charge a super capacitor - Page 1
Forum section: electronics / beginners

author: nickeevblog10
author_rank: Contributor
timestamp: May 21, 2019, 11:50:06 pm
content: Hi, I purchased a 2.7V 500F capacitor to use as an experimental spot welder for battery tabs. I will not spot weld many battery tabs and I have no idea if this Youtube project will really work. After some looking at the specifications it seems this capacitor can store something like 30Amps and so may have a good little whack of energy needed to melt light aluminium strips for tabbing AA AAA and other batteries.My question, I have a variable power adapter which has 12-3V settings and charges @ .500A and puts out 4.4V. Can I safely charge this capacitor using this power adapter setting.I also understand I need to monitor the rate of charge with my multimeter so the capacitor does not overcharge and dangerously explode or something like this.This capacitor has no dangerous chemicals according to is specification .pdf file.http://www.samwha.com/electric/product/list_pdf1/DB.pdfThis capacitor was dented during transit, the packaging was damaged but because there is no damage other than a closed dent I am going to use it. I also asked for a replacement and the person offered me a refund because he no longer had any left. I took the refund, not knowing if it would work or not. I am not sure what I can do about this now.

author: Psi
author_rank: Super Contributor
timestamp: May 22, 2019, 12:12:03 am
content: If you put a diode in series with the 3V from the wall adapter this will drop the voltage by ~0.5V and prevent any capacitor over-charge.Something like a 1N4007 diode or similar.You also must protect against drawing too much current from that wall adapter because i doubt that one has a current limit.It may overheat if you don't limit the initial current somehow. I recommend you find some sort of dummy load, like a resistor or light bulb which draws the max current available (500mA at 3V).Put that resistor/lightbulb in series too, it will prevent over-current when the capacitor is first connected in a flat state. It will also prevent the diode from being damaged by large currents.Ideally you should find a better power supply that has adjustable voltage and current limit.If you set a proper lab power supply to output 2.7V with its current limit set to the max rated current of your cap (30A) it will charge up the super cap as fast as possible. It will also be safe from overcurrent or overvoltage.NOTE: A psu that can do 2.7V at 30A is going to be hard to find. Most lab power supplys do 0-30V at 0-5A or something like that.You can use a 5A one, it will just take longer to charge the cap at 5A than 30A.The important thing is to set the voltage to match the cap voltage of 2.7VI also have my doubts you are going to get enough energy from that cap to spot weld a battery tab.I tried spot welding with a 2.5V 2600F maxwell supercap and couldnt get it to work very well.

author: mariush
author_rank: Super Contributor
timestamp: May 22, 2019, 12:32:54 am
content: I simply used a 1117 adjustable linear regulator, configured at 2.65v with 2 resistors:https://www.digikey.com/short/p4b890Some have 0.8A max current, others 1A... and internal current limit at around 1.2-1.5A and input voltage must be ~1v higher than output, so it works fine with 5v from USB, phone chargers etc...If you want you could use regulators with fixed 2.5v output, it's a super common voltage... and sometimes cheaper ex 15 cents for 1117-2.5v :https://www.digikey.com/short/p4b85r...or 11 cents for the MIC5504-2.5 :https://www.digikey.com/product-detail/en/microchip-technology/MIC5504-2.5YM5-TR/576-4880-1-ND/5209408This one's 300mA max but has only 0.38v drop at 300mA so you could use the 3v output and get 2.5v out.... but charging 500F would be slooow.. like 12000s at ~300mA (6000 seconds to 2v)At 0.8A, 500F would charge to 2.5v in ~4200s (half to reach 2v)

author: nickeevblog10
author_rank: Contributor
timestamp: May 22, 2019, 01:41:14 am
content: Just now realizing I can measure Amperes with my multimeter @ 3V reads 0.06A. This is a DC power adapter so I think the voltage and current are both going to be steady and slow charging, very slow if you like.So charging the capacitor seems to me to be possible safely if I monitor the charged capacitor so the charge does not go over 2.8V which is the absolute maximum rated voltage, but I may just like to charge up to 2.6V to keep things safe.What do you think? I do not mind even letting the charge go slowly as this is my first time at this.Could I even charge at a higher Voltage. I notice the Amps double @ 4.5V.This is an experiment and the metal for the tabs is very thin, thank you China.

