datanaut
Posts: 11
Joined: Mon Jan 21, 2013 10:05 pm
Location: London W9

240v 500ma MOSFET questions.

Fri Feb 01, 2013 2:02 am

I want to switch a light 240 volts up to 100 watts.
Image
I have used relays like this up till now but the size of the relay is now becoming an issue and I only have a 10mm x 20mm x 25mm space. I have searched around and it looks like a MOSFET is the way to go and I have found the P7NK80ZFP that with switch 800V at 5.2A, way more than I think need. I've looked at the datasheet and I'm lost. I've learnt a bit about transistor switching form places like this one HERE and think I can work it out. One problem is everything I have found thinks 24v is a lot of volts!

So a some of questions:
1) Is this even feasible.
2) Is the P7NK80ZFP the best MOSFET for this task and will it switch of the GPIO directly.
3) How hot might the MOSFET get if I run .5 amps through it.
4) Is a Darlington Pair a better option, and if so which one.
5) Can you point me to any projects already doing this.
6) Are there any unknown, unknowns, I should know.

I'm quite pleased with what I have learnt so far but a little knowledge can be a dangerous thing (especially with 240 volts).

Thanks........

gritz
Posts: 449
Joined: Sat Jan 28, 2012 2:33 am

Re: 240v 500ma MOSFET questions.

Fri Feb 01, 2013 2:43 am

I just typed a measured and lengthy reply, but the forum ate it and presented me with a 1040 error... :x

Find more room and use a mechanical relay. Seriously. We don't want to lose anyone here.

Mains is alternating current, so it needs a device that can control current in two directions. A single transistor (either bipolar or fet) won't do.

You'll struggle to squeeze anything meaningful into that space, but if you're curious have a google for "triac" and "solid state relay" and do some research on power dissipation and stuff. I wouldn't advise using anything other than a professionally made complete module of a suitable rating that just needs connecting up though. And I wouldn't advise even doing that without supervision from someone wot know about these things.

skiddaw
Posts: 9
Joined: Sat Oct 29, 2011 6:19 pm

Re: 240v 500ma MOSFET questions.

Fri Feb 01, 2013 3:40 am

I've often salvaged RELAYS out of old microwave ovens. As an example the OMRON G4W-2212P-FD-US-TV5 has a 12VDC coil and the contacts are rated 10A 250VAC/24VDC.
They are also a good source for micro-switches(3) and large magnets(2)

The size is approx: 30mm x 18mm x 30mm (1.125 x 0.75 x 1.125)

I use one to control the router on my CNC unit.

datanaut
Posts: 11
Joined: Mon Jan 21, 2013 10:05 pm
Location: London W9

Re: 240v 500ma MOSFET questions.

Fri Feb 01, 2013 6:09 pm

gritz wrote:I just typed a measured and lengthy reply, but the forum ate it and presented me with a 1040 error... :x

Find more room and use a mechanical relay. Seriously. We don't want to lose anyone here.

Mains is alternating current, so it needs a device that can control current in two directions. A single transistor (either bipolar or fet) won't do.

You'll struggle to squeeze anything meaningful into that space, but if you're curious have a google for "triac" and "solid state relay" and do some research on power dissipation and stuff. I wouldn't advise using anything other than a professionally made complete module of a suitable rating that just needs connecting up though. And I wouldn't advise even doing that without supervision from someone wot know about these things.
So glad I didn't press ahead regardless. It never occurred to me, but more surprisingly the electronics shop that I discussed my project with while buying the mosfet, that you can't use a transistor/mosfet with an AC current.

I'll look into the Triac thing and solid state relays. Thanks for saving me from the smoke and BANG!

datanaut
Posts: 11
Joined: Mon Jan 21, 2013 10:05 pm
Location: London W9

Re: 240v 500ma MOSFET questions.

Fri Feb 01, 2013 6:12 pm

skiddaw wrote:I've often salvaged RELAYS out of old microwave ovens. As an example the OMRON G4W-2212P-FD-US-TV5 has a 12VDC coil and the contacts are rated 10A 250VAC/24VDC.
They are also a good source for micro-switches(3) and large magnets(2)

The size is approx: 30mm x 18mm x 30mm (1.125 x 0.75 x 1.125)

I use one to control the router on my CNC unit.
I'll look again for miniature relays and keep my eyes open for old microwaves. Thanks :-)

simonpb1
Posts: 25
Joined: Thu Sep 13, 2012 11:25 am

Re: 240v 500ma MOSFET questions.

Fri Feb 01, 2013 6:19 pm

I once made some disco lights using a good old pic processor. There are some IC's that provide optical decoupling between the pi and the mains side. Here is one http://www.panasonic-electric-works.es/ ... an_030.pdf

You still need a triac to provide the current, and these devices also provide zero crossing detection so the Triac only fires when the AC signal passes through 0v so you do not get any Interference from it.

Again you need to know what you are doing with these things.

It's an option that I would use..

datanaut
Posts: 11
Joined: Mon Jan 21, 2013 10:05 pm
Location: London W9

Re: 240v 500ma MOSFET questions.

Sat Feb 02, 2013 3:43 pm

simonpb1 wrote:I once made some disco lights using a good old pic processor. There are some IC's that provide optical decoupling between the pi and the mains side. Here is one http://www.panasonic-electric-works.es/ ... an_030.pdf

You still need a triac to provide the current, and these devices also provide zero crossing detection so the Triac only fires when the AC signal passes through 0v so you do not get any Interference from it.

Again you need to know what you are doing with these things.

It's an option that I would use..
Liked the link and it's sent me off to look for a chip that will do the switching and some "learn electronics for beginners" sites. Thanks

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