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Charging a capacitor? 

 

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 Post subject: Charging a capacitor?
Posted: Wed Dec 08, 2010 10:10 pm 
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Alright i know how to charge it i know how to install it but it says i need a 12v light or a resistor to charge the cap so i dont blow any fuses because the cap will draw huge current if connected straight to the battery.

Where do i get a 12v light or what value resistor? would a test light work fine?
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 Post subject: Re: Charging a capacitor?
Posted: Wed Dec 15, 2010 3:40 pm 
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try d**k smiths or a caravan accessory supplier for a 12v light and d**k smiths or any electronics shop for a resistor. i cant remember what value resistor we used to use when i was installing, but i will see if i can find out

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 1:34 pm 
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Just any ok size 12v light will do. The 12v light acts as a resistor. It just limits the initial current.

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 4:10 pm 
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A completely drained cap draws a huge amount of current for not very long... I have the formulas here somewhere to work out how long the cap takes to charge...

I'm guessing we are talking about an audio capacitor (for amps and the like)?

Usually you run your 4 gauge straight into it and out into your amp or distro block... If you go putting a light or resistor in series I can't see it working real well? Most of your current is supplied from the alternator/battery... Sudden spikes that require a massive amount of current are handled by the capacitor... So if you go putting a current limiter in its circuit... It doesn't sound like its gonna work really well?

Say you limit it to a 12V 60W bulb... Thats 5 amps... A spike on an amp might be 100amps or more for a tiny fraction of a second... How does the cap supply that with a 5 amp limiter?

I really can't follow how this is supposed to work... So unless someone can fill me in a little bit?

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 4:18 pm 
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This is for when the capacitor is new and not charged.

If i connect the capcitor to the battery (directly) it will fry fuses and stuff because the current draw will be huge.

NOTE OF CAUTION: Power capacitors store a large amount of energy and they charge very quickly. You must first "charge" your power capacitor before connecting it directly to +12 volts. This is done with the a 1k ohm resistor and a voltmeter. The exact value of the resistor is not critical but I would keep it in the 500-1k ohm range. I would recommend getting a 1 watt resistor if possible (your capacitor may have come with a resistor for charging). A lower wattage resistor will heat up too quickly. Also, do not hold the resistor with your bare hand. The current flowing through the resistor will cause the resistor to heat up and you could be burned. A good place to put the resistor is in the main power wire fuse holder (the one installed near the battery). Simply substitute the resistor for the fuse. A diagram for the capacitor charging setup is shown below. You will need to place a voltmeter across the capacitor to monitor the voltage. Once the voltmeter reads 12 volts (or close to it) you can remove the voltmeter and replace the resistor with the power fuse. Alternatively you can measure the voltage across the charging resistor. It should start around 12 volts and slowly work its way down to 0 volts. When the voltage stops changing you have charged the capacitor completely.

Another method for charging involves using an old style test light instead of a resistor. The connection is similar (alligator clip on one side, probe on the other) but you don't need a voltmeter to monitor voltage. When the bulb goes out the capacitor is charged (because the voltage across the bulb went from 12volts to 0volts).

I found this info. so its al lgood i know how to do it now :D
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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 6:12 pm 
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Don't all large capacitors have an internal bleed resistor to protect against bodily harm? As an electrician I know large motor start caps have one... Car caps are huge (1 to 2 Farad commonly?)...

So when the potential is lost across the cap the resistor becomes a load and drains the cap... So if you ever touch the terminals it doesn't discharge through you...

This means every time you start the car the cap is discharged... Hence you are right back where you started from?

The theory makes sense... They slow how long it takes to charge...

For car audio you run the boot power on its own wire and usually with a massive fuse/circuit breaker... So there is no issue... The inrush current doesn't bother anything... Its on its own circuit and the fuse/breaker is large enough to withstand the inrush for the short period of time to charge... You don't put the cap on an existing circuit...

When I was in retail we had a sound wall... Wired to a 20amp power supply which plugged into the power point... It had a 2 Farad cap... When you turned it on the cap would charge and it would come alive... When you turned it off there was no power at all... the cap would keep the amps turned on (not accy like in a car) and this would last until the cap was totally flat (20 or 30 secs)... Next time you turned it on the cap would charge from nothing... No issues with it what-so-ever... This was a 20amp PSU... Your alternator is a 70 or 100amp unit... Still not seeing what the issue is?

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 6:30 pm 
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I've got a broken hand so bare with my left handed typing lol.. The resistor is used to prevent damage to the cap as opposed to directly connecting to 12v for its initial charge. It does this by slowing the rate at which the cap is pretty much fully charged which takes 5 time constants (5t). You can calculate by how long it will take to charge by using the equation:

t = resistor value x capacitor value.

then times t (time constant) by 5.

Whenever charging or discharging a cap you should use a resistor preferably but a lot of people are lazy and simply touch the two legs.

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 6:36 pm 
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Thats right... 5 TC's is 99.6% or something like that from memory...

So you've covered two types of resistor...

One in series to limit charge current... And one in parallel as a bleed resistor... The one in parallel is a must I agree... Making a spot welder out of a cap is never a good idea...