author: Psi
author_rank: Super Contributor
timestamp: May 22, 2019, 07:30:13 am
content: You can't measure amps when the red DMM probe is in the volts socket.But it's good that you did that wrong because you had the DMM probes connected directly across the power supply!If you had used the DMM amps socket you would have shorted the power-supply and maybe damaged something.You can only measure the amps that a particular load draws. You can't measure the amps of a powersupply

author: nickeevblog10
author_rank: Contributor
timestamp: May 22, 2019, 10:39:01 am
content: Ok, lesson received, thank you.I see I need to do some reading.The power adapter Current reading @ 3V is now 1.920A.So now how does the power adapter look as a charger for the capacitor.I am not worried how long it will take as long as the capacitor gets charged.I noticed on Youtube videos charging seemed to stop at a certain voltage and I would not want to over charge the capacitor.Maximum voltage for the Capacitor is 2.8V.It seems to me the multimeter read the output current of the Power Adapter @ 3volts. Would this be correct and the reading on the meter is 1.920 Amperes.

author: Psi
author_rank: Super Contributor
timestamp: May 22, 2019, 12:14:19 pm
content: An unregulated power supply, like the one you have, is rated to supply a max amount of current safely. Your one is 500mABut... any unregulated power supply will happily supply MUCH more current until it gets hot and starts to smoke/burn out.I suspect you have just tried to do that. Creating a short circuit across the 3V by using your DMM on amps setting.That 1.920A is your power supply screaming "omg im going to die"When using a multimeter with the red probe in the amps socket there is a big short circuit between the red and black probs!Never connect them across power.The must go in series, never in parallel.To measure current you remove a wire from a working circuit and replace that wire with the meter.On amps mode the red and black probs should never be connected to the two terminals of a powersupply

author: nickeevblog10
author_rank: Contributor
timestamp: May 23, 2019, 12:42:24 am
content: Does a regulated power adapter mean it is rated at a limit such as 500mA is for this power adapter?I think I see you are saying this power adapter is rated at 500mA and I should not be getting any reading for Amperes any greater than 500mA, unless the adapter is now faulty.More testing: I had the meter set to DC thinking a DC power adapter would give a consistent output.This time I set the meter to read AC and the reading was 1.200A which kept dropping very slowly.What might these results tell me?

author: piguy101
author_rank: Regular Contributor
timestamp: May 23, 2019, 01:17:31 am
content: Most likely the 500 mA number is the highest the power supply can safely deliver to the load without damaging itself or causing a fire. However, the power supply may be not current limited (as we just saw) and can possibly exceed its rated output current if you short its outputs.Measuring current of a power supply requires having a load. Measure the current in line with the load. Never measure the current directly by shorting the terminals of the power supply. This is incorrect and dangerous.

author: nickeevblog10
author_rank: Contributor
timestamp: May 23, 2019, 01:31:50 am
content: hi piguy,Isn't this how a meter reads any power and amps and so on.If I check the pins on a laptop power supply for Voltage I turn the laptop power adapter on and put the probes on the two terminal on the pin that goes into the laptop socket and I usually get a seemingly correct result from a 19 V power adapter of around 19 V. Is this shorting also?I mean the way I have used the meter seems to be correct to me.The manual tells me if I try to to measure too many amps the fuse can blow or the operator could harm himself.EDITED after retesting using the correct meter setup: there is now a reading of 1.2 A using the same correct setup as before but in Amps reading mode, I was using DC mode last time and getting the 1.920A reading.Also my meter gives O.L. when I use too lower a settings uA and mA to read AC.

author: nickeevblog10
author_rank: Contributor
timestamp: May 23, 2019, 01:49:53 am
content: Now just thinking then, this power adapter may be ok for charging the capacitor and I could safely begin charging and test the output of the power adapter @ 3V under load on the Capacitor.How does this sound to you guys?Thank you very much for your feedback.

author: StillTrying
author_rank: Super Contributor
timestamp: May 23, 2019, 10:26:56 am
content: Nothing can be done safely with power supplies, batteries or charged caps until you learn how to measure current without shorting things, it's possible you've already damaged the DVM and/or the 3V supply!https://www.allaboutcircuits.com/textbook/experiments/chpt-2/ammeter-usage

author: Psi
author_rank: Super Contributor
timestamp: May 23, 2019, 01:56:21 pm
content: Quote from: nickeevblog10 on May 23, 2019, 01:31:50 amNo that is not shorting. You can do whatever you like on Volts mode.The danger is measuring Amps.You must connect the meter correctly when measuring amps or you might blow something up.Those 1.2A or 1.9A readings you are getting are dangerous. You are shorting out the power supply.

