# replace relay with Mosfet transistor

#### mah

Joined Mar 15, 2010
337
i want to replace a relay to switch Ac load that will draw about 20 Amps . i used IRFP250 and bridge rectifier as in the picture . i still didn't simulate or do it . the voltage gate come from a plc (programmable logic controller) 24 Volt positive or negative .
will this circuit need any thing else ? will the mosfet need heat sink to act as a switch ?
if i replaced all of this with a triac will it work?

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#### tcmtech

Joined Nov 4, 2013
2,867
I would suggest buying a off the shelf solid state relay and do it the right way.

Joined Jul 18, 2013
20,405
You appear to be switching an AC induction motor from DC?
Usually no can do!
Use an off the shelf Opto, such as Opto22 type.
Or Triac if needed, the Opto22 gives isolation.
Max.

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#### mah

Joined Mar 15, 2010
337
i don't want to use SSR because i need some thing small and cheaper . what is thee problem with this circuit?

#### bertus

Joined Apr 5, 2008
20,446
Hello,

In your circuit the AC motor will get pulsating DC.
Is that what you want?

Bertus

Joined Jul 18, 2013
20,405
i don't want to use SSR because i need some thing small and cheaper . what is thee problem with this circuit?
You show AC motor, a simple 1ph induction motor does not run on DC.
If a Universal motor, it will run on AC/DC.
A triac circuit should power it off AC.
Max.

#### mah

Joined Mar 15, 2010
337
You show AC motor, a simple 1ph induction motor does not run on DC.
If a Universal motor, it will run on AC/DC.
A triac circuit should power it off AC.
Max.
i would use it to turn on ac heater not specifically motor but i didn't find a heater in proteus if this is the problem.

Joined Jul 18, 2013
20,405
Ensure the Heater consists of an element only.
I would still tend to use a Triac or SSR.
Max.

mah

#### mah

Joined Mar 15, 2010
337
would you recommend a triac and the gate voltage will be +-24 v ? will it be a simple triac connected in series with the heater ? . i want the triac gate to operate on negative or positive voltage from the plc .

Last edited:

Joined Jul 18, 2013
20,405
. i want the triac gate to operate on negative or positive voltage from the plc .
What kind of output is that on a PLC?
Max.

#### mah

Joined Mar 15, 2010
337
it is 24 volt dc .the output differs related to programming so i want the circuit to be flexible with both positive and negative 24 v if required. positive and negative won't happen at the same time so i think i will need invert gate to operate the triac in both directions

#### tcmtech

Joined Nov 4, 2013
2,867
i don't want to use SSR because i need some thing small and cheaper . what is thee problem with this circuit?
But thats what you are making and a poor one at that.

Your IRFP250 is only rated for 200 volts and your schematic shows it running from a 220 VAC power source of which already has a peak voltage of around ~320 volts.
On top of that the device has a 30 amp peak rating of which most any DC motor that takes 20+ amps at 310 volts is likely to have a huge startup current way way past 30 amps.

Cost and size wise an basic SSR that will exceed your current schematics capacities is a round. $5.37 including shipping and Smaller than a deck of cards. http://www.ebay.com/itm/250V3-32VDCNEW-Solid-State-Relay-SSR-25DA-25A-250V-3-32VDC-KA-/261623363074?pt=LH_DefaultDomain_0&hash=item3ce9f7b602$1.02 more ($6.39) gets you a 50 amp unit from china. http://www.ebay.com/itm/1-Solid-State-Relay-SSR-50-DA-50A-250VAC-Input-3-32VDC-Output-Volatge-24-380VAC-/251509390370?pt=LH_DefaultDomain_0&hash=item3a8f20bc22 it is 24 volt dc .the output differs related to programming so i want the circuit to be flexible with both positive and negative 24 v if required. positive and negative won't happen at the same time so i think i will need invert gate to operate the triac in both directions Any random turn on SSR can do phase angle shift voltage/power control just by varying which part of each half of the cycle it turns on at. Highly effective and very reliable. Last edited: Thread Starter #### mah Joined Mar 15, 2010 337 But thats what you are making and a poor one at that. Cost and size wise.$5.37 including shipping and Smaller than a deck of cards.

http://www.ebay.com/itm/250V3-32VDCNEW-Solid-State-Relay-SSR-25DA-25A-250V-3-32VDC-KA-/261623363074?pt=LH_DefaultDomain_0&hash=item3ce9f7b602

Any random turn on SSR can do phase angle shift voltage/power control just by varying which part of each half of the cycle it turns on at. Highly effective and very reliable.
but this SSR working on just positive 3-32 v but i need it to be flexible with also negative input .

#### tcmtech

Joined Nov 4, 2013
2,867
but this SSR working on just positive 3-32 v but i need it to be flexible with also negative input .
Umm?????????????? What???????????

I am backing away now.

Joined Jul 18, 2013
20,405
would you recommend a triac and the gate voltage will be +-24 v ? will it be a simple triac connected in series with the heater ? . i want the triac gate to operate on negative or positive voltage from the plc .
I don't get it?
If it is a heater element, why do you need bi-directional current control ?
Also what is the exact nature of the PLC output that has a +- output?
Max.

#### mah

Joined Mar 15, 2010
337
the heater is working on AC so if i used just half cycle it will work with half power but i need full power. the plc output differs from plc to another, some plcs output is positive another type is negative i want the circuit to work on both of them.
but what triac gate will operate on 24 volt?!

Last edited:

Joined Jul 18, 2013
20,405
but what triac gate will operate on 24 volt?!
A triac gate is traditionally fed off of the 110/240 AC as long as you use the appropriate gate resistor?
Max.

#### mah

Joined Mar 15, 2010
337
ok , recommend a triac for this application , i will connect the ac source in series wirh bridge rectifier and optoisolator on the gate of triac as in the last picture . so the gate will work on +ve and -ve cycles but the plc will control this on/off operation.

#### tcmtech

Joined Nov 4, 2013
2,867
Anyone else thinking absolutely none of this adds up from one end to the other?

First it's a motor then it's a heater but a solid state relay is too expensive although they proposed cheap circuit is a solid state relay anyway but will not work for any of the parameters listed on the schematic. Next the PLC's are both power source and power sink type and the ......... Heck I give up.