Electric Fence-Capacitors?? - Page 1
Forum section: electronics / general

author: Joshua
author_rank: Regular Contributor
timestamp: June 20, 2011, 03:29:16 am
content: Hey everyone.First off, let me publicly state that myoldusername was PianoKid1994. My new username, as you can see is 'Joshua'. I felt that my old name was a little childish, and I see no reason to not make my username my actual name. I will leave it as Joshua from here on out.Anyway... I am trying to fix an electric fence charger that is not putting out the charge signal. I opened it up and took a look around and see two capacitors that might be of issue. It looks like this thing might have gotten wet as the inside of the case is all rusted, and it is also quite old. I've read on here before that a capacitor might be tested to the right capacitance but its ESR has greatly increased causing the circuit to fail. Well on neither of the capacitors can I get a capacitance reading off of. I am using a Fluke 87-3 to measure it.I tried googling the numbers on them but did not gather any useful information, except that 'Z5U' means like the type of ceramic used or something...I figure that if it's not too much money to replace the caps, I will order them and see if it works. Does anyone know what value these caps are, and what kind of replacement I can use?(I edited some of the pictures to make the numbers easier to read)Thanks,Joshua

author: thmjpr
author_rank: Regular Contributor
timestamp: June 20, 2011, 07:09:54 am
content: The first is 0.27uF, 600Vdc. Metalized polyester film type:http://www.datasheetcatalog.org/datasheets/50/235884_DS.pdfSecond is 500pF, 20kV. Z5U is a temperature limit/tolerance spec for ceramics as you stated:http://en.wikipedia.org/wiki/Z5UCapacity you want to stay close, voltage can be higher when buying replacements of course.The first is quite common, I see some on ebay/mouser/digikey. Second is more rare, there are some onebaybut from china. I might be tempted to series some lower voltage parts instead.You may want to try testing them again out of circuit before completely writing them off. But they do look bad..

author: Joshua
author_rank: Regular Contributor
timestamp: June 20, 2011, 10:04:07 pm
content: Thanks for the reply.I found the Metal Poly on Mouser, so that's taken care of.For the ceramic, when you say 'series' lower voltage caps together, let me make sure I understand.The original is rated for 20kV, so I should meet or exceed this rating to be safe, even if they already over-spec'd it. I can therefore put two 1000pF 10kV caps in series. Because the caps are identical the capacitance is equal to 1/2 of one of them, giving me my 500pF. Since they are identical they will have the same voltage drop across them. If the actual voltage is 20kV, the voltage across one of them will be 10kV meeting the voltage requirement. If I then actually choose a 15kV cap(30kV total) , like thisone, I should be perfectly safe, forgetting that if one of them is leaky it will transfer voltage to the other...?

author: thmjpr
author_rank: Regular Contributor
timestamp: June 21, 2011, 07:07:49 am
content: Quote from: Joshua on June 20, 2011, 10:04:07 pmNice find.I believe you are correct. There will be some variance due to the tolerance of the capacitor (+/-20%), as well as the leakage.There is no spec for leakage, so I can't say for sure, but I suspect it will be fine.

author: Neilm
author_rank: Super Contributor
timestamp: June 21, 2011, 06:58:35 pm
content: You could also put a high value resistor in parallel with the capacitor. This will ensure that the voltage is shared equally between the capacitors.YoursNeil

author: Joshua
author_rank: Regular Contributor
timestamp: June 21, 2011, 08:19:53 pm
content: Quote from: Neilm on June 21, 2011, 06:58:35 pmNeil, that sounds interesting. Is there a link you can provide with information regarding this? I'd love to learn more about it. A google search returned RC circuits and did not explain to me how this worked.Are you saying to put a resistor in parallel witheachcapacitor, or one resistor in parallel with both capacitors as a group?

author: thmjpr
author_rank: Regular Contributor
timestamp: June 21, 2011, 10:55:41 pm
content: Quote from: Joshua on June 21, 2011, 08:19:53 pmParallel with each cap, example image here:http://www.coilgun.info/theorycapacitors/capacitors2.htmIts hard to find 10kV rated resistors though.. and they would have to be up in the Gig ohms.

author: Zero999
author_rank: Super Contributor
timestamp: June 22, 2011, 04:43:00 pm
content: Don't worry about the resistors too much, just keep the maximum voltage below 20% of the capacitor voltage rating (or whatever the capacitor's tolerance is) and all will be fine.

author: Neilm
author_rank: Super Contributor
timestamp: June 22, 2011, 06:56:52 pm
content: Quote from: thmjpr on June 21, 2011, 10:55:41 pmI have done a quick search on the Farnell website and found thishttp://uk.farnell.com/ohmite/sm104031007fe/resistor-10kv-1-gig-1/dp/1550776Not too bad for a 10kV 1G resistorNeil

author: Zero999
author_rank: Super Contributor
timestamp: June 22, 2011, 11:56:32 pm
content: Look at the price though!It's cheaper to just buy the correct voltage rating in the first place or even connect a couple of over rated capacitors in series than buy expensive resistors. To clarify what I said earlier: if you have two 20kV capacitors with the same value and a tolerance of 20%, the maximum safe working voltage is 40kV - 20% = 40kV*0.8 = 32kV but it's still good to derate a little more for reliability's sake, so knock another 10% or so off and you're pretty safe.These rules apply because when capacitors are connected in series, the smallest capacitor always charges up to the highest voltage. Don't worry about the leakage current, the above formula takes it into account because it will be roughly proportional to the voltage and inversely proportional to the capacitance so the two effects will cancel. Providing the capacitors are of the same type, there shouldn't be a problem, as long as they're suitably derated.

author: Joshua
author_rank: Regular Contributor
timestamp: June 23, 2011, 04:05:12 am
content: Thanks for the links Neil and thmjpr!Quote from: Hero999 on June 22, 2011, 11:56:32 pmI would like to buy the correct voltage, but couldn't find it on digi-key and mouser. Alright so here's my calculations.Two 15kV caps at 1000pF. 20% tolerance. 30kV*0.8 = 24kV - 10% = ~21kV. The original cap in the device was rated at 20kV. I assume that they would've added a margin of safety already. I think I should be fine then. Would you agree?

author: Zero999
author_rank: Super Contributor
timestamp: June 23, 2011, 04:50:47 pm
content: Yes, you're probably safe, two 15kV 20% tolerance capacitors connected in series will certainly be fine for 20kV.