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Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Hi guys. I normally use a 2kW variac for bringing the DC cap up to full charge and then run the coil through the variac. I am looking to remove the variac from the equation and just run it on 120v mains. Problem is the initial current draw is so sudden and high when turning on power that it trips the breaker (>20A). So I want to bring the voltage up slowly until the DC caps are charged and then switch over to the main line before operating the coil. Rather than try re-invent the wheel, was wondering if anyone has come up with a suitable method?
It's a portable demo coil, and I am trying to keep the control console small and portable too, hence the need to get rid of the heavy and bulky variac.
Hi guys. I normally use a 2kW variac for bringing the DC cap up to full charge and then run the coil through the variac. I am looking to remove the variac from the equation and just run it on 120v mains. Problem is the initial current draw is so sudden and high when turning on power that it trips the breaker (>20A). So I want to bring the voltage up slowly until the DC caps are charged and then switch over to the main line before operating the coil. Rather than try re-invent the wheel, was wondering if anyone has come up with a suitable method?
It's a portable demo coil, and I am trying to keep the control console small and portable too, hence the need to get rid of the heavy and bulky variac.
Yep one easy way is to use a pre-charge resistor as dextor suggested.
In fact, this was the very reason why I developed this board: which performs both pre-charge and auto discharge, and adds some additional features like very basic in-built EMI filtering and over-temp cut-out.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
if you want to replicate the voltage control capabilities of the variac without the weight, then you could build a fairly simple phase angle controlled rectifier with a couple of SCRs and a control circuit. Nice and small, just replaces your existing rectifier.
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
Hi Graham,
If I may add.. I use capacitor (reactance) to charge capacitors with no problems..
From one of the AC leg mains, I do series a capacitor 10-20 uf 600v non-polar (PP) capacitor. (before, I used this capacitor as a bypass capacitor in my bridge)
then a relay will bypass it when voltage is high enough to not pull such current that trips the breaker.
I used this method to charge 6000 uf @ 630vdc voltage doubler - filter cap of my medium sized drsstc) and it also works well with my 20000 uf QCW drsstc.. before, I did use resistors too but I don't like the way it heats up.. :)
You can also try series a incandescent lamp in one of the two AC legs.. I haven't tried this, but theoretically, it will lit bright at first (since the capacitor is acting as short when at ~0v) then slowly dim as the impedance of the capacitor getting bigger - at the dimmest, it indicates the capacitor is already not consuming current/ then it is charged.
of course let's don't forget the rectifier after the charger before entering the lytics. Thanks!
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Thanks Julian, I had thought about the incandescent light too - also a good visual indicator. Had not considered the capacitor option though. I like that too. I have some lying around and will give that a try.
Thanks Julian, I had thought about the incandescent light too - also a good visual indicator. Had not considered the capacitor option though. I like that too. I have some lying around and will give that a try.
The lightbulb is a good resistor and provides a great way to know if the bank is done charging or not. Likewise for discharging the capacitor bank, but do be careful of the bulb voltage and do not exceed it. In my DRSSTC 3 you can see that I used a light-bulb as a precharge resistor:
In my newer precharge board I'm just using a resistor since it isn't fragile like a bulb.
I'm also using carbon composition resistors which are specifically designed to handle low duty cycle energy dumps like these, and also added some features on the board since they were some extra features I wanted. :)
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Never noticed the light bulb in that photo before. It is a simple solution. I like it. Do you have a relay to switch to mains when voltage has leveled out?
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
You're welcome Graham!
Loneocean is correct, light bulb is fragile, if you're thinking "resistive" charging, then use resistor, if "reactive" charging then go to capacitor instead of inductor since larger reactance is easier to achieve using C vs L. remember that Xl goes proportionally with L @ given freq ... Xl=2PifL, so for bigger Xl, you need large L
If you google, voltage line dropper you would see capacitors are usual approach, low resistive element, low ESR, low heating..
And yes, there should be a bypass relay to direct the caps to the mains.. you don't want any resistance from the supply coming to your beast ;)
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DRSSTC Capacitor Charging
