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General electronics => Beginners => Topic started by: plasma on August 02, 2020, 07:13:54 AM

Title: current limiting to parallel resonance circuit
Post by: plasma on August 02, 2020, 07:13:54 AM
Hi I have been looking at tank circuit, simulators and books. On the sim there's quite a large in rush current, it doesn't matter so much with low voltage, but when trying to find a high voltage high current power supply, I'll probably trip breakers.
Will be starting with 24acv and go up to 240acv, starting the construction of the inductor will be 100mH and capacitor 100uF at 50hz.
I  don't really want to uses mechanical switch's to change resistance, but will settle for that.
Any ideas I could look at it, triac and...

Thanks
Title: Re: current limiting to parallel resonance circuit
Post by: davekni on August 02, 2020, 06:49:59 PM
The standard solution for limiting inrush current is NTC thermistors designed for that purpose, called "inrush current limiters".  Here's the Digikey page for those:
https://www.digikey.com/products/en/circuit-protection/inrush-current-limiters-icl/151?k=inrush%20current

For improved efficiency, many power supplies have a mechanical relay that shorts across the inrush current limiter a few seconds after startup, after the inrush event is past.
Title: Re: current limiting to parallel resonance circuit
Post by: Zipdox on October 17, 2020, 11:26:45 PM
Sorry for necroposting, but I'm having issues as well.
I have a simulation with a 3300 uF cap being charged from 230VAC, so 325VDC, across a 680 Ohm resistor. I plotted the voltage on the cap, and the power that the resistor dissipates, which is quite drastic.


I tested this circuit with a relay delay circuit which bypasses the resistor after 1.4 seconds. It tripped the breaker. I'm at a loss here.
The charging is too slow and the resistor generates too much heat to dissipate.
I also don't know how to delay the relay any longer without the need of a massive cap across its coil. Currently I'm just using a capacitive dropper and full bridge rectifier on the mains to power the relay.
Title: Re: current limiting to parallel resonance circuit
Post by: davekni on October 18, 2020, 05:15:50 AM
Looks like you are simulating with a 325Vdc source.  Average power will be lower if you simulate a sine-wave source fed through a bridge rectifier as your real circuit presumably is.  Charging will take longer too, especially nearer the end (long slow tail to charging).

If you are tripping your breaker, you may also have welded relay contacts together.  Check relay operation before further use.

3300uF at 325V is 174J.  Unless using a PFC or other fancier input stage (large inductor or switching regulator), there's no way around dissipating energy to charge.  With a DC source as in your simulation, total resistor dissipation will be 174J (power integrated over time).  With rectified AC input, total resistor energy will be less than 174J, but still significant.  However, you don't need a resistor rated for 150W continuous to handle 150W peak.  A 25W rated wire-wound resistor is likely plenty.  I'd suggest 50 or 100 ohms for faster charging, or an NTC inrush current limiter for even faster charging.  Total resistor energy remains the same.  Make sure relay delay is long enough for charging, and that your load isn't on until after the relay closes.
Title: Re: current limiting to parallel resonance circuit
Post by: plasma on October 18, 2020, 07:31:58 AM
A capacitor in series on the AC side of the bridge will limit current to a max.
The impendence of the capacitor at 50hz with 240V limited to 9A.
X = 1/((2*3.142*50*H))
Title: Re: current limiting to parallel resonance circuit
Post by: Hydron on October 18, 2020, 12:08:36 PM
NTC (or a few of them) seems like it's still the best option to me - they have the most resistance at the start of the charging period when it's needed and least at the end when it's not (meaning faster charging without excessive current), are rated for pulse loads in exactly this application, and can be bypassed with a relay once their job is done.

Another option would be to use a couple of SCRs in the bridge and increase the conduction phase angle to soft start it. More complicated but if there was a desire for a variable voltage output then you could do two jobs at once.
Title: Re: current limiting to parallel resonance circuit
Post by: Zipdox on October 19, 2020, 09:48:07 PM
Will putting a choke after the rectifier help improve the power factor and reduce the current spike?
Title: Re: current limiting to parallel resonance circuit
Post by: davekni on October 19, 2020, 11:02:11 PM
"Will putting a choke after the rectifier help improve the power factor and reduce the current spike?"

Yes.  But, before I say any more details, try it in simulation.  Sine-wave voltage source, bridge rectifier, inductor into 3300uF cap.  Plot current and voltage for different values of inductance.  That's the way to learn.
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