Capacitor charging circuit

[calbiterol]

I'm currently harvesting a large amount of capacitors off of an old vcr. I don't know exactly what I'll use them for, but I will need to know how to charge them. So, long story short, what is needed for a capacitor charging circuit?

[insane_alien]

a battery/dc voltage source and a resistor. if you want to go really high tech then you could add a switch.

[Lance]

Keep in mind the caps can only charge up to the charging voltage. Don't expect to charge 400V soda can caps with a 3.4V wall wart.

[MetaFrizzics]

Definitely use a resistor to limit the current when you throw on the power, since the capacitors will act like a short circuit (zero resistance) when uncharged and presented with a voltage. Once a capacitor is charged, you can bypass the series resistor with a switch. This is done for instance (via a relay or transistor -timer circuit) in power supplies to allow rapid replenishment of charge in the caps and provide a steady (or slow changing) voltage supply with varying loads.

 

Keep the maximum D.C. voltage below 90% of the rated value of the cap and at least as high as 1/3 of the rated voltage, to prevent arcing and failure of the capacitor(s) especially if you are going to use them for 24/7 duty cycles in a power supply left on.

 

Modern caps will also have a 'surge' voltage rating about 20% higher than the sustained rating, but this is not a safe voltage for use, only a statistical failure rate for parts placed under stress for very brief durations (something on the analogy of a 'slow-blow' fuse).

[Lance]

Most power supplies he has access to will have some internal resistance. Adding an external resistor is pretty pointless. He shouldn't be using a power supple capable of supplying lots of power when he doesn't know what hes doing with it anyway.

[DQW]

But aren't typical output impedances of the order of some milliohms ? How will that sufficiently serve as a current limiter ?

[MetaFrizzics]

But aren't typical output impedances of the order of some milliohms ? How will that sufficiently serve as a current limiter ?

that's right. While sophisticated power supplys do have current limiters, and very cheap power supplys may simply trickle-charge capacitors, most moderate and home-made supplys will need an external current limiter like a resistor for the simple reason that it is cheaper to just use a resistor than to repeatedly replace fuses or reset breakers. Why risk damage to your charging supply and cap when such a simple solution is readily available?

 

Also, this doesn't address home-made power supplys or projects he may build with the capacitors he has rescued from the old VCR. In these circuits he would be well advised to add resistors if they are not already indicated in the schematic he might be copying.

[Lance]

Also, considering your experience level, I would advise NOT attempting to charge any capacitors directly from mains, regardless whether you're using resistors and diodes.

 

I think the short answer to charging a capacitor is you really don't need anything other than the supply voltage itself. The long answer depends on how safe you want the set up to be (safe from component harm and bodily harm).

 

What were you planning to do with the charged cap?

 

Also, keep in mind that electrolytic caps do not like reverse voltage and will violently fail similarly to if you over volt them.

[YT2095]

just a quick tip, for current limiting in a Reactive manner, use a bulb in line as the resistor.

the bulb will also go out when it`s charged enough, thus providing a visual indication as well as reactive current limiting

[Douglas]

But aren't typical output impedances of the order of some milliohms ? How will that sufficiently serve as a current limiter ?

The capacitor is an open ckt, it doesn't matter if you charge it through the internal resistance of the supply or a 1 megohm resister.

[YT2095]

it`ll only affect the charge time.

[MetaFrizzics]

The capacitor is an open ckt, it doesn't matter if you charge it through the internal resistance of the supply or a 1 megohm resister.

Actually, this seems to be a misunderstanding of the function of an uncharged capacitor in a circuit.

 

In fact, uncharged caps act as *CLOSED*(short) circuits, not open circuits!

 

The transient voltages and currents that take place during rapid charging change rates rapidly following exponential curves.

 

There is no true steady-state 'D.C.' current in the behaviour of these circuits until the capacitor is actually fully charged up to close to its rating/and the voltage of the charging supply. Only then (when all the smoke has cleared and the cap is charged) will the circuit behave as a simple D.C. circuit with a counter-E.M.F. and almost zero current.

 

At the end of the charging cycle, you can indeed treat the capacitor as an 'open circuit' as far as relatively unchanging applied D.C. voltages is concerned. But getting there is the whole problem.

 

It'll only affect the charge time.

...and the safety and longevity of the circuit and components as well.

