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Old 20th Sep 2018, 11:14 pm   #1
Philips210
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Default Electrolytic capacitor reformer meter circuits

Hi.

Many years ago I built the Radio Constructor electrolytic capacitor reformer which was originally published in the May 1969 issue. This unit has proved to be invaluable and whilst some question the point of reforming old electrolytics I can vouch for its effectiveness.
I decided to add meters for both capacitor voltage and leakage current. I had a couple of Monocor moving coil meters to hand which were pressed into service.
For voltage measurement, I used a 50uA FSD meter which has a coil resistance of 3k Ohm. I decided on having four ranges 0 to 10V, 0 to 50V, 0 to 100V and 0 to 500V which should cover all possible requirements. Calculations were made for the necessary multiplier resistors and testing revealed quite accurate results.
Similarly, for current measurement, I used a 100uA FSD meter. This has a coil resistance of 1k Ohm. Three ranges were thought to be adequate viz 0 to 100uA, 0 to 1mA and 0 to 10mA. The shunt resistors for the 1mA and 10mA ranges were then calculated. Again, this proved to be quite accurate when compared with my Fluke 23 multi-meter.

Something worth considering, is the switch for the current range ought to be a make before break MBB type to protect the meter. The voltage range switch should conversely be a break before make (BBM) type again to protect the meter.


By having meters for voltage and current, the reformer is also particularly useful for checking non electrolytic capacitors for leakage by subjecting them to their maximum working voltage whilst monitoring the current.

Attached pics showing the original RC circuit diagram, my meter circuit add-on, and some pics of the completed unit.

I hope these notes will be useful.

Regards
Symon
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Old 21st Sep 2018, 12:00 am   #2
Craig Sawyers
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Default Re: Electrolytic capacitor reformer meter circuits

That is a really neat implementation of the classic reformer schematic.

Craig
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Old 21st Sep 2018, 12:00 pm   #3
Philips210
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Default Re: Electrolytic capacitor reformer meter circuits

Thanks for that Craig.

Regards
Symon.
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Old 21st Sep 2018, 12:02 pm   #4
Philips210
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Default Re: Electrolytic capacitor reformer meter circuits

Hi

In the attached pics, I've shown a couple of worked examples for calculating the values of the shunt and multiplier resistors. I used trimpots to achieve good accuracy and replaced these by selected fixed resistors.

Regards
Symon
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Old 21st Sep 2018, 1:12 pm   #5
sven_swe
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Default Re: Electrolytic capacitor reformer meter circuits

What a great build and thanks for the notes, very helpful!
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Old 21st Sep 2018, 2:39 pm   #6
Nickthedentist
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Default Re: Electrolytic capacitor reformer meter circuits

Very nice.

My reformer has two little MC meters which I salvaged from the tape deck part of a 1970s music centre. They still have their original VU meter scales, and as a temporary measure, I pencilled-on a few useful values, pending making proper replacement scales. That was 10 years ago!

With halfway decent multimeters being available for not very much (see Mark Hennesey's site), I would be sorely tempted to simply attach a couple to any reformer I built nowadays, though your way is so much more elegant.

N.
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Old 21st Sep 2018, 4:13 pm   #7
Philips210
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Default Re: Electrolytic capacitor reformer meter circuits

Hi.

Thanks everyone for the positive feedback, it's much appreciated. I hope the notes will inspire others to build something similar.

A few other points about the reformer:

The mains transformer came from a German valve radio, I think it may have been a Telefunken model. A 100mAT fuse protects the HT winding. The heater winding not being used, though I guess it could be a useful 6.3V ac supply. I may well add this feature by way of a couple of 4mm sockets.

The selenium rectifiers in the original circuit were replaced by BY133 silicon diodes with a series 560 Ohm 11W wirewound resistor. This value will depend on the transformer's HT winding and will need to be experimented with to obtain the correct HT voltage across the divider chain. I placed a 2n2 Y ceramic cap in parallel with the rectifier in both cases to help reduce RFI to other equipment, (ie my AM radios) not that it really matters that much.

To keep reasonably close to the specified capacitor values in the voltage doubler circuit, I used two 33uF 400V electrolytics in series to achieve the quoted 16uF values. I should have included voltage sharing resistors across each cap to avoid the possibility of exceeding the working voltage of the cap. In practice, the Panasonic caps were well matched so there have been no problems. It's not difficult to calculate the required sharing resistor values, typically 10M Ohm will be suitable in this case.

