Demonstration of an electrolytic capacitor charge (excursus for those interested in electronics ;-)

Andreas Thaler

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The flash capacitor removed and discharged from a Minolta Auto 132X fascinates me:



I'm thinking about what I can do with it that's safe.

I have access to my laboratory power supply with a maximum output voltage of 20 VDC (= volts direct current) for charging.

This should be a safe voltage.


You should only open flash units or cameras with inbuilt flash if you know exactly what you are doing. The flash capacitor of even small compact cameras can charge to a dangerous voltage.

The nominal voltage of the electrolytic capacitor is 350 VDC.

Such high voltage is dangerous!


Does the LED light up?

I'm interested in how far I can get with the achievable load.

An LED offers a good comparison option - because you are used to it.

Can I with a load of


Q = C * U [Coulomb]

Q = 800 uF * 20 V

Q = 16 millicoulombs


make an LED light up?

That should work.

Because the 16 millicoulombs correspond to 16 milliamperes if they flow through the LED in 1 second.

This gives a bright glow for 1 second.

If I limit the current through the LED to 5 milliamps, the LED lights up weakly, but for longer, namely 3.2 seconds.


Tame the voltage

Now, when discharging a capacitor, most of the current flows at the beginning and the voltage of the capacitor decreases quickly.

Since my LED can only handle 2 volts, anything above that voltage would overwhelm and send it into the electronic afterlife Because the electrolytic capacitor is charged to 20 VDC.

Therefore, the voltage for the LED must be regulated. Namely, so that it receives the maximum amount of voltage that you get.

The easiest way to do this is to use a Zener diode, which supplies 3 volts of output voltage and thus supplies the LED appropriately.

The LED also has a series resistor that reduces the input voltage from 3 to 2 volts.


A circuit for the LED

Here is a simple circuit that works in circuit simulation:



Simulation with

EveryCircuit: Animated interactive circuit simulator

Interactive real-time circuit simulation and animated visualization inspired students and engineers to design 2.6 million circuits online and in mobile app.
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  • On the left there is the laboratory power supply as a voltage source with 20 VDC.
  • The electrolytic capacitor is charged via a switch and a resistor. This takes 4 seconds.
  • The electrolytic capacitor discharges via a second switch via a voltage divider with a Zener diode.
  • The output voltage of the divider finally supplies the LED, which lights up for approx. 4 seconds.
  • The resistance in the voltage divider is chosen so that no current flows through the Zener diode that remains for the LED. Because there is only a limited charge on the electrolytic capacitor.

Here are the calculations:



Charging the electrolytic capacitor.




Voltage limitation with Zener diode.

Calculations with

Electronics Engineering ToolKit PRO

The home of the EE ToolKit! A highly rated and recommended productivity App. This powerful tool is suited for anyone interested in electrical engineering, from students to hobbyists to electrical engineers.
ee-toolkit.com


I am interested in the following questions:
  • Can I switch the charge on the electrolytic capacitor so that the LED lights up evenly and then goes out? So that the LED comes down from the discharge curve and a consistent current flows?
  • Apart from an LED - what load could be used to demonstrate the electrolytic capacitor's charge just as clearly? I would like to get a better feel for charge values. Electrolytic capacitors are prominently featured in the Minolta X cameras, for example.
+++

All information provided without guarantee and use at your own risk.
 
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Jim Jones

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For the LCD to provide even illumination while it discharges the capacitor, you need a current regulator in series with it instead of a voltage regulator (zener diode) in parallel.
 
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Andreas Thaler

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Jim Jones said:
For the LCD to provide even illumination while it discharges the capacitor, you need a current regulator in series with it instead of a voltage regulator (zener diode) in parallel.
Click to expand...

Thanks!

Does the discharge then proceed as a rectangular curve, thus ON, OFF?
 

albada

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The fun part is to charge the cap to about 300 volts, which is the voltage the flash unit uses.
Then short the cap's wires together.

BANG!

It sounds like a gunshot! And leaves the wires welded together.

When working with such caps, I always ensure they're discharged by placing a high-power 100 ohm resistor across its wires.

Mark
 
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Andreas Thaler

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albada said:
The fun part is to charge the cap to about 300 volts, which is the voltage the flash unit uses.
Then short the cap's wires together.

BANG!

It sounds like a gunshot! And leaves the wires welded together.
Click to expand...

I don't do that kind of nonsense. This is dangerous. I'm interested in how I can make my LED light up.

albada said:
When working with such caps, I always ensure they're discharged by placing a high-power 100 ohm resistor across its wires.
Click to expand...

Sounds more sensible.
 
