Cap Re-former

This is one I made for my own use, I am not an expert with circuit design and others may be able to advise on improvements.
I added the zener diode to limit the voltage across the meter and its series resistor to about 5v.
I also added an on off switch by using the other pole of the discharge switch. Not shown in photo.

When the current drops to below 1 mA I find the capacitor is probably ready for use. I normally check it by doing a discharge test as well.

Mike

Yes, looks good.

I happen to have some shaver transformers I got at €4 each and a spare new 1mA meter.

I just fit a resistor to the preamp power supply on a valve amplifier and it provides entertainment while the capacitor is reforming. An AVO 9 provides the metering as it does not waste batteries on these sort of long monitoring jobs.

It's also useful to have some means of regulating the output voltage. At the start, a large current might flow. If the cap reforms slowly it will heat up which is not good. In practice I just run mine from a variac and slowly increase the input voltage, keeping the reforming current under say 10mA.

Hi Andy,

As a relatively newcomer to the cap reforming brigade, I've spent considerable time reading many posts elsewhere on the subject, digesting the many views and arguments that surround this subject.

The conclusion was that many people have many opinions and many solutions, but no one solution is the definitive one. It is a confusing minefield for newcomers due to all those conflicting viewpoints and methods.

The following is just my observations and what has worked for me. I'm not advocating it, just reporting what I did and my findings.

I spent the entire weekend reforming a couple of 400uF+400uF caps, these have been the most stubborn ones I've ever had to reform, the pair took a little over 48hours, they now seem to have reformed fine. In most cases I have put an NOS cap on the Hunts run it through the voltage cycles and in under 4-5 mins the cap is good to go, these however were not having any of that.

To clarify I use my Hunts CRB-3, I fed the cap via the first range 1. 20-100V only moving on to the next ranges of 2. 100-200V, 3. 200-300V, 4. 300-400V, 5. 400-500V ( did not use this stage for this cap) when the previous one had been completed, which is when the neon extinguishes and the voltage for that stage has been achieved.

In the example here I monitored it via three methods, the Hunts neon, the voltage being held and leakage current. Until recently I never bothered monitoring the leakage current as the other two seemed enough. I stand corrected on this matter by Mike W so I've adapted my process to include an additional meter. Although I think personally I was getting the info I needed by observation of the first two indicators, namely voltage rise and neon behaviour.

Anyway back to the process, on the first range 20-100V it took a little over 2 hours to reach 100V , the Neon was very bright at the outset and the leakage current showed as 1.9mA. As time passed and the voltage held increased, the neon decreased in brightness, flickered and then extinguished. Whilst that was occurring the leakage current was dropping all the time until the meter read zero, no current being drawn.

At this point I moved on to the next stage (100-200V) and the whole process was repeated until the cap was running at rated voltage, in this case 325V and this is where all the time was taken up. On each of the stages the neon was indicating leakage by being extremely bright then changing its state as the voltage grew. The voltage was very slow to climb but it was in a steady upwards direction, the leakage each time showed as 1.9mA dropping to .1mA when the neon extinguished, that's when I moved to the next stage (200-300V). Whilst this was going on I frequently checked the cap was not hot, it was cool temperature wise throughout.

Once the final stage was running and it reached the rated voltage, I then ran through a cycle of charging and discharging. Each time this was done the cap would reach rated voltage in a much quicker period than before. Now the caps reach full voltage charge in under 40 seconds.

As I said at the beginning I'm not advocating this method just reporting what I did, this is just my way and it worked for me. I thought it might be useful to share in this thread which although looking for a circuit, seems to have developed into discussing cap reforming as a process in general.

Andy I'm sure there are many opinions, many methods and whatever one you use that works for you is good. I look forward to you sharing your project when you decide on which one to build. I found the comment below by a very competent and respected engineer, to be useful on the subject in hand for me, others may disagree with his statement.

Everyone seems keen to measure the current. I regard this as a bit pointless.

Firstly, you must limit the current to a maximum small value or you just don't do the capacitor any good. For most high voltage capacitors you should not exceed about 10mA (says a number of expert sources). Make a circuit that just can't deliver more than that, and don't let the capacitor get hot.

The best way to monitor progress is to measure the voltage using a high-impedance (i.e. electronic) meter. This tells you the working voltage that the capacitor has reached (given the current isn't large).

The point is this: it takes quite a small quantity of electricity to actually form the dielectric layer. All the rest is just burning up the electrolyte, never to be replaced. The aim is to make progress in building up the holes in the dielectric without too much electrolyte loss. If the voltage is just about rising then you are doing it, so that is the aim. When it reaches the required working voltage then you are done. Extra current speeds the process up at a cost of damaging the capacitor for the long-term.

If you are impatient, I have found that cycling the charge on the capacitor, by fully discharging it from time to time, often makes the process faster.

Sorry to have waffled on with this long monologue, but finally a picture of my hideous lash-up. Loads of bare exposed potential dangers and everywhere

It's also useful to have some means of regulating the output voltage. At the start, a large current might flow. If the cap reforms slowly it will heat up which is not good. In practice I just run mine from a variac and slowly increase the input voltage, keeping the reforming current under say 10mA.

