Capacitor Ripple current. - UK Vintage Radio Repair and Restoration Discussion Forum

PWH · 5th Mar 2006, 9:46 pm

Hi. Is there an easy way of working out the ripple current of a capacitor?

The ones I have only mention their value and working voltage. Some look way too small to withstand a rectifier across its terminals!

YC-156 · 5th Mar 2006, 10:11 pm

To my knowledge there is no general way of determining maximum ripple current for a given cap. It depends on the materials/chemicals used, and its internal physical construction, among other parameters. Two seemingly similar sized capacitors of same voltage and capacity rating can have very different ripple current tolerances. Check Farnell's online catalog under electrolytic caps for examples of this.

Hope this helps a bit.

Frank N.

paulsherwin · 6th Mar 2006, 12:59 am

While it's good to be aware of the ripple current issue, I would make the following two points.

Firstly, I've never had a replacement capacitor fail in a restored radio, for excessive ripple current or any other reason. They run completely cool. That's with standard commodity electrolytics that I have to hand, a mixture of manufacturers and ratings. The position may be different with equipment that draws a much higher HT current, such as a power amplifier or transmitter.

Secondly, modern electrolytics are designed to vent pressure in a controlled manner rather than explode if they overheat, so a failed electrolytic won't be a catastrophe, especially as it's likely to be mounted under the chassis.

Best regards, Paul

PWH · 6th Mar 2006, 8:20 am

Thanks. Would there be a hum problem if the ripple current was underrated?

I have just replaced some 4uF caps with new tiny 4.7uF @ 400V in a DKE38 german radio and it seems to be more 'hummy' than other types of radio. Mind you its a very simple circuit.

oldeurope · 6th Mar 2006, 9:03 am

In this case an MKP is a better replacement than an electrolytic.
Don't change values in this set, it is hum compensated!

Kind regards,
Darius

Ed_Dinning · 6th Mar 2006, 9:04 am

Hi Peter, the other correspondants have covered the topic, but I would add that ripple current is generally more important in switch mode power supplies where the frequency is much higher and the ripple effect correspondingly greater. The only way you can check, without the capacitor data sheet and a means of measuring the ripple current THROUGH the capacitor, is to look at it's temperature. Most caps are rated to 85C, some to 105C; this is usually stated on the sleeve. The life of the cap follows the usuall law. reduce temp by 10C and double cap life.
In practice you should always be able to keep your fingers on the cap can, even after many hours of use.

Your new caps may have a higher ESR (which is generally related to ripple current), but this is unlikely. Check if you have connected them in the same position and that andy clamping bolts for negative connections are tight and corrosion free. Chassis return currents going in a different direction can be a vause of hum.

HTH Ed

paulsherwin · 6th Mar 2006, 9:42 pm

Quote:

Originally Posted by peterwhughes

Thanks. Would there be a hum problem if the ripple current was underrated?

No, the caps just run warm and have a short life. If severely underrated they'd get very hot very quickly and vent lots of hot gunk - you'd certainly know about it!

Best regards, Paul

PWH · 7th Mar 2006, 10:12 pm

quote "In this case an MKP is a better replacement than an electrolytic.
Don't change values in this set, it is hum compensated!"

What is an MKP sorry for my ignorance! Will 0.7uF make a difference to this set? It does hum quite a lot and if I null it out with no signal using the variable resistor on the back, the hum is very prominent when tuned in. I may be drifting off topic but if the author of this message could PM me perhaps i can sort out my underlying problem

thanks all

ppppenguin · 8th Mar 2006, 8:26 am

MKP: I think we have a German to English translation problem here. my guess is that MKP is some kind of plastic dielectric capacitor. I really don't know what kind. Maybe motor run or PFC compensation? This will certainly have an ample ripple current rating.

With modern dielectric materials a 4uF motor run capacitor might be no bigger than an ancient 4uF electrolytic.

jjl · 8th Mar 2006, 11:59 am

Darius will probably correct me if I am wrong, but my understanding is that

MKP is metallised polypropylene
FKP is foil & polypropylene
MKT is metallised polyester

John

oldeurope · 8th Mar 2006, 5:16 pm

Quote:

Originally Posted by jjl

Darius will probably correct me if I am wrong, but my understanding is that

MKP is metallised polypropylene
FKP is foil & polypropylene
MKT is metallised polyester

John

Yes, that is it John. Thanks!

