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Author Topic: Mosfet controlling ac power  (Read 2931 times)

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Offline JdogTopic starter

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Mosfet controlling ac power
« on: April 12, 2009, 06:01:24 PM »
I want to be able to control 120 volt AC power coming from a wall socket. Could I use a mosfet for this application, and if so, what mosfet do you recomend. I'm drawing no more than 5 amps. Would this work?
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=SPB21N50C3INTR-ND

EDIT:
I also would like to do this with a low voltage low current signal going to the gate. Or is there such a thing as a photo-sensitive MOSFET that I could just encapsulate with an LED in a chamber so that when the low voltage low current signal powers the led, that switches the MOSFET, like the ssr on this site http://zone.ni.com/devzone/cda/tut/p/id/2774. I looked for assembled SSRs but they are expensive, and I need this to be very cheap ($1-$2)
« Last Edit: April 12, 2009, 08:30:44 PM by Jdog »

Offline HDL_CinC_Dragon

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Re: Mosfet controlling ac power
« Reply #1 on: April 12, 2009, 08:27:41 PM »
That device wont work because the maximum rated power is 208W which means using 120V you would be able to use a maximum of 1.73A.
I recommend using an actual relay for that sort of thing.

Also: That part you linked to requires you to order a minimum of 1,000 per order and I doubt you want to pay $1663 for 1000 of these
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Offline JdogTopic starter

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Re: Mosfet controlling ac power
« Reply #2 on: April 12, 2009, 08:31:54 PM »
That device wont work because the maximum rated power is 208W which means using 120V you would be able to use a maximum of 1.73A.
I recommend using an actual relay for that sort of thing.

Also: That part you linked to requires you to order a minimum of 1,000 per order and I doubt you want to pay $1663 for 1000 of these

The only problem is it needs to be driven by a low current low voltage signal.

Offline Razor Concepts

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Re: Mosfet controlling ac power
« Reply #3 on: April 12, 2009, 08:40:19 PM »
There are some cheap ($8) shipped SSRs on eBay, rated at 25a with a 3-24v dc input. I ordered one and will see if they are of good quality or not.

Offline HDL_CinC_Dragon

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Re: Mosfet controlling ac power
« Reply #4 on: April 12, 2009, 08:43:54 PM »
I have 3 Solid State Relays:
Input Side - Load Side
3-32vdc - 24-280vac @ 25A
90-280vac - 48-660vac @ 25A (2x)
theyre good stuff
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Offline JdogTopic starter

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Re: Mosfet controlling ac power
« Reply #5 on: April 12, 2009, 08:46:42 PM »
I have 3 Solid State Relays:
Input Side - Load Side
3-32vdc - 24-280vac @ 25A
90-280vac - 48-660vac @ 25A (2x)
theyre good stuff
But how expensive are they? I hope to use it on a commercial application, and I need it to be very very cheap. Also, how much current does the input side need? Also, looking at how they work, they seem like they would be easy enough to build, I just need to find photo-sensitive MOSFETs which I'm imagining are cheaper than an assembled ssr.
« Last Edit: April 12, 2009, 08:50:49 PM by Jdog »

Offline Razor Concepts

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Re: Mosfet controlling ac power
« Reply #6 on: April 12, 2009, 08:51:23 PM »
Dragon - the 3-32vdc input one, does it look like this?

That is what I ordered, cost me $8 shipped so a pretty good deal if it works well.

Offline Soeren

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Re: Mosfet controlling ac power
« Reply #7 on: April 12, 2009, 08:55:47 PM »
Hi,

The only problem is it needs to be driven by a low current low voltage signal.
That's not the only problem, it's the least of the problems!

It's nominal 208W with a pristine cooling arrangement that's only available in a research facility (think liquid Helium).
For common use, a good rule of thumb is 1/3 of it's nominal max. power i.e. 70W absolute max. and that still needs a hefty cooling arrangement.
With a current draw of 5A, it means it can be used down to 14W under the max. rating of the mains voltage and up.
120VAC + 10% is 132V, so the possible area would be 118V to 132V (@5A).

But the real problem here is IMO, that if you have to ask about something, you must be inexperienced in that field, which is a very bad thing when you want to play around with mains voltage.

If you don't really really really need the noise free dimming that this approach can give, go with a phase angle control instead.

And remember to allways keep (at least) one hand in your pocket when working with mains.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Razor Concepts

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Re: Mosfet controlling ac power
« Reply #8 on: April 12, 2009, 09:03:38 PM »
Looking further into SSRs...

$4 1 amp SSR
Build your own SSR
« Last Edit: April 12, 2009, 09:05:48 PM by Razor Concepts »

Offline JdogTopic starter

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Re: Mosfet controlling ac power
« Reply #9 on: April 12, 2009, 09:07:23 PM »

Offline SmAsH

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Howdy

Offline JdogTopic starter

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Re: Mosfet controlling ac power
« Reply #11 on: April 12, 2009, 09:27:58 PM »
I found a $1.50 one that I might go with, I realized I only need .7 amps. This is it:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=425-2363-5-ND

Just one thing, according to the datasheet http://document.sharpsma.com/files/pr29mf11_e.pdf
Would I connect the positive dc to pins 1, 2, or 4 the negative dc to pin 3 and one of the ac lines to pin 6 and the other to pin 8?

Offline Razor Concepts

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Re: Mosfet controlling ac power
« Reply #12 on: April 12, 2009, 09:35:02 PM »
Bottom of page 10 in the datasheet shows standard circuit.

Also if you need .7 amps I would not use a relay rated for .9 amps, .9 amps is the absolute maximum. Running at .7 may take some extra cooling.

Offline JdogTopic starter

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Re: Mosfet controlling ac power
« Reply #13 on: April 12, 2009, 10:27:16 PM »
Bottom of page 10 in the datasheet shows standard circuit.

Also if you need .7 amps I would not use a relay rated for .9 amps, .9 amps is the absolute maximum. Running at .7 may take some extra cooling.

Should I go for something like this? http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=425-1206-5-ND
Or should I get one rated for higher voltages than 120 because I'm using it for 120 volts.

Offline TrickyNekro

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Re: Mosfet controlling ac power
« Reply #14 on: April 12, 2009, 10:35:12 PM »
What about a TRIAC instead of all these..... :-\
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline JdogTopic starter

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Re: Mosfet controlling ac power
« Reply #15 on: April 13, 2009, 04:10:05 PM »
What about a TRIAC instead of all these..... :-\
I didn't know what thy are so I just read up on them and I have one question. For it to turn off, does the electricity have to drop below the threshold current at the gate, or is it pertaining to the current flowing through the TRIAC?

Offline JdogTopic starter

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Re: Mosfet controlling ac power
« Reply #16 on: April 16, 2009, 11:22:04 PM »
Anybody?

Offline Razor Concepts

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Re: Mosfet controlling ac power
« Reply #17 on: April 17, 2009, 06:12:23 AM »
Just use a SSR, you won't need a lot of external circuitry. Some SSRs, depending on the type, may even have triacs built in to them.

Offline TrickyNekro

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Re: Mosfet controlling ac power
« Reply #18 on: April 17, 2009, 12:59:17 PM »
I truly lack any confidence with TRIACs...
I just know them, know what they do and that they exist...
Maybe some google will save you from much trouble....
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

 


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