capacitor leakage current - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 07:01:55 am
content: I am designing a 2 minute shut off timer with a comparator, using a capacitor and resistor in parallel to give a time delay. Now i used a 100uF cap and got a good time, then I switched to a 10uF cap as it was too long and it went off nearly instantly. Would this be different leakage currents ? I have a 1M discharge resistor so yes it gets a bit critical maybe. The unit will be potted, will i need to worry about the resistance of the potting compound ?

author: Kremmen
author_rank: Super Contributor
timestamp: October 24, 2012, 11:35:34 am
content: Mica is the lowest leaking dielectric, followed by plastic film (polyester, polystyrene, polypropylene). It can be tough to find them in those capacitances however, especially mica.Would it be possible to use a dual opamp for the comparator part and use the other amp to create a constant current source to charge the capacitor. That would make adjustment of the time and compensation of various biases/losses easier as well.I would think any "normal" potting compound would have a quite high resistance so not a problem. YMMV.

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 11:43:30 am
content: I don't need any timing precision, just to be able to achieve the timing required give or take some seconds without ending up with massive parts

author: SeanB
author_rank: Super Contributor
timestamp: October 24, 2012, 11:51:20 am
content: RC with electrolytic, you are looking at pain for sure. Use a lower value and a CMOS counter and oscillator. Look at the ZN1034E datasheet for examples and appnotes.

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 11:53:07 am
content: needs to be compact, I can always go to an MCU if it gets any more complicated.

author: Zero999
author_rank: Super Contributor
timestamp: October 24, 2012, 11:54:51 am
content: What's stopping you for going for an MCU?If you need to avoid an MCU you could use the CD4060 which is an oscillator and counter in one package.

author: deephaven
author_rank: Frequent Contributor
timestamp: October 24, 2012, 11:57:54 am
content: With your new PIC experience, you could go for a PIC10F204 which costs 39p and is in a compact SOT23 package. No need for any external crystal.

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 11:58:39 am
content: Quote from: Hero999 on October 24, 2012, 11:54:51 amlike i said to you on the phone the other day it was an off the top of my head thing as i was asked whilst doing something else.It's for a 24V (mil) vehicle so I'd need plenty of transient protection so by the time i have an MCU, a reg all the caps i need anyway I've made it more complicated and expensive. I need electrolytic for the reg/mcu anyway don't I ? so what changes. I've got a good working concept I just need to make considerations like caps that are good enough

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 11:59:22 am
content: Quote from: deephaven on October 24, 2012, 11:57:54 amuh got enough on my plate in that department already

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 12:02:34 pm
content: This all also needs to be very low current as i have to keep an old boy happy who is scared I'll discharge the vehicle battery that is of an undisclosed capacity. The comparator is 0.4mA at rest

author: deephaven
author_rank: Frequent Contributor
timestamp: October 24, 2012, 12:07:23 pm
content: Quote from: Simon on October 24, 2012, 12:02:34 pmPIC is <175 uA running at 4 MHz and 100 nA standby

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 12:14:33 pm
content: and it will need a 5V reg that will have some current draw too although I'm aware I can get low power parts. I can look at that option if i have time. Essentially we just need a rough delay to keep something running for 2 mins after switch off, if I need an electrolytic cap either way I might as well use my current simple solution rather than start more trouble for myself.

author: SeanB
author_rank: Super Contributor
timestamp: October 24, 2012, 12:23:14 pm
content: Electrolytic means leakage that varies with time, voltage, capacitance, temperature, age and how it bloody feels. Time will be a little variable, anything from 10 seconds to 20 minutes, depending.

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 12:25:30 pm
content: well I can take the +/-10 seconds but not the 20 minutsI just wonder, what if I just put a 0.1-1uF ceramic or other non polarized in on it's own and let the comparator discharge it ? just a wild thought as i guess the chips will all be different

author: deephaven
author_rank: Frequent Contributor
timestamp: October 24, 2012, 12:37:27 pm
content: Quote from: Simon on October 24, 2012, 12:25:30 pmThat won't work, it would be all over the place!OK, here is my latest offering which I think ticks most of your boxes:http://tinyurl.com/99hxvxj

author: Gall
author_rank: Frequent Contributor
timestamp: October 24, 2012, 01:11:58 pm
content: It is possible (and it was done somewhere) to use an RC circuit with ceramic capacitor and many MOhm resistor. But this is quite unstable.A small MCU (may be even ATtiny10 with only 6 pins in SOT-223) would be much better. If you need more stability with badly filtered power, use 4060 and a reasonable RC circuit. This is a single chip solution.

author: Simon
author_rank: Unknown
timestamp: October 24, 2012, 01:58:25 pm
content: really it is either the comparator circuit or the MCU, nothing in between.

