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Registered Member #1617
Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139
Hi Everyone,
My current TC project has 3600 uF of smoothing caps across the 'DC bus' (340 volts). In my area, I can't buy caps like this locally, so I went to ebay, hence they were fairly expensive. I dont want to blow them up, and I cant afford a suitibly sized variac. So would some sort of inrush limiting really be necsessary in this situation? If so, i was thinking of having a power resistor in series with the whole thing, and a relay to short it after a small delay. but if its not absolutely nescessary, I wont bother, I just dont want to blow my caps up/have the breaker trip every time I hit the CW button. Suggestions/comments?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You won't blow them up. Compared to the abuse you put them though running a DRSSTC, mains inrush is nothing.
However, you will probably blow fuses and breakers, so you'll need the inrush limiting.
You'll also need to vary the power supply voltage, though. It's naive to expect the thing to work properly at full DC bus voltage the first time it's turned on. A variac would probably pay for itself in blown semiconductors.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Use a "relay delay" circuit, this is the circuit I use and it works well:
First you need to measure the relay coil resistance and calculate the coil current. Then you size the series foil capacitor (labeled X2 in the schematic) to have such reactance to supply the desired relay current. The electrolytic cap across the relay coil just sets the delay time (calculate RC constant), anything in the range of 200-500ms should work.
If you don't have a variac, you can test your circuit by putting incandescent lightbulbs in series with the mains, start with lower wattages and go higher (you can use high power halogens and if everything works right, bypass it).
Registered Member #1617
Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139
You'll also need to vary the power supply voltage, though. It's naive to expect the thing to work properly at full DC bus voltage the first time it's turned on. A variac would probably pay for itself in blown semiconductors.
Thanks, yes i realize this, I will be initially running from a low voltage initially; there would be no way in hell i would connect it up full throttle first go! However, there is no such thing (that I have been able to find so far) in South Australia as a variac bigger than about 300 VA, and I can't afford to have one shipped from anywhere, ( I do have a slightly larger one although I need a new brush for it.)
Dr. Kilovolt, thats exactly the kind of circuit i was thinking of! Thanks!
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Inrush Current Limiting
