Dropper capacitor replacement - UK Vintage Radio Repair and Restoration Discussion Forum

Tractorfan · 5th Oct 2014, 8:16 pm

Hi,
For a while now we have been using an 8 watt fluorescent inspection lamp for illumination when camping. Last week it finally failed and investigation showed that the dropper capacitor had ejected its dielectric through a split case. The markings say '225J' and '250v'. this lamp was made long before the advent of X2 caps.
I tried replacing it with a modern 0.22uF 275VAC X2 device,but the tube now only glows very dimly (I tried a new one). So, what value would be the modern equivalent of '225J'?
Thanks.
Cheers, Pete.

Al (astral highway) · 5th Oct 2014, 8:26 pm

Erm, it is 2.2uF, so ten times larger than the one you have used.

paulsherwin · 5th Oct 2014, 8:42 pm

Exactly.

You may find a motor run capacitor is the most cost effective replacement if you can find one that physically fits.

Tractorfan · 6th Oct 2014, 9:24 am

Hi,
Many thanks for your help. I've always had a 'blind spot' when it came to capacitor markings.

I have amassed a small collection of X2 caps salvaged from various dead appliances, and I think there's a 2.2uF among them somewhere.
Cheers, Pete.

David G4EBT · 6th Oct 2014, 10:43 am

If you don't trawl one up Pete, you can get them here for £1.96 inc VAT, but with an MOQ of five.

Still, at least orders over a fiver ex vat are post free in the UK:

http://cpc.farnell.com/1/1/87153-cap...25m-epcos.html

Maarten · 6th Oct 2014, 3:28 pm

2u2 X2 will be hard to find. Given the original was a 250VDC capacitor and lasted reasonably long, a 630VDC MKP (not MKT) capacitor will be a good replacement in this particular case (better than original, while an X2 rated capacitor may do worse than the original since most aren't made with series applications in mind). Depending on the actual ciruit, a 400VDC MKP capacitor might do just fine (the original was rated 250VDC after all!)

turretslug · 6th Oct 2014, 4:59 pm

Is it a tight PCB situation? Otherwise, 3x 680nF X-type should be straightforward to find and at least give graceful degradation redundancy.

daviddeakin · 6th Oct 2014, 8:39 pm

2.2uF X2 caps are pretty common and easy to find these days.

http://www.rapidonline.com/electroni...acitor-10-2556

julie_m · 6th Oct 2014, 9:55 pm

If there is a metal film (or designated fusible) resistor in series with the capacitor, then it need not actually be X-rated; the resistor will act as a fuse if the capacitor ever fails low-impedance.

Maarten · 8th Oct 2014, 5:28 pm

A series dropper capacitor doesn't need to be X-rated by definition, since it isn't "across the mains". Especially in this case where a lower working voltage is probably sufficient (the 250VDC capacitor lasted for a long time) though I can't say for sure without a schematic of course - that's why I recommended 630V.

Most of the time it won't hurt to use an X-rated capacitor, but since they are mostly not specified for series use (the datasheet will mention if they are! If you want to be on the safe side and use an X-rated capacitor, look for this) they sometimes tend to fail a bit too safe: i.e. go open circuit or loose capacity in only a few years time. In the Netherlands, we call this the Senseo syndrome, since Philips Senseo coffee makers are very prone to this faillure mode.

P.S. We all assume this lamp is mains powered, but is it? Just asking to make sure.

Tractorfan · 8th Oct 2014, 10:41 pm

Hi,
Thanks for your replies. Yes, Maarten, the lamp is mains powered and, oddly, it carries a warning not to shorten the mains cable. Thus, I assumed that it was some kind of resistance cable, but it's just ordinary 0.5mm2 two core cable.
I will trawl my junk box and see what lurks within.
Cheers, Pete.

5th Oct 2014, 8:16 pm	#1
Tractorfan Dekatron Join Date: Mar 2008 Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.) Posts: 3,184	Dropper capacitor replacement Hi, For a while now we have been using an 8 watt fluorescent inspection lamp for illumination when camping. Last week it finally failed and investigation showed that the dropper capacitor had ejected its dielectric through a split case. The markings say '225J' and '250v'. this lamp was made long before the advent of X2 caps. I tried replacing it with a modern 0.22uF 275VAC X2 device,but the tube now only glows very dimly (I tried a new one). So, what value would be the modern equivalent of '225J'? Thanks. Cheers, Pete. Attached Thumbnails __________________ "Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..."