author: mariush
author_rank: Super Contributor
timestamp: May 23, 2019, 02:33:25 pm
content: To measure voltage, it's enough to put each probe on the positive and negative terminals.To measure current going to a device, the multimeter must be placed in-circuit : this means the energy must go through the multimeter and into the device.So for example, one probe goes to the positive of your power supply and the other probe goes to your device, the other probe becomes the positive of your power supply.Depending on what range the multimeter is set, there will be some amount of voltage drop inside the multimeter, because the actual measurement is done with a resistor, and you have the basic formula (Ohm's law) Voltage = Current x Resistance.So for example, on the 10A range of your multimeter a 0.1 ohm resistor may be used but if your multimeter is on the 1A range the resistor may be 1 ohm.With this example, if you set the multimeter on 1A range and the multimeter has a 1 ohm resistor inside, then if you measure a 5v power supply, at 100 mA there will be 5v - 0.1Ax1 ohm = 4.9v and at 500mA it may be 5v - 0.5a x 1 ohm = 4.5vPower supplies can provide a limited amount of power, where power means the product of voltage and current.In your particular case, your power supply may only guarantee 3v at 500 mA , which means it can give you 3v x 0.5A = 1.5 watts of power. There may be some "reserve", some tolerance, so in reality it may give 550-600mA.If a device needs more, then the power supply may increase the current, but lower the voltage so for example, you may measure 2A of current, but the voltage may only be 0.75v, because the power supply is still only capable of 1.5 watts of power ( 0.75v x 2 = 1.5 watts)A fancier power supply may turn itself off if a device takes too much current or if the voltage goes below some threshold, to protect devices connected to it.So when you placed the multimeter directly across the positive and negative wires of your power supply, you more or less created a short circuit between the wires... in reality the very tiny resistor used by the multimeter to measure the current plus the resistance of the multimeter probes were the only things connected.So the power supply sent as much current through the "load" between the positive and negative wires, which probably was 1.92 A of current .... but you don't know what voltage that was, could have been 1v.Also, you also don't know if the power supply was actually capable of sustaining that "effort" for long periods of time. Components inside could be damaged or broken after some period.Another thing you have to understand.A lot of power supplies regulate a voltage only when something is connected to the power supply and that something consumes a tiny bit of power.Some power supplies will output a higher voltage with nothing connected to them, but as soon as something is connected, the voltage gets closer to the actual specified voltage.For example, cheap wall wart adapters that use old transformers may be configured to output 9v but they'll output 10-11v with nothing connected to them. If you connect something as simple as a led (with its current limiting resistor) that consumes 10-15mA of current you may notice that the voltage gets closer to 9v.Other power supplies use cheap techniques to regulate the output voltage, which can cause power supply to output higher voltage at very low currents. For example, you may see a phone charger output 5.5v with nothing connected to it, but as soon as something consumes power, the voltage would go down closer to 5v.There's various levels of quality when it comes to power supplies

author: nickeevblog10
author_rank: Contributor
timestamp: May 26, 2019, 05:01:41 am
content: To test the power adapter @ 3 volts I bought a 3 Volts LED and understand I need to break the circuit between the power adapter and the load (the LED) to measure the Amps @ 3 Volts. Do I need to use a transistor in the circuit also?EDITED: now looking back at the first reply to my question I see the meter must have a resistor in it to get any Ampere reading so presuming I need not add a second resistor to the test circuit. How am I doing here?Thanks for taking the time.Here is the result of the power adapter test fro Amps using a 3 Volt LED see attachmentI am concluding the power adapter is ok although very low Amps to charge the super capacitor.But how can I test the power adapter to see if it will put out 500mAs?

author: StillTrying
author_rank: Super Contributor
timestamp: May 26, 2019, 12:28:55 pm
content: You might have to believe in Ohms Law, to see how you can limit the current to safe levels.www.allaboutcircuits.com/textbook/experiments/chpt-2/ohms-law"But how can I test the power adapter to see if it will put out 500mAs?"If you had a 6 Ohm 2 Watt resistor you could put it across the 3V output, then 3V/6R = 0.5 Amps would flow, and the resistor would get quite warm.A 6.8 Ohm 2 Watt resistor in series with the 3V and super cap would limit the charging current to 440mA, you'd still have to watch that the cap doesn't eventually charge to over 2.6V, charging will be slow towards the end.You might be able to use a small 6V bulb instead of a resistor to limit the cap charging current down to ~0.5A.