The 2k Ohm wirewound resistors in the potential divider generate a fair amount of heat if the unit is on for extended periods. For each, I used two 1k Ohm Vitreous wirewound types in series from Welwyn, type W23. They're all laid out on the tagboard but standing clear of the base to allow air circulation, so good ventilation in the case is preferable. The wiring to the divider tagboard has silicone coated fibre glass sleeving.

The main difficulty with building this reformer will be sourcing a decent 2 pole 12 way rotary switch. I was lucky to have something suitable in my junk box of spares. The mains transformer will probably be best taken from a write off valve radio.

Having the meters fitted gives you the reassurance that reforming process is going to plan or not.

Quite a lot of the time, the reformer is used for checking paper caps for leakage and it certainly has weeded out any below par examples. It could be regarded as being an insulation tester but with more ranges. It shows up problems with those Hunts paper types and those now grotty mixed dielectric caps. Most of the latter are swollen/cracked in appearance. It's also worth seeing how much those cracked Rifa class X types leak. Some examples appear fine while others are terrible. Not that I'd want to risk using any across the mains in any case!

Regards
Symon.
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Old 21st Sep 2018, 8:26 pm   #8
TrevorG3VLF
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Default Re: Electrolytic capacitor reformer meter circuits

To keep reasonably close to the specified capacitor values in the voltage doubler circuit, I used two 33uF 400V electrolytics in series to achieve the quoted 16uF values. I should have included voltage sharing resistors across each cap to avoid the possibility of exceeding the working voltage of the cap. In practice, the Panasonic caps were well matched so there have been no problems. It's not difficult to calculate the required sharing resistor values, typically 10M Ohm will be suitable in this case.


You do not need voltage sharing resistors, the circuit is a pair of half wave circuits in series, there is no way the normal voltage can be exeeded.

Why is a smoothed supply required in a reformer? The capacitors are needed for the doubler of course.
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Old 21st Sep 2018, 9:01 pm   #9
Philips210
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Default Re: Electrolytic capacitor reformer meter circuits

Yes, I now see it's true that the working voltage of the electrolytics is not going to be exceeded but it is normally good practice to fit voltage sharing resistors.

Regards
Symon
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Old 24th Sep 2018, 7:02 pm   #10
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Default Re: Electrolytic capacitor reformer meter circuits

I have just come across this thread.
I recently decided I needed a capacitor reformer and decided to use the circuit Radio Constructor circuit as a basis, but like Symon I wanted added meters.

My wish list was:
• It should use as many components from the junk box as possible
• Since the objective is to prevent the bang at switch on by reforming the oxide layer it is not necessary to use the exact rated voltage but a minimum of four voltage settings should be available.
• Voltages below 60V are not needed as I have a bench power supply that goes up to 60V that I can use.
• The voltages should be derived either by transformer taps or by doubling as I want to minimise the heat and wanted to avoid dropper chains.
• A current meter with ranges of 10mA, 1mA and 100uA is desirable to indicate progress and final leakage but the flashing neon is unnecessary.
• Volt meters to indicate the HT volts and test capacitor volts are desirable.

The biggest problem was the transformer, while I do have some transformers in the junk box from old valve tape recorders I have stripped for parts, I want to keep them for audio projects.
I did however have a couple of spare transformers with dual 12V secondaries, one with dual 120V primaries so decided to use the old trick of running them back-to-back with the secondaries connected together and the dual 120V windings providing isolated high voltage outputs – 120V or 240V if connected in series.
Additionally if the “output” transformer is under-run by feeding its (series connected 12V) 24V winding with 12V from the “input” transformer I have 60V available from one of the 120V windings.
Finally I decided I could either simply rectify or double the 240V output.
After rectification I now had the following voltages available
• 82V
• 165V
• 330V
• 650V

All the above are actual measured voltages, off load, from the finished unit.
For the Doubling circuit I used a trick I discovered when I was a design engineer some 40 years ago, I do not claim originality but have not seen it used elsewhere.
See the pdf of the (hand drawn I am afraid) schematic..
It uses four diodes connected in a standard bridge configuration but with two reservoir capacitors connected in series (plus voltage sharing resistors). One end of the transformer winding is connected to the bridge circuit as normal. The other end goes to the common connection of a simple changeover switch. The changeover switch connects the other end of transformer winding either to the diodes as a standard bridge, or to the junction of the reservoir capacitors to form a doubler circuit. When used in the doubler configuration the two unused diodes are reverse biased and essentially out of circuit.
The transformer tap selection and Bridge/Doubler switching I combined into a 3 pole 4 way rotary switch to make the voltage selection easier.

