I am more comfortable with mosfet's....
well another option is IGBT but they might be expensive...

 

[alexan_e]

vvenky88 is using for motor

vvenkyl88 didn't say anything about a motor, unless he has another thread where he explains more.

Alex

 

[hussain_kiet]

he clearly said that he has to drive relays not motors....

hi hussian_kiet,
Yours circcuit is correct but in my project i have to drive 12 relays (which are available in the outside),i should provide two lines from my circuit, one is gnd from 110V supply and another one is 110V suppply through mosfet...So for this configuration how should i use it?????

 

[vvenky88]

dear all,
Now i got confused..So please tell which is better for driving relays???

Whether N-channel or P-channel MOSFET or Triac?????