5th Oct 2014, 8:26 pm	#2
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Dropper capacitor replacement Erm, it is 2.2uF, so ten times larger than the one you have used. __________________ Al Last edited by Al (astral highway); 5th Oct 2014 at 8:33 pm.

5th Oct 2014, 8:42 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Dropper capacitor replacement Exactly. You may find a motor run capacitor is the most cost effective replacement if you can find one that physically fits.

6th Oct 2014, 9:24 am	#4
Tractorfan Dekatron Join Date: Mar 2008 Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.) Posts: 3,184	Re: Dropper capacitor replacement Hi, Many thanks for your help. I've always had a 'blind spot' when it came to capacitor markings. I have amassed a small collection of X2 caps salvaged from various dead appliances, and I think there's a 2.2uF among them somewhere. Cheers, Pete. __________________ "Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..."

6th Oct 2014, 10:43 am	#5
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,768	Re: Dropper capacitor replacement If you don't trawl one up Pete, you can get them here for £1.96 inc VAT, but with an MOQ of five. Still, at least orders over a fiver ex vat are post free in the UK: http://cpc.farnell.com/1/1/87153-cap...25m-epcos.html __________________ David. BVWS Member. G-QRP Club member 1339.

6th Oct 2014, 3:28 pm	#6
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,203	Re: Dropper capacitor replacement 2u2 X2 will be hard to find. Given the original was a 250VDC capacitor and lasted reasonably long, a 630VDC MKP (not MKT) capacitor will be a good replacement in this particular case (better than original, while an X2 rated capacitor may do worse than the original since most aren't made with series applications in mind). Depending on the actual ciruit, a 400VDC MKP capacitor might do just fine (the original was rated 250VDC after all!)

6th Oct 2014, 4:59 pm	#7
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,400	Re: Dropper capacitor replacement Is it a tight PCB situation? Otherwise, 3x 680nF X-type should be straightforward to find and at least give graceful degradation redundancy.

6th Oct 2014, 8:39 pm	#8
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Re: Dropper capacitor replacement 2.2uF X2 caps are pretty common and easy to find these days. http://www.rapidonline.com/electroni...acitor-10-2556

6th Oct 2014, 9:55 pm	#9
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Dropper capacitor replacement If there is a metal film (or designated fusible) resistor in series with the capacitor, then it need not actually be X-rated; the resistor will act as a fuse if the capacitor ever fails low-impedance. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

8th Oct 2014, 5:28 pm	#10
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,203	Re: Dropper capacitor replacement A series dropper capacitor doesn't need to be X-rated by definition, since it isn't "across the mains". Especially in this case where a lower working voltage is probably sufficient (the 250VDC capacitor lasted for a long time) though I can't say for sure without a schematic of course - that's why I recommended 630V. Most of the time it won't hurt to use an X-rated capacitor, but since they are mostly not specified for series use (the datasheet will mention if they are! If you want to be on the safe side and use an X-rated capacitor, look for this) they sometimes tend to fail a bit too safe: i.e. go open circuit or loose capacity in only a few years time. In the Netherlands, we call this the Senseo syndrome, since Philips Senseo coffee makers are very prone to this faillure mode. P.S. We all assume this lamp is mains powered, but is it? Just asking to make sure. Last edited by Maarten; 8th Oct 2014 at 5:37 pm.

8th Oct 2014, 10:41 pm	#11
Tractorfan Dekatron Join Date: Mar 2008 Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.) Posts: 3,184	Re: Dropper capacitor replacement Hi, Thanks for your replies. Yes, Maarten, the lamp is mains powered and, oddly, it carries a warning not to shorten the mains cable. Thus, I assumed that it was some kind of resistance cable, but it's just ordinary 0.5mm2 two core cable. I will trawl my junk box and see what lurks within. Cheers, Pete. __________________ "Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..."