For the voltage measurement I used two, modified, three digit DVM modules.
The basic module is available from the well-known auction site and is described as “Three Wire LED Panel Meter Mini Digital Voltmeter DC 0V To 99.9V”. They cost around two or three pounds. They have three wires, Black is common 0V, Red is a power input for the meter and takes 4 to 30V, the yellow wire is the measuring input, 0-100V.
As supplied the meter has a relatively low input impedance, I wanted to connect a meter directly across the capacitor under test so needed to minimise the current drawn by the meter. The IC used in these modules has an input sensitivity of 2V for full scale and uses a potential divider to scale the input to 100V. To increase the input impedance it is necessary to change the lower resistor of the divider from 2K to 220K. Additional resistors can now be added in series with the yellow lead, two 47M ohm and two 10M ohm in series to give a full range sensitivity of 999V. This will give a current drain of about 1uA per 100V which will be indicated on the meter, at 650V with no capacitor it will give a “standing” reading of 6.5uA. I decided that the advantage of seeing the capacitor voltage outweighed the disadvantage of the standing current.
I also cut the track to pin 3 of the display which is the decimal point LED.
I attach photos to show which resistor needs to be changed. This a surface mount resistor and quite fiddly to change, I managed to fit a standard resistor rather than SM types.
The meter supply volts I derived by half wave rectifying one of the 12V secondaries on the input transformer, while not strictly necessary, I also added a 12V regulator to stabilise the supply.
Since I was not using the neon indicator I decided to only use one series current limiting resistor (in the original design the series resistor is chosen to drop about 60V at the desired final leakage current which would result in the neon going out). I chose to use 81K made of three 27K, 2W resistors in series. This gives a maximum available current at 650V of 8mA.
For the current meter I used a movement from an old cheap analogue multimeter, this has an f.s.d of 41.1uA and a resistance of 2K ohm. Shunts were calculated to give the required ranges. The current selection switching is slightly complex due the fact that I only had break-before-make switches to hand. If make-before-break are available it would make the switching much easier.

Finally a biased switch was used to switch the plus end of the capacitor under test from the reforming circuit to a load consisting of two 2K2 2W resistors in series to discharge it.

I have successfully used this to reform the 16uF capacitors on my Mullard High Speed Valve Tester (Although I did later change them anyway)

I attach a pdf of the final circuit and some photos of the finished unit. Sorry about the quality of the hand drawn schematic, I drew it for myself, not for sharing. If anyone is interested I will redraw it on the computer and post it.
Peter
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Old 25th Sep 2018, 12:30 pm   #11
crackle
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Default Re: Electrolytic capacitor reformer meter circuits

If anybody wants to follow the original Radio Constructor project it can be found on page 664 here https://www.americanradiohistory.com...RC-1969-05.pdf

Mike
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Old 25th Sep 2018, 11:03 pm   #12
Philips210
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Default Re: Electrolytic capacitor reformer meter circuits

Hi

That's a very useful reformer Peter. I like the voltage doubling technique you've employed and the digital voltmeter modules. The back to back mains transformers is also a good alternative too. Thanks for sharing the details.

Regards
Symon
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Old 30th Oct 2018, 4:14 pm   #13
avocollector
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Default Re: Electrolytic capacitor reformer meter circuits

Quote:
Originally Posted by crackle View Post
If anybody wants to follow the original Radio Constructor project it can be found on page 664 here https://www.americanradiohistory.com...RC-1969-05.pdf

Mike
Thanks for the link Mike - was looking for something similar.
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Old 30th Oct 2018, 10:20 pm   #14
Ed_Dinning
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Default Re: Electrolytic capacitor reformer meter circuits

Hi Symon, I built a similar unit some years ago and used a variac and a 240/650v transformer.
The switching is interlocked to discharge the caps at switch off and I used diodes across the meters to prevent damage on overloads or large reversals at switch off.

Ed
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