AC power control.

I'm considering a general purpose 'dimmer'. (more on-off ATM, but the functionality could be added later).

The power control device would be a 400V FET (that I happen to have a bag of that a pick and place machine ate half of the drain wire) inside a bridge rectifier on the live side.

A FET, due to the inherent current limit, which is nice as it's coming from a UPS that might otherwise trip on a fault when on battery.

Of course, isolated by a couple of optoisolators, with power coming from a small cap to neutral. Turned on by one opto, and the other indicates an overcurrent has happened, and the FET is off.

Assuming I can turn the FET on fast enough and off fast enough that it never gets hot, even if shorted, and I have an overvoltage device across the FET to catch spikes, and adequate snubbing across the bridge to stop the diodes being destroyed, is there anything I'm missing?

Reply to
Ian Stirling
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Consider syncronizing the on/off changes with the 60Hz power cycle. Each cycle (or half cycle), decide if it's going to be on or off, and set it during the zero crossing.

Reply to
DJ Delorie

"DJ Delorie"

** The OP indicated he need a "dimmer".

Zero crossing switching CANNOT be used for lamp dimming as it creates a VERY noticeable flicker in the light.

Lamp dimmers use "phase control" or else chop the AC wave at high frequency to vary the effective output while maintaining a sine wave voltage.

........ Phil

Reply to
Phil Allison

Even though you have a bag of free fets given the extra bits you need to drive it you would do better to use a triac like everyone else but dont let me stop you inovating.

Reply to
cbarn24050

It will be zero crossing initially, using it as a dimmer is something that might be done in the future.

Reply to
Ian Stirling

"Ian Stirling"

** Why bother at all ??

Zero crossing causes massive inrush surges with all transformer loads as well.

You have not got the bloody faintest - have you ?

......... Phil

Reply to
Phil Allison

The OP never said he wanted a lamp dimmer. He just said a "dimmer," which could be attached to a heater. If you want to correct others (in simple things like this), at least make sure you are not assuming anything.

Reply to
Mochuelo

"Mochuelo" = another posturing f****it

** Dimmer = lamp dimmer.

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** No way.

That is a heat controller or thermostat.

** You need to take you own pompous advice.

Plus stop being a f*****ad anytime.

......... Phil

Reply to
Phil Allison

It will be used as a zero crossing switch of course f****it.

Zero crossing dimming of transformer based loads can of course be done, but you have to use either a whole cycle, or get into the nasty buisness of modelling the flux, which is rarely going to work well.

Reply to
Ian Stirling

He also said "more on-off ATM" and worried about switching heat.

If you switch "off" at a zero crossing, and "on" partway through the phase, you can avoid that too, but then you're back to the switching heat issue.

But a 120Hz flicker won't be that noticeable. A 60Hz flicker is produced by your TV all the time, people don't seem to complain.

Yes, I know. And people use triacs instead of FETs, and lots of other normal things. He's trying something different, so I suggested he

*consider* it. It was just an idea.
Reply to
DJ Delorie

You are right :-)

Reply to
Mochuelo

The 2 are somewhat different though. TV light output is not a 60Hz sine wave, the screen is always putting out light, but the distribution across the screen is what is varying. This has a very different effec ton the eye/brain: eg compare a single L ballasted fl light with a twin fitting with one tube on L ballast, one on C. Both will produce mains frequency flicker, but the difference to the eye is substantial.

FWLIW you can chop high current incandescent lamps at low speed, as the thick wire is slow to change temp. But thats probably not whats wanted here.

Really though I'm not sure if there is a universal solution, since different loads like different control methods. Some loads are happy with triac lamp dimmers, some with variacs, some with zero switching, some with series L or R, some need full power on then reduced power, and so on. Not sure how you'll get anything universal unless you're willing to have a switch for which control method it uses. Building a set of each type would be easier - buying some would be easier still.

NT

Reply to
meow2222

"DJ Delorie"

** The OP indicated he need a "dimmer".

He never mentioned "switching heat" ??

** Huh ?

That is phase control !!!!

** Huh ?

You delusional as well ?

** Huh - you on drugs ?

** I strongly suggest he FORGETS any TRIPE that comes from you.

......... Phil

Reply to
Phil Allison

"Ian Stirling"

** So is it a religious notion of yours that "zero crossing is always best " ??

What faith is that part of then ??

Scientology ??

** Fools do lots of silly things too.
** Be in MASSIVE trouble if you feed half cyles (or odd numbered chains of half cycles) into a transformer.

Something called a " DC component" gets in the way.

** You have not got the bloody faintest - have you ?

Piss off.

......... Phil

Reply to
Phil Allison

** Phase control ( with trailing edge) will work with nearly all AC power loads.
** What puke.

** Just use trailing edge phase control.

Easy enough with a MOSFET as the switch.

........... Phil

Reply to
Phil Allison

Why else would he care about switching the FET fast enough? If you switch too slowly, the resistance of the partially activated FET could generate lots of heat.

Duh. But if he can manage to switch the FET fast enough only on one edge, synchronizing the other edge with the line phase may help. Or if both edges are slow, syncing one edge might reduce the stress to the device.

Are you looking at your computer monitor right now? Yes? Good. What's its refresh rate? Is it 120Hz or faster? No? Didn't think so. Can you see the flicker? Didn't think so (or you're too stupid to turn the refresh up to 75Hz or so).

Recall that old fluorescent lights had a 120hz flicker all the time, and that TVs have a 60Hz flicker all the time. Since he didn't say what his target device was, we can't know what a 120Hz duty cycle would result in. I can't, you can't. At least I tried to help.

Oh, and a standard triac-based dimmer set to half results in a 120Hz flicker too. Nobody seems to notice in all those houses around the world that are using those low cost dimmers on their lights.

But he should listen to your overzealously punctuated zippy phrases? "Oh, please, nobody listen to the guy who's giving you ideas to think about, just do what the rest of us sheep are doing."

Reply to
DJ Delorie

"DJ Delorie" = another, know nothing idiot

** To save it from a dead short load, he said.
** Why did you snip the context ?

The issue was "zero crossing" power control causing serious lamp flicker.

** Why did you snip the context ?

The issue was "zero crossing" power control causing serious lamp flicker.

** Wrong context - imbecile.

The issue was "zero crossing" power control causing serious lamp flicker.

** Got any idea what "zero crossing " AC power control is?

Got any idea why it makes lamps flicker badly?

Then post them.

....... Phil

Reply to
Phil Allison

Phil, please place both hands on your gluteus maximii and push very hard.

--
JosephKK
Reply to
Joseph2k

Will you just calm down?

Of course it's not. Initially these will be used on-off, with zero crossing switching.

Reasonability.

if you're just going for on-off, then not zero crossing in the face of possibly capacitive loads is rather silly.

That's why I said "whole cycle". Whole is not half.

Of course it does, if you supply one.

Reply to
Ian Stirling

"Ian Stirling"

( snip more utter s**te than a Koala can Bear )

** Dear Ian,

You are one, vile, illiterate, know nothing, over snipping, context shifting congenital f****ng liar.

And those are your best points.

Go to hell.

........ love, Phil

Reply to
Phil Allison

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