[YT2095]

...and the safety and longevity of the circuit and components as well.

nit-picking is Sooooo unecessary and adds nothing to your point at all, considering that this is largely theoretical in principal

[Douglas]

T=RC, as I recall, it takes about 2pi*t to charge a capacitor. There is no power supply or battery that can be damaged by putting a capacitor directly across the terminals. The supply will current limit for 1 time constant, but so what?

[calbiterol]

Also, considering your experience level, I would advise NOT attempting to charge any capacitors directly from mains, regardless whether you're using resistors and diodes.

 

Wasn't planning on it. If/when I do decide to do that, I'll have a chat with my dad, or get him to help - one of his bachelors is in electrical engineering.

 

What were you planning to do with the charged cap?

 

Also, keep in mind that electrolytic caps do not like reverse voltage and will violently fail similarly to if you over volt them.

 

I'm not entirely sure. Perhaps (seeing as I also have 2 photo-flash caps) build a simple (and very small) railgun. We shall see. I am pretty well-versed in safety, as far as electricity/electronics goes, and I know some basic theory, I just don't know as much as I'd like to.

 

As far as reverse voltage, do you mean reverse polarity? If yeah, then that's a bit of a no-brainer... Anyways, is there anything I should know about simultaneously discharging/charging cap's with different voltage ratings in the same circuit?

[YT2095]

flashgun caps have a fast duty cycle, so discharge isn`t a issue too much with them, getting up to the required (usualy 400+ volts) might be, you`ll need an oscilator and a step=up transformer and then a rectifier diode(s). after that the rest is really quite simple

[Thomas Kirby]

YT2095: Thank you, I was about to recommend the use of a light bulb myself.

 

Douglas, Lance: The voice of experience and knowledge can tell you this: There are many power supplies that can be damaged by hooking a large capacitor directly across the terminals. At best you might have a shut-down condition or blow a fuse. If you do something like that with a wall transformer, it is usually permanent. If your power supply isn't carefully protected, and a lot of hobbyist power supplies aren't, placing a discharged capacitor directly across the leads can zap the rectifier diodes, the series pass transistor, the three-terminal regulator, and in some cases even the wire itself before the capacitor charges. There is also the fact that it is difficult to protect fairly high current low voltage power supplies.

 

The price of one small part protects all of your electronics from damaging conditions. I've bought new resistors for as low as USD 0.015 each in small quantities.

[MetaFrizzics]

agreed. its not nitpicking, its a safety and economics issue.

 

By the way, when salvaging caps off old radios etc., you'll find many older caps which are loaded with PCBs, highly carcenogenic insulating oils. When these caps burn or explode, deadly toxins are released into the air as gases and vapours. These caps must be disposed by special 'toxic taxi' or industrial/hazardous waste centres, according to law.

 

Another reason not to be sloppy with old capacitors.

[calbiterol]

As far as the caps go, so far I have collected the following (all electrolytic):

2 photo-flash, rated at 330v160µF and 330v200µF;

1 rated at 200v120µF;

3 rated at 18v330µF;

1 rated at 6.3v1000µF;

1 rated at 200v4.7µF;

1 rated at 50v47µF;

1 rated at 50v4.7µF;

1 rated at 6.3v100µF

There's a bunch more smaller ones, as well as some that are of a different type (I suspect ceramic, see below) that I haven't harvested from the VCR yet. I'm really surprised at the difference a few millimeters makes in capacitance. The 50 volt caps, which vary in capacitance by a factor of ten, are - at most - a millimeter or two bigger in diameter. As far as transformers go, I still have the majority of the camera-flash circuitry left, and I was planning on pulling off the circuitry. Would an LED serve the same purpose as a bulb, or do I need an actual bulb?

 

They aren't very old. The VCR itself can't be more than 7 years old; my gut says it's closer to 5. Also, what's the difference between the mini-soda can caps and the ones that look like coins wrapped in blue tape? Are the former electrolytic and the latter... Ceramic? Tantalum (electrolytic)?

 

[Edit: my dad says that transformers only work on AC. How do they step up the voltage from a 1.5 volt battery to the 330v (it's probably more like 160 volts) needed to charge the capacitor quickly? I see what looks like a transformer on the circuit, but would they really go to the trouble of using an alternator on such a small circuit?]

[Lance]

First of all: Flash caps are no good for a railgun. The voltage is nowhere near high enough and you will just succeed in welding your projectile to the rails. They will however work very well for coilguns.