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Andreas Thaler

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I will build the circuit for my LED with a Zener diode and then with a voltage regulator and measure the voltage curves on the LED with the oscilloscope. This is going on my project list, it requires some work if you don't do it every day.
 
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UnderwoodPhoto

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Using that capacitor, the best thing in my opinion is a neon bulb oscillator. You already have 3 neon bulbs, just need three 1 Megohm resistors and three small capacitors.
 

Chan Tran

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UnderwoodPhoto said:
Using that capacitor, the best thing in my opinion is a neon bulb oscillator. You already have 3 neon bulbs, just need three 1 Megohm resistors and three small capacitors.
Click to expand...

Using neon bulb indicator you will have to charge the capacitor to at least 90V or higher. I don't think the OP has a power supply that can supply that high voltage.
 

Chan Tran

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DWThomas said:
Enh -- could always use the cap to build a spot welder for doing cell replacements of NiCads, etc.
Click to expand...

800μF is a small capacitor for such an application. Spot welder requires very high capacitor and low voltage. I have seen they use the 1F (but can be charged to 2.7V max) super capacitor for that. A 1F capacitor is 1250 times larger than the 800μF capacitor.
 

DWThomas

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Hmm, think that depends on what you are spot-welding -- joules per cubic whatchamacallit. In the 1960s a company I worked for had a small bench top unit used to stick very small and thin mostly nickel alloy parts together. As I remember it, it used an adjustable and significantly higher voltage and fairly modest caps. (I was not a regular user of the machine, but occasionally used it to tack thermocouple wires together.)

Now when you get to automotive body/frame assemblies . . . . .
 
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Andreas Thaler

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If I short-circuit a 150 uF/4 V electrolytic capacitor with the pliers, it sticks to the metal. Some energy is already being transformed at certain points.
 
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Andreas Thaler

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Andreas Thaler said:
If I short-circuit a 150 uF/4 V electrolytic capacitor with the pliers, it sticks to the metal. Some energy is already being transformed at certain points.
Click to expand...

Energy of a capacitor:

Eel = 1/2 * Q * U [J]

Q = C * U [C]


Q = 150*10^-6 F * 4 V

Q = 600 uC

Eel = 1/2 * 600*10^-6 C * 4 V

Eel = 1,2 mJ


You would now have to illustrate this amount of energy in order to be able to imagine what it means. Since the energy conversion takes place within a very small area on the pliers, this would explain the sticking effect.

What do you all mean?
 

Reginald S

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I believed that you meanwhile had finished this experiment?

Best,
Reginald (Digi Club)
 

albada

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An experience a few decades ago: A couple of benches away, I heard two loud BANGs. It turns out that an engineer a couple of benches away from me had assembled a couple of power supplies, and turned them on. But he had put the electrolytics in backwards. Electrolytics have polarity (positive and negative wires), and they hate being charged backwards. At least those did.

That plus my BANG experience described above have made me very distrustful of and careful with large electrolytics.

BTW, large electrolytic capacitors are found in power supplies and Xenon flash units. Power supplies use them to store DC energy from the mains, and to smooth the output. Flash units use them to power the flash tube.

I regard electrolytics that are over 15 years old to be dangerous because they dry out, some faster than others. When drying out, they can short out. Or they can blow out their tops and smoke out, which happened to me once when turning on a PC. Old audio equipment develops hums due to leaky caps. I had an HP oscilloscope and a Pentax flash die with POP sounds as the electrolytics were shorting. If you have an old flash unit, I suggest replacing its electrolytic.

Mark
 

Bushcat

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If you're worried about the capacitor holding charge, you might want to pop a bleed resistor permanently across it.

In your 20V circuit, and say you want it to be discharged within 30 seconds, something like a 12 kilohm, 0.5W resistor would be fine, having a peak dissipation of 30 mA. If you want a resistor suitable for the mas capacitor rating, then something around 6.5 kilohm, 20W would be fine. I'm assuming a "safe" level of 1V, but you could also decide on 5V, of course. At 20V, almost any resistor will work after a fashion, as long as its resistance isn't low enough to affect circuit operation. At 350V, ultimately it's the physical size of the resistor that is probably the decider.

It's natural to want to discharge things quickly, but don't go too low with the resistance: it will be dissipating power the whole time the circuit is powered up, so 600mW at 20V and a noticeable 20W at 350V. So you might decide to bleed the charge off over, say, 10 minutes or more. Then you can pop any old high-value resistor in and the job is done. 1 megohm, 1/4 watt would do it in under 80 minutes.

Our HV lab had bleed resistors on everything by default. There was also a box of old flourescent tubes at the entrance with an idiot's guide: You must be holding a tube to enter. If a tube glows, retreat. Don't touch anything unless you have seen someone else touch it first.

Obligatory ElectBOOM:
 
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