I also sometimes run my "reformer" via the variac to "ease the cap in".
But I find it useful to have a dedicated reformer, so that other power supplies are not taken up for what can sometimes be days on end.
I use my variable 30v bench supply to ease in some "old" transistor sets.
Mike

My first reformer (built into Tesco wooden bread bin) was simply a variac a BY127 diode and a 5MA meter
later fitted with a switchable neon .it was arranged that the reformer bit could be turned off then I had a boxed variac.
PS had to use an external volt meter
Rob t

I think if you have a simple current limiter (1 to 5mA that can cope with a short circuit) and supply voltage equal to rated voltage (or less, up to half as much may do) then just measuring the voltage works too. But the 1mA FSD meter with protection (a pair of back to back 1N4148 or 1N400x assuming a short is less than 10mA) does let you see the Quality.

Assume the capacitor initially is a short circuit in calculating parts.

I can theorise that half wave is better than DC.

Also once current is under 100uA there may be advantage in short period (current limited to 500uA) at 20% more than rated voltage.

I use a 100K 2W resistor and an ordinary HT power supply. For lower voltage capacitors in the 100V to 250V range I just plug into the Variac.
If I am doing something else while the reformer is working I add an AVO for glancing at as it carries on its work. Sometimes I add a diode and capacitor and plug directly into an isolating transformer.
I have had some capacitors that have taken three days.

here is a proto type using a nixie tube HT buck supply

robt

I have given some thought to making such a circuit using a sine wave oscillator with the coil from an old wall wart in it.
It would run from either 6V or 9V or perhaps 12V depending on the type of batteries to hand.
Sine wave being chosen so that it could also be adapted to run the HT in a battery radio.

I have given some thought to making such a circuit using a sine wave oscillator with the coil from an old wall wart in it.
It would run from either 6V or 9V or perhaps 12V depending on the type of batteries to hand.
Sine wave being chosen so that it could also be adapted to run the HT in a battery radio.

this will run quite well from a 9volt battery supply the current is very limited at 280 volt output but that was the intention. great for checking caps but takes a time to reform electrolytics but I have not had one fail after reform and the cap being reformed never gets hot.
Rob t

I find that reforming is best at about 1W for a large smoothing can from a mains set.
Just take the voltage you measure across it and multiply by the currant. Using a resistor in series gives something not that far from constant power until the capacitor is almost fully reformed.

hi Rob
I like the idea of the neon, where does it go in the circuit? Have you got a drawing of the circuit to show the neon and any limiting resistors.
Cheers
Mike

hi Rob
I like the idea of the neon, where does it go in the circuit? Have you got a drawing of the circuit to show the neon and any limiting resistors.
Cheers
Mike

the neon is simply wired In series with the cap being tested, obviously will not work below 70 volts but because the current is so low(on this design) low voltage caps wont suffer.
rob t
PS I will dig out the circuit.

How much current can a small neon pass.
I thought a a neon tended to regulate a voltage across it to about 70v. So would this voltage across the neon effect the voltage supplied to the cap being reformed. I think I need a circuit and a little help to understand this.
Mike

Yes it will reduce HT by about 65V. Neons are about 65V on and about 90V strike. Sustain currents vary. This is why with parallel capacitor and high enough series resistor you get a sawtooth generator with about 25V pk to pk amplitude and 65v Offset.

Larger modern neons rated about 2mA max, smaller ones 1mA approx. At high currents (More than 500uA) they blacken faster and at higher still they may crack or pop. Typical series resistor in mains indicators about 470K, approximately 0.35mA (350uA). Old appliances may use 220K. Ensure resistors have adequate pulse voltage ratings.

I would only put a neon in series with a capacitor with a 220K minimum series resistor, ideally 470K. A leakage tester for capacitors up to maybe 10uF and non-electrolytic (i.e. paper, plastic film, ceramic, mica etc) would maybe use 200V to 600V HT, a 1M to 2M* series resistor to capacitor under test terminal A and then terminal B returning via neon and parallel 100nF 100V rated (min) very low leakage capacitor (poly film most types or ceramic). With a short the neon flashes more than 10Hz, appears solid on. With 100M Ohms it flashes slowly and 10 M Ohms about 1s (adjust parallel capacitance for 1/10th Hz at 100M Ohm if you like).

(* I use a pair of 1M resistors in series to ensure safe operation with 600V HT. Note that with NO capacitor and 2M it's doubtful that you can even feel a voltage on the terminals but a 2uF Motor capacitor tested for leakage, if good could be lethal!)

So am I right in thinking that using a neon in series will limit the current (leakage) in the capacitor to say 500uA. depending on the series resistor used for the neon. And that the neon will go out and stop conducting altogether when the capacitor voltage is approximately 50-65v less than the supply voltage. To me that seems it will cause the capacitor to take much longer to reform, less energy going into capacitor, and not give a good indication of when the capacitor is fully reformed i.e. the voltage across the series resistor is practically zero. Or in other words the voltage on the capacitor is almost the same as the supply.
Or have I got the wrong idea.
Mike

HI mike I added the neon to mine not for reforming caps but as a simple leakage test mainly for paper caps.
I did say in the post switched ,what I meant was a simple switch in parallel with the neon that takes it out of circuit for reforming purposes.
rob t