Darius

5th Mar 2006, 9:46 pm	#1
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	Capacitor Ripple current. Hi. Is there an easy way of working out the ripple current of a capacitor? The ones I have only mention their value and working voltage. Some look way too small to withstand a rectifier across its terminals!

5th Mar 2006, 10:11 pm	#2
YC-156 Retired Dormant Member Join Date: May 2005 Location: Aarhus, Denmark Posts: 281	Re: Capacitor Ripple current. To my knowledge there is no general way of determining maximum ripple current for a given cap. It depends on the materials/chemicals used, and its internal physical construction, among other parameters. Two seemingly similar sized capacitors of same voltage and capacity rating can have very different ripple current tolerances. Check Farnell's online catalog under electrolytic caps for examples of this. Hope this helps a bit. Frank N.

6th Mar 2006, 12:59 am	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,957	Re: Capacitor Ripple current. While it's good to be aware of the ripple current issue, I would make the following two points. Firstly, I've never had a replacement capacitor fail in a restored radio, for excessive ripple current or any other reason. They run completely cool. That's with standard commodity electrolytics that I have to hand, a mixture of manufacturers and ratings. The position may be different with equipment that draws a much higher HT current, such as a power amplifier or transmitter. Secondly, modern electrolytics are designed to vent pressure in a controlled manner rather than explode if they overheat, so a failed electrolytic won't be a catastrophe, especially as it's likely to be mounted under the chassis. Best regards, Paul

6th Mar 2006, 8:20 am	#4
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	Re: Capacitor Ripple current. Thanks. Would there be a hum problem if the ripple current was underrated? I have just replaced some 4uF caps with new tiny 4.7uF @ 400V in a DKE38 german radio and it seems to be more 'hummy' than other types of radio. Mind you its a very simple circuit.

6th Mar 2006, 9:03 am	#5
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: Capacitor Ripple current. In this case an MKP is a better replacement than an electrolytic. Don't change values in this set, it is hum compensated! Kind regards, Darius

6th Mar 2006, 9:04 am	#6
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Capacitor Ripple current. Hi Peter, the other correspondants have covered the topic, but I would add that ripple current is generally more important in switch mode power supplies where the frequency is much higher and the ripple effect correspondingly greater. The only way you can check, without the capacitor data sheet and a means of measuring the ripple current THROUGH the capacitor, is to look at it's temperature. Most caps are rated to 85C, some to 105C; this is usually stated on the sleeve. The life of the cap follows the usuall law. reduce temp by 10C and double cap life. In practice you should always be able to keep your fingers on the cap can, even after many hours of use. Your new caps may have a higher ESR (which is generally related to ripple current), but this is unlikely. Check if you have connected them in the same position and that andy clamping bolts for negative connections are tight and corrosion free. Chassis return currents going in a different direction can be a vause of hum. HTH Ed

7th Mar 2006, 10:12 pm	#8
PWH Heptode Join Date: Mar 2003 Location: Waiheke Island, New Zealand Posts: 503	Re: Capacitor Ripple current. quote "In this case an MKP is a better replacement than an electrolytic. Don't change values in this set, it is hum compensated!" What is an MKP sorry for my ignorance! Will 0.7uF make a difference to this set? It does hum quite a lot and if I null it out with no signal using the variable resistor on the back, the hum is very prominent when tuned in. I may be drifting off topic but if the author of this message could PM me perhaps i can sort out my underlying problem thanks all

8th Mar 2006, 8:26 am	#9
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Capacitor Ripple current. MKP: I think we have a German to English translation problem here. my guess is that MKP is some kind of plastic dielectric capacitor. I really don't know what kind. Maybe motor run or PFC compensation? This will certainly have an ample ripple current rating. With modern dielectric materials a 4uF motor run capacitor might be no bigger than an ancient 4uF electrolytic.

8th Mar 2006, 11:59 am	#10
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: Capacitor Ripple current. Darius will probably correct me if I am wrong, but my understanding is that MKP is metallised polypropylene FKP is foil & polypropylene MKT is metallised polyester John