Renewable Energy Questions/Discussion > Renewable Energy Q&A

dis -charging capacitors

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artv:
Hi Oz,...starting to clear
In your example ,you discharge 320000 watts in .000028 secs....so .5 sec=18watts...,   .25 sec=36watts and so on (correct)?
"very very quickly via a SCR"...this sounds like what I'm looking for...as it is I just connect and disconnect the caps to the battery by hand, but I can't switch fast enough ,probably 2-3 dis-charges/sec.
Can I measure the current between the, cap + out, and the battery +, but it would be a peak reading that would drop to zero very fast,...would that peak reading be accurate??
Also to find the circut resistance, connect to the common ground, but where should I connect the red lead,at the + caps out???
Is there a rule about ,time to charge and discharge,...ex. 1sec charge 1sec dis- charge??

In the fencer ..how long does it take to charge the cap? and what is the source??
It just got cloudy again......artv

WooferHound:
Didn't you look at the Pulse Charger links that I posted above ?
Didn't you search google for more information about Pulse Chargers ?
You are talking about Pulse Charging, there is tons of information out there  ! !

bvan1941:
ARTV,
Capacitor don't charge or discharge in a linear line. They do charge and discharge over a period of time, in whats called TC'S (time constants). While the time constants are equal in time, the chrg/d'chrg per each TC varies in the following percentage rates: 
TC # 1 =   63%
TC #2 =    86%
TC #3=     95%
TC #4=     98%
TC #5=     99%

So to summarize, each TC period is a combination of the capacitance and the resistance in that circuit combined. Normally, the period is in milliseconds/ microseconds. It is a formula that can be found online. It's use is by electronoic techs / engineers to compute.
If you new the TC period, you would know then that in the first TC, the charge on that capacitor would be either 63% charged or discharged.
Hope that clears up some confusion and give you a perspective of that characteristic of capacitors.
Bill





















 

artv:
Hi Woofer,...I did look at the links, and thank-you very much for those..Just skimmed them so far...not alot of time for the internet
I don't want to charge batteries............I want to pulse electromagnets
The more volts ,more capacitance,.....the stronger the electromagnet???
I need to get more caps ....real caps...
You always give the best links......any suggestions where to get them, high uf, high voltage..
Right now the stored caps will bunt ...the alt ,..after shut-down..
I need bigger caps though.........artv

oztules:
"In your example ,you discharge 320000 watts in .000028 secs....so .5 sec=18watts...,   .25 sec=36watts and so on (correct)?"

Thats what the equations say. It is not linear, ( more like exponential) but the energy dissipated over those time frames give us the power. It will start out hard and dribble to an end. If the load is a LED, it will stop discharging before zero volts (led will drop out before then) or for  a battery, will only discharge to that voltage.


""very very quickly via a SCR"...this sounds like what I'm looking for...as it is I just connect and disconnect the caps to the battery by hand, but I can't switch fast enough ,probably 2-3 dis-charges/sec."

Yes you can use an SCR..... but it only switches off when it drops below it's turn off threshold. So with AC no problem ( it disappears through zero every cycle, with DC as in the fencer, the driving input impedance is only mild, so when the SCR shorts, it sees almost zero volts for a finite time, and can turn off. If the source is too strong, you may find turning the scr is not so simple as "discharge and go again".. like I have done in this instance. Driving a battery it won't turn off, as the volts don't drop to zero.


"Can I measure the current between the, cap + out, and the battery +, but it would be a peak reading that would drop to zero very fast,...would that peak reading be accurate??"

The peak reading will be just that... a peak. The output will decay from that peak at the start to the dribble at the end.



"Also to find the circuit resistance, connect to the common ground, but where should I connect the red lead,at the + caps out???"

I am not familiar with your circuit, so can't really comment sensibly.

 

"Is there a rule about ,time to charge and discharge,...ex. 1sec charge 1sec dis- charge??"

Bvan1941 covered this..... but maybe we should see how to make an equivalent water solution you may find easy to understand what is going on.

If we think of a capacitor as a water tank with a value of the capacitance as the square footage/inches of the base of the tank X the voltage as the height of the tank, and gravity as a constant.... we may be able to describe this.

As we fill the tank, the volume will be a factor of the floor area times the height.... but the height will also give us water pressure.. dependent on height. A High V capacitor would be tiny surface area and very tall height, and a low voltage one ... a bigger floor area and low height for the same capacitance.

Now as we fill the tank, the volume fills, but the back pressure attributable to the height will resist the incoming water... the higher it gets, the more pressure it will exert against the incoming water force... until the pressure of the height of the water equals the driving force behind the filling water.... were fully charged.

Now remove the filling force (hose) and let the water run out. When the tank is full, it will run out quickly.   But as the level drops, so does our volume, and our pressure.... so the flow rate slow down some.... this continues until we get to the bottom, where the pressure is now zero, and only the water laying on the bottom of the tank remains.... and there is no more pressure to push the dregs out.... unti it drips out, or evaporates over time.

The fill time and empty time will also depend on the pipe size on the input and output if they are not the same. (if not using the inlet for the outlet???) For the fencer, it comes in through a small hole, and to empty, we pull down an entire wall to empty it almost instantly.... close it back up and fill it again... etc.

Now I'm not an electrical engineer, but thats as good as I can do in the water analogy stakes....

Anyone else  have a better idea for the water analogy?


"In the fencer ..how long does it take to charge the cap? and what is the source??"

 This breaks a lot of the rules Bvan1941 mentioned.

It runs off 12v DC. It is a little oscillator driving a small ferrite transformer. The transformer aims for about 800v.... but never gets there.

When the cap is empty, it charges up to about 400v rapidly, and then eases off as the differential between the transformer output gets nearer to the cap voltage... but as the cap gets fuller, the oscillator runs harder, and so it does not follow the exponential time frame that you would think.... because the voltage is not a constant source. The terminal voltage is the cap voltage, but the driving EMF is dependent on the oscillator.

It takes about 1 second to charge up to 600v, at which time the SCR triggers into a very very low impedance (9 turns of 13 gauge wire) which pulls the cap down to zero (near enough) in a very very short time. This will start as an exponential  curve thing..... although the transformer saturates very very heavily, and that will effect the discharge curve. One moment it drives an inductor, and next a virtual short when saturated. This brings the voltage way below threshold... SCR releases, and the whole thing starts the slow recharge.

It is the saturation that lowers the output wattage so dramatically, and threw the time calcs out. The input power is much higher then the recovered output power.

It has to take at least a second to charge...... thats an occupational safety thing.... so is the very short duration pulse.



...............oztules

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