Capacitor Questions

[AXI Motor Models]

1) If you charge up a large capacitor and connect it to a load, will the load only draw the current it needs, or will the capacitor force its current through the load and damage it?


2) If you have a coil such as a solenoid and it is rated at 12V 1A, and you connect a 12V battery with a max discharge rate of 60A, will the solenoid only draw 1A and operate or will it burn up with 60A running through it?
Now if you connect a charged large capacitor will it burn up the coil in the solenoid, or will the solenoid operate and still only draw 1A?

Thanks.

[vintage1]

1) only what the load needs,.

2) Only 1A. As long as the capacitor is not charged beyond 12v, that will also be the peak current through the coil when connected to it..

[AXI Motor Models]

Thanks

[Ron van Sommeren]

... Now if you connect a charged large capacitor will it burn up the coil in the solenoid, or will the solenoid operate and still only draw 1A?...Your current will oscillate (and the oscillations will get smaller and smaller) but it will not exceed 1Ampère. You created a LC-oscillator :)

Vriendelijke groeten ;) Ron van Sommeren

[The Fonz]

Ron,Vintage, AXI

First, I like these types of questions as they help me further my amature electronics dabbling. I only respond to get feedback and not meant to argue or hi jack this innocent thread.............Both of you (vintage) seem to post alot on these types of queries and this one peaked my curiosity as to the responses, so I am trying to understand.......

Are you basing that oscillation idea on that the inductor (solenoid) is opposing the gradual change in current from the capacitor discharging and charging from that opposition. I can see the idea of caps used in ac circuits for things like ripple control as seen with non switching power supplies on the output but across an inductor..............would it not appear from the solenoid as a type of suppression like a free wheeling diode. Certainly , unless the capacitor was rated in terms of many farads not milli or micro, would it really create any useful current.

I understand the question and I agree with both your and vintage 1 answers but would it really draw rated current if the capacitor was capeable. I see a graph in my head that appears to show current versus time and it is oscillating...............the properties of the inductor would prevent it from showing a gradual decrease over time......is this what you meant?

Thanks

The Fonz

[pmackenzie]

I suspect it would only oscillate if the "Q" was very high, i.e. the total resistance of the circuit was low ( relative to L and C)
If the Q is low then the circuit is well damped and the voltage would just decay.
Probably some simple relationships between L,C and R that determine all this, (long since forgotten by me :o )...

Here is what google found:
http://www.greenandwhite.net/~chbut/lc_oscillator.htm

Pat MacKenzie

[vintage1]

It's a classic text book case actually.

Response of coil to step input.

It wont oscillate or even ring when connected, because the damping (resistance) will be high on account of the resonant frequency with the capacitor being very low.

When disconnected tho, its a different story. Disconnecting a live solenoid is how spark ignition works. Expect many hundreds of volts for an instant!

The effect of the coil inductance when connected will be to slow the initial current..so it builds up gradually..peak value will be about the amp mark followed by a decay as the capacitor loses charge.

[Ron van Sommeren]

Yep, oscillation was the wrong word, it implies a long time.

[The Fonz]

thanks,

I value your responses!

The Fonz

[AXI Motor Models]

The effect of the coil inductance when connected will be to slow the initial current..so it builds up gradually..peak value will be about the amp mark followed by a decay as the capacitor loses charge.

Thats perfect - just what I wanted.
I was thinking of switching the energy from the capacitor to the coil through a MOSFET, If there is any current running back into the capacitor will this damage the MOSFET, should I add a diode?
Thanks

[vintage1]

Thats perfect - just what I wanted.
I was thinking of switching the energy from the capacitor to the coil through a MOSFET, If there is any current running back into the capacitor will this damage the MOSFET, should I add a diode?
Thanks

Yes..you MUST have a reverse connected (schottky?) diode across the solenoid...this will clamp any 'flyback' type voltage spikes that occur on switch off.