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DRSSTC Build again

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colin heath

Sun Jun 07 2009, 12:45PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

thanks guys :) I was pleased the toroid worked also. it only broke out on brerakout point

Extreme Electronics

Sun Jun 07 2009, 06:32PM

Registered Member #74 Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99

Colin, It'l be great to see this coil in action. 28" Sounds A good start.

Re: Variac taking out breakers.
I had the same problem with an 8A variac, I put an inrush limiter in series with the mains in.

It now only trips 1 in every 5 times, I did consider upgrading my breakers, but you never know where you need to use the coil and/or variac or what breakers a venue have.

Derek

colin heath

Sun Jun 07 2009, 07:09PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Thanks for the tip Derek. I hadn't thought of using this method. I was off down some soft start thyristor circuit avenue:-O

Maybe a thyristor control for the future. I have some massive Thyristors crying out to become a solid state variac.

I am right in assuming it's some sort of varistor?

I will go and have a look

EDIT: ok I see they use thermistors but am struggling to find anything beefy enough. it's a 20a variac and is only 0.8 across the windings DC. I see RS do them upto 25ohm but only up to 3-5A if i series then resistance drops. Either need bigger thermistor or perhaps put a resistor in series then drop out with contactor and timer?

Extreme Electronics

Sun Jun 07 2009, 07:24PM

Registered Member #74 Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99

I am right in assuming it's some sort of varistor?

They are actually a thermister, the ones Ive played with start at about 100Ohms and go down to about 4 on load. The cheapest source for small ones is a PC power supply. Maplin used to do a 16A or bigger version http://www.maplin.co.uk/DiscontinuedModule.aspx?ModuleNo=44113, but they don't stock them any more. RS still do smaller versions. Not sure where else too look, I suspect the usual suspects of Farnel / Rapid.

Unfortunatly you can't parallel small ones up to increase the current capability as the hottest one takes all the power away from the others.

Otherwise its a Thyristor and a zero cross circuit.

Cheers

Derek

colin heath

Sun Jun 07 2009, 07:30PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Thanks Derek, it seems maplin still do the large ones you state online so will look into that:-)

I assume I could use a switched relay instead and put resistor in series before turn on and then once core magnetises then drop resistor out. How does that sound? I have some large wirewounds i could use and switch gear such as contactors and timers.

Cheers

Extreme Electronics

Sun Jun 07 2009, 09:20PM

Registered Member #74 Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99

The ideal way would be to turrn on the variac at zero cross. I'd probably have a try with a triac and a zero cross opto like the MOC3043 or similar.

Derek

colin heath

Mon Jun 08 2009, 08:50AM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

I had a look at data sheet for those and look a nifty little chip :) I am going for a real simple just to get me running then will look at something decent such as the zero crossing a bit further down the line.

Hoping to get the resistor/ timer setup done this week then may get a little time to play the weekend. if not will be a couple of weeks time.

colin heath

Thu Jun 11 2009, 08:23AM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

OK I have taken Dereks advice on board (pardon the pun lol) and decided to do it properly with a zero crossing triac driver and triac for turn on.

I have some 600v 26A triacs on the way along with MOC3020 which maplin had in stock. Just need to figure out the trigger which shoul dbe simple as just need a low voltage supply on tx side when I power up. Didnt really want to use trnasformer for such a small supply so looking how else to do it.

Hopefully the turn on at zero will solve if not will add in a PWM chip and soft start the variac. I may even get chance to breadboard this weekend and try:-)

I also ordered some rs232 interface chips but thats a whole different thread;-)

Extreme Electronics

Thu Jun 11 2009, 09:51AM

Registered Member #74 Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99

I have some 600v 26A triacs on the way along with MOC3020 which maplin had in stock

The MOC3020 is not a zero crossing opto, so you will need some ZC triggering system to give you zero crossing turn on of your variac.
MOC3020 PDF

But if you are building a phase controller they are ideal. Just remember that you need to control the turn on delay (from zero cross), not the on time for phase control to work with a triac.

Derek

Wolfram

Thu Jun 11 2009, 03:08PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Zero crossing is actually the worst time to turn on a transformer, contrary to common belief.

Anders M.

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