author: Neilm
author_rank: Super Contributor
timestamp: October 26, 2012, 03:27:25 pm
content: Quote from: Simon on October 24, 2012, 01:58:25 pmIf you want a possible alternative you could consider a 74HC123 dual monostable and a cheap counter can generate a slow signal quite accurately.Neil

author: Simon
author_rank: Unknown
timestamp: October 26, 2012, 03:33:49 pm
content: well the 12F615 has a very low consumption at 31.5 KHz which is plenty for my need and i can write a simple timer program. I'll have them not ground the pump but connect it between the circuit switch and +24V so one logic level mosfet connected straight to the pic will do the switching, then I just need a reg and some smaller caps

author: ejeffrey
author_rank: Super Contributor
timestamp: October 26, 2012, 05:31:51 pm
content: Quote from: Simon on October 24, 2012, 12:02:34 pm0.4 milliamp will not discharge any vehicle battery I am aware of faster than the self-discharge rate. At least if by vehicle you mean passenger car, truck, or boat. I am not sure about motorcycles. I guess some small motorcycles / mopeds might have a battery small enough that 0.4 milliamp constant draw gives a non-negligible change to the discharge rate. It will still only be a problem if you mainly plan to store the vehicle, not drive it. If I were you I would force him to give me specs before spending too much time engineering a solution to what might be a non-problem.Obviously if there is more to it, you have to consider the total system. If you add 100 parts of 0.4 milliamp they are no longer negligible. But if you are spending a lot of time on this single timer circuit, I suspect you are way off the edge of the cost-benefit curve.

author: Simon
author_rank: Unknown
timestamp: October 26, 2012, 05:47:53 pm
content: It's a tank! so i'm sure the battery is plentiful. with a pic solution i can knock out any electrolytics, which also avoids risks when some stupid supplier uses cheap Chinese caps with an atrocious self discharge rate.

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2012, 07:10:07 pm
content: Just because it is an armoured vehicle does not mean the battery is anything other than a normal automotive 12V unit ( 2 in series) in the battery bay. I did have 3 weeks of fun rewiring one that had an electrical fire caused by the wiring loom wearing through in a panel cutout. 36 fire extinguishers later the battery cover was unscrewed ( a thin 1/4 inch plate held by 4 M8 bolts) and the battery cable was torn off by a hammer. A mod was immediately instituted to include a battery switch next to the battery with a handle chained to the vehicle so the battery could be disconnected. I made the hole in the panel to the instruments bigger ( cutting torch and sand to make the 40mm hole in the 25mm plate a 100 by 100 hole) and after cooling and grinding off the loose bits a flexiform grommet strip was glued into the hole to pass the new loom through. Made the new loom using the best guess from looking at the undocumented wiring of the other one in for brakes and got it working perfectly, everything from new fuel gauge and sender, new indicators and flashers and even got the brake lights and horn working. i did not lace the new loom, just used a lot of spiral wrap and cable clamps to make the panel passes non moving and well insulated. Added the one thing these never had, actual fuses in a fuse box, and proper generator wiring to the regulator. Even got an award for it, on my last day......... Bittersweet memories.

author: Simon
author_rank: Unknown
timestamp: October 26, 2012, 07:17:33 pm
content: well never you mind what I do and don't know about the vehicle. This could also be used in anything else we make so no harm in a one fix for all.

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2012, 08:37:23 pm
content: The 16 ton tractor was not the worry, it was the fully fuelled and armed plane it was towing at the time. 36 fire extinguishers were all that were in a 100m radius of the little fire. Fire brigade was there 10 minutes later.......... 5km drive at 160kph. 100l of diesel, 6000l of JetA1 and 1 ton of HE are not good companions to an electrical fire.

author: Simon
author_rank: Unknown
timestamp: October 26, 2012, 08:40:29 pm
content: Quote from: SeanB on October 26, 2012, 08:37:23 pmindeed

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2012, 09:10:30 pm
content: Fitter in the plane had a reaally good view of the whole thing, as he was about 2m above the fire......... with an ejection seat as well, and no ladder to get out with.

author: Simon
author_rank: Unknown
timestamp: October 27, 2012, 07:22:26 am
content: I'm toying with using a 12F615, it has 11uA consumption @ 2 V although I'd be working at 5V when working at 32KHz, well the internal clock is only 4MHz I need to go as low as possible so i need to use an external RC, naturally microchip don't put all of the info in the same place (making it simple is obviously illegal at microchip).Anyone know the formula ? I work out that using the limit values (100K) I'd not be using more than 50uA for the clock gen.All i need now is a low quiescent reg and I could match or beat the comparator option with much better timing.