The one in series I can't see the guts off... The cap is seen my DC as a short circuit for only a fraction of a second... One its own circuit It isn't going to hurt anything... Once it goes flat (as per the bleed resistor) it will need the series resistor again to limit inrush...

If you have a permanently connected series resistor it will also limit discharge back through the amp... Or if amp is connected to the cap side of the resistor then the amp will be limited as the pull from the alternator will be limited by the series resistor...

I can't see it working... Every time it goes flat (which it will if it has a bleed resistor)... You need to add this series resistor to limit the current in... which will limit the working function of the cap... if your doing to do that why bother fitting such a large cap in the first place?

Seems like a complete waste of time and money... :?

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 6:51 pm 
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I didn't think they had bleed resistors for this reason so they are always charged. Hence why you only need the series resistor at the start for the initial charge and then later discharge..

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 6:57 pm 
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the flux capacitor wil need percisly 1.21jigawatts to make it work, might need a few more altenators :P

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 9:11 pm 
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DormanAU wrote:
the flux capacitor wil need percisly 1.21jigawatts to make it work, might need a few more altenators :P


haha... I feel this figure is precisely .01% incorrect so I would like to see all your calculations :P

One Drone wrote:
I didn't think they had bleed resistors for this reason so they are always charged. Hence why you only need the series resistor at the start for the initial charge and then later discharge..


As far as I aware by law all capacitors over "x" amount or micro farads are required to have an internal bleed resistor... So I would have naturally assumed that a 1 or 2 farad capacitor would have had one...

But...

Usually the cap is on its own 4 gauge wire... So even with the car off the cap is charging off the battery... As a cap is charged based on TC's it can never 100% charge due the basis that each TC is 83% or remainder of the last, so it is always charging... Plus if it has an internal bleed resistor this contributes to battery drain when the car is off too... So people who have a cap are more prone to getting a flat battery?

Hence, the only way the cap will go flat is when the battery itself goes flat too... So when you jump start somebodys car with a cap your saying you should again put this resistor in series with the cap to prevent damage to both cars and the jumper leads?

Cheers,
Tim

 

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 Post subject: Re: Charging a capacitor?
Posted: Thu Dec 16, 2010 11:28 pm 
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Maybe for certain applications such as your line of work for safety. I haven't seen bleed resistors as a standard for over a certain size cap. For applications with big caps such as RF for example, that resistor if it was a standard would greatly affect the performance of the circuit i.e. a filtering characteristics, frequency roll off not to mention signifcantly making circuits bigger and more complex for "safety". Just have signs saying beware high voltage lol.

Yep, car caps also dont have that resistor for that reason; it would drain the battery. Once a cap reaches the same voltage of the battery, it should stay the same voltage and not draw any more current from the battery so it shouldn't drain it. Over time, other electronics of the car are more likely to drain the battery i.e. alarm, radio standby. Remember that when the voltage across the cap gets higher matching the source voltage, the current gets lower; it's inverse.

 

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 Post subject: Re: Charging a capacitor?
Posted: Fri Dec 17, 2010 8:43 am 
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Yeah thats right... But as you are aware with TC's that they never reach 100% so there is always a small drain (well very very small drain) same as things like KAM and the clocks etc use a tiny amount of juice...

Should this be right and they don't have an internal bleed resistor... If the car does go flat, are you saying you need this resistor again when you jump start the car? So part of your jump starting procedure is to disconnect the cap and then insert the resistor to charge it again... Like I said for the tiny fraction of a second that its a short circuit I honestly can't see why the resistor is needed... The battery itself and the alternator should not be phased...

Esp when your car battery is rated at close to 600cca anyway? This means it will discharge at 600 amps for 30 seconds at 0 degrees F while maintaining 1.2V per cell...

I guarantee you the cap isn't not going to draw 600amps for 30seconds... it will charge way quicker than that... So in that case... Charging the cap is really no worse on the battery than starting the car... The starter motor is virtually a dead short anyway... Massive copper, not many turns...

Cheers,
Tim

 

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 Post subject: Re: Charging a capacitor?
Posted: Fri Dec 17, 2010 2:03 pm 
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You don't have to do this. If you were anal you'd keep a resistor in the car if you had to change batterys or if the battery was dead flat which is rare. But if its a bigger sort of cap like what you probably deal with (maybe above half a farad) then you do charge it slowly is to reduce the possibility of arcing involved with fast charging. High voltages and a small gap between terminals maybe? This arcing won't hurt the cap so much but it might damage the chrome/gold plating on the connectors. Not a biggy.

 

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 Post subject: Re: Charging a capacitor?
Posted: Fri Dec 17, 2010 2:19 pm 
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Guys are you talking about a polarising resistor?
With large caps that are left on the shelf to long a fast charge can cause them to explode, very spectacular to see a 2 farad drive cap go off (not). by doing a slow initial charge the cap can then deal with fast discharges. We used to use a lot of big caps in the lift industry and and found it was normally the new caps that had been sitting on a shelf for a while that would go bang on first power up, after the inital charge no problem with rapid charging.
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