Capacitor ESR how, high is too high? - UK Vintage Radio Repair and Restoration Discussion Forum

newlite4 · 31st Jan 2023, 6:48 pm

I have been testing some surplus car boot caps today, mostly 50s 350V 32uF types. Some brands seem OK at 1 to 1.5 ohms but others come in at around 6.5 ohms. Are the higher resistance ones useable or should they be discarded?
Neil

Cruisin Marine · 31st Jan 2023, 6:58 pm

Depends on the C value and the working voltage.
Here is one chart I just found quickly (there will be many more)
https://1.bp.blogspot.com/-o5LO3MoMO...0/esrchar2.jpg

newlite4 · 31st Jan 2023, 8:22 pm

Interesting chart. Seems as though the lower the voltage or the higher the capacitance the lower the ESR. Maybe the caps testing at 6.5 ohms may be OK.
Neil

Station X · 31st Jan 2023, 8:40 pm

When it comes to valve equipment I've never bothered about the ESR of electrolytic capacitors. So long as you don't have mains hum and the caps don't get warm they'll be OK.

If you want to check them out of circuit I suggest using a capacitor reformer to check the leakage current.

David G4EBT · 31st Jan 2023, 8:50 pm

There have been various tables and countless homebrew and commercial designs for both analogue and digital ESR meters over the years.

In the early 90s there was a design in Wireless World.

Some ten years or so ago various homebrew designs were also doing the rounds, and I posted my own homebrew analogue meter o the forum, but Chinese multi-testers have pretty much made such projects hardly worthwhile. At the time, I used a chart which Bob Parker, and ESR 'guru' in Australia had supplied. Similar charts give small variations.

Bob Parkers ESR meter site:

www.bobparker.net.au/esr_meter/esrhints.htm

I've attached an analogue meter dial from a typical homebrew meter and you will see that it's graduated 'SMPS' (for low ESR caps for use in switched mode power supplies), 'Good', 'Compare', which means compare the result with a new capacitor, and 'Bad'.

Do remember that with the 32uF 350V caps you have, which would typically be used as reservoir and smoothing capacitors in valve radios and amplifiers, it isn't ESR which you should be concerned about, but leakage, which is really parallel resistance. You can only measure than with the capacitor in circuit operating at its rated voltage.

In a new good condition 32 uF capacitor at its rated voltage, the leakage should be under 1mA, and as a general rule, when reforming an existing one, the maximum permitted would be no more than 1 mA per 30 uF. See:

https://www.vintage-radio.com/repair...%20each%2030uF

I recently had a friend who couldn't understand why the anode and screen voltages of a radio were low, yet with by putting a voltmeter across the HT load resistor, noting the voltage drop and working out the HT current flowing, it seemed about right. The reason the anode and screen voltages were low was because some of the current was going to ground via a leaky smoothing capacitor, which had signs of bulging at the end. A replacement cap was fitted and the radio worked fine. (No prizes for guessing what radio it was!).

31st Jan 2023, 6:48 pm	#1
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,732	Capacitor ESR how, high is too high? I have been testing some surplus car boot caps today, mostly 50s 350V 32uF types. Some brands seem OK at 1 to 1.5 ohms but others come in at around 6.5 ohms. Are the higher resistance ones useable or should they be discarded? Neil __________________ preserving the recent past, for the distant future.

31st Jan 2023, 6:58 pm	#2
Cruisin Marine Heptode Join Date: Jul 2018 Location: South Coast, UK Posts: 730	Re: Capacitor ESR how, high is too high? Depends on the C value and the working voltage. Here is one chart I just found quickly (there will be many more) https://1.bp.blogspot.com/-o5LO3MoMO...0/esrchar2.jpg __________________ "Behind every crowd, there's a silver Moonshine"

31st Jan 2023, 8:22 pm	#3
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,732	Re: Capacitor ESR how, high is too high? Interesting chart. Seems as though the lower the voltage or the higher the capacitance the lower the ESR. Maybe the caps testing at 6.5 ohms may be OK. Neil __________________ preserving the recent past, for the distant future.

31st Jan 2023, 8:40 pm	#4
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 19,896	Re: Capacitor ESR how, high is too high? When it comes to valve equipment I've never bothered about the ESR of electrolytic capacitors. So long as you don't have mains hum and the caps don't get warm they'll be OK. If you want to check them out of circuit I suggest using a capacitor reformer to check the leakage current. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

31st Jan 2023, 8:50 pm	#5
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,212	Re: Capacitor ESR how, high is too high? There have been various tables and countless homebrew and commercial designs for both analogue and digital ESR meters over the years. In the early 90s there was a design in Wireless World. Some ten years or so ago various homebrew designs were also doing the rounds, and I posted my own homebrew analogue meter o the forum, but Chinese multi-testers have pretty much made such projects hardly worthwhile. At the time, I used a chart which Bob Parker, and ESR 'guru' in Australia had supplied. Similar charts give small variations. Bob Parkers ESR meter site: www.bobparker.net.au/esr_meter/esrhints.htm I've attached an analogue meter dial from a typical homebrew meter and you will see that it's graduated 'SMPS' (for low ESR caps for use in switched mode power supplies), 'Good', 'Compare', which means compare the result with a new capacitor, and 'Bad'. Do remember that with the 32uF 350V caps you have, which would typically be used as reservoir and smoothing capacitors in valve radios and amplifiers, it isn't ESR which you should be concerned about, but leakage, which is really parallel resistance. You can only measure than with the capacitor in circuit operating at its rated voltage. In a new good condition 32 uF capacitor at its rated voltage, the leakage should be under 1mA, and as a general rule, when reforming an existing one, the maximum permitted would be no more than 1 mA per 30 uF. See: https://www.vintage-radio.com/repair...%20each%2030uF I recently had a friend who couldn't understand why the anode and screen voltages of a radio were low, yet with by putting a voltmeter across the HT load resistor, noting the voltage drop and working out the HT current flowing, it seemed about right. The reason the anode and screen voltages were low was because some of the current was going to ground via a leaky smoothing capacitor, which had signs of bulging at the end. A replacement cap was fitted and the radio worked fine. (No prizes for guessing what radio it was!). __________________ David. BVWS Member. G-QRP Club member 1339.