Controlling A/C loads with transistors

Hello.

This isn't a question for a specific project, but I was wondering if it were possible to control AC voltages with a mosfet and saw that most of the time people use triacs for AC but that it was possible using two mosfets in series (or with one mosfet and a rectifier)

page with discussion/diagram

I had two questions

I see he said the circuit is not suitable for inductive loads. Are ballasts like hps or mh inductive?

Two mosfets seems simpler and cheaper (and without the associated hum of a coil) than a relay/contractor so why aren't they used a lot more often in lighting controls for buildings.

Are ballasts like hps or mh inductive?

Yes.

so why aren't they used a lot more often in lighting controls for buildings.

Because despite you saying:-

Two mosfets seems simpler and cheaper

they are not.

1.75 x 2 = 3.50

3.5 x 4 poles = $14

https://www.etundra.com/restaurant-parts/electrical-parts/contactors/commercial-hartland-110-120v-30-40a-4-pole-contactor/?scid=scplp11600166&sc_intid=441073&catargetid=320013490001158671&cadevice=m&gclid=CjwKEAjwqJ67BRCzzJ7Hy-LYlFYSJABwp9PGVJxVz9xcH_xEQUTAONAndE1jqTg8XEBD967tu6cHHRoCY_vw_wcB

$38 bucks.....

Plus the humming issue?

So, I think maybe they are cheaper.... They seem simpler. Could you elaborate?

And why aren't mosfets suitable for inductive loads.

(and without the associated hum of a coil) than a relay/contractor so why aren't they used a lot more often in lighting controls for buildings.

A coil isn't necessarily going to hum, and/or you can run a relay coil from DC.

I added a relay with a 12VAC coil to my home furnace because it didn't have a fan-only switch (the thermostat does have a "fan" switch.) I don't remember the cost, but it wasn't expensive and it was a super-easy project. With all of the other noise in there, I don't hear any relay hum coming from the furnace closet. :wink:

In a "building" you may find a relay/contactor or a solid state relay. You may not know what's inside a solid state relay (some use MOSFETs and some use TRIACs).

Electro-mechanical relays are electrically rugged... You can short-out the circuit (occasionally) and it might even survive a lightning strike. Solid state relays don't have any mechanical parts, so they can last forever if not abused, but in general they can be more easily burned-out by electrical overloads.

Since solid state relays are isolated (as are mechanical relays) there's a little more to the circuit than a TRIAC or a couple of MOSFETs. And with any significant power, they need to be heatsinked.

A mechanical relay is almost always cheaper unless you are driving it with something like an Arduino where you usually need a driver circuit for the coil. In that case a solid state relay may be cost competitive (and it may be a lot easier to wire-up if you don't have a relay board with the driver already built-in).

Qdeathstar:
And why aren't mosfets suitable for inductive loads.

Well, more correctly, not suitable for inductive loads without a suitable snubber circuit to
control the inductive spike. TRIACs don't switch off unless the current has already fallen to
(near) zero, so they don't have such an issue. Its only switch-off that is at risk, whatever the
device.

Thank you all. Would the mosfets get damaged or would the load stay on?

So, I think maybe they are cheaper....

Comparing a 30V FET with a mains 4 pole contactor is rather like comparing an abacus with a bowl of fruit.

those are $3.70

60a 600v

Quoting prices from some distributor is not indecitave of the real price of anything. They are way way cheaper than that.

Also it is not just the cost of the individual components but of all the other stuff that you have to put round it to make a product. It is also about the reliability and robustness of the finished product.

Grumpy_Mike:
Quoting prices from some distributor is not indecitave of the real price of anything. They are way way cheaper than that.

Also it is not just the cost of the individual components but of all the other stuff that you have to put round it to make a product. It is also about the reliability and robustness of the finished product.

I understand what you are saying. It is obvious that it is not cheaper or better, if it were they would be more popular. My question is why. On paper, it seems like they are.