 

If you're planning on using these caps in any circuit that requires a fast discharge time such as a coil or rail gun then the vcr caps wont work. They will fail fairly quickly.

 

In photo-flash boards the 1.5 is pulsed through a transformer via the little transistor you see. For your first project I would recommend simply using the board itself to charge the caps rather than building a new power supply. You Don't even have to remove the cap from the board. Just solder some wire to the cap's leads.

 

Douglas' date=' Lance: The voice of experience and knowledge can tell you this: There are many power supplies that can be damaged by hooking a large capacitor directly across the terminals. At best you might have a shut-down condition or blow a fuse. If you do something like that with a wall transformer, it is usually permanent. If your power supply isn't carefully protected, and a lot of hobbyist power supplies aren't, placing a discharged capacitor directly across the leads can zap the rectifier diodes, the series pass transistor, the three-terminal regulator, and in some cases even the wire itself before the capacitor charges. There is also the fact that it is difficult to protect fairly high current low voltage power supplies.

[/quote']

 

Funny. I consider myself to have quite a bit of experience in this area and I have NEVER had any power supply fail from charging a capacitor with zero protection. The only power supplies that would have any problem with this were badly made. Even common wallwarts wont have any problem with this.

 

Keep in mind that all components also have a surge rating that is much higher than the continuous rating. Although it may seem to you that the a diode is receiving currents above it's rating usually the current is still below its surge rating.

 

Wall transformers are simply a transformer and a bridge with a cap in series with the output to limit current.

[MetaFrizzics]

The mini-soda cans are electrolytic caps. These can only be used in circuits which have a net D.C. voltage of about 30% or more of their continuous rating voltage, and in the correct orientation for polarity. Caps like this must have a minimum voltage applied to them in order to maintain an insulating layer between the plates, which is held in place by the electrostatic forces from the charge. If such caps are constantly run below about 30% of their rating, they will short out as electricity jumps across the gap and discharges, damaging the cap.

 

Some special caps are 'self-healing', that is, they burn off bits of themselves to prevent shorts from remaining more than a fraction of a second. However, each time an arc or internal spark occurs inside the cap, its total capacitance goes down, until it must be replaced as unreliable.

 

Other caps, like the 'blue pennies' are non-polar, usually mica or simple plates. These can be wired in any orientation, and will work in circuits without a D.C. charging voltage to keep them functioning properly. They can transmit A.C. signals and are often used as either D.C. blockers (allowing a.c. signals) or as high-speed clipping caps intended to take the edge off of on/off spikes.

 

Usually large caps are polar (electrolytic) and are used in D.C. power supplies. The small ones are signal caps, and can be lower capacitance and voltage since signals in most circuits are small voltages and large impedances.

 

There are actually about a dozen different kinds of caps but you will mostly see electrolytics and micas.

 

In speaker crossovers, you'll find non-polar electrolytics. These are made to pass A.C. voltages without needing to be charged up. They work by taking two ordinary electrolytic caps and wiring them in series internally back to back (+ to +). This allows the caps to form insulating barriers and also pass A.C. without a D.C. voltage superimposed upon them. But they are specialty caps and rarely found elsewhere.

[calbiterol]

First of all: Flash caps are no good for a railgun. The voltage is nowhere near high enough and you will just succeed in welding your projectile to the rails. They will however work very well for coilguns.

 

I dunno about that first part - I have heard of (and googled) a lot of experiments with photoflash caps being used for railguns, in addition to coilguns.

[YT2095]

coilguns yeah, they`re not too bad for low power coil types

I`m not sure about the rail gun though, I think you`de need considerably more "whack" than a photo-flash cap for a result (I`de like to be proven wrong though, I may just make a small one myself if that`s the case) )

[calbiterol]

Not just one, the ones that I've seen had about 15-20 wired parallel.

[Edit: here's one: http://cas.umkc.edu/physics/sps/projects/railgun/railgun.html ]

 

While I'm on the subject, can I charge multiple capacitors in parallel with one circuit? I ask because one of the charging circuits got fried. So I have one (partially) operational circuit, with most of the circuitry intact, and another that's fully operational.

[Lance]

Trust me on this, if you want a decent rail gun you're not going to be building it with flash caps.

 

Yes, you can. It won't be fast though.