Polar vs Non-Polar Caps - Page 1
Forum section: electronics / beginners

author: Legion
author_rank: Frequent Contributor
timestamp: March 29, 2014, 02:50:52 am
content: I always thought that all else being equal, polar caps could be substituted with non-polar caps. I thought the polarity was just a limitation of electrolytics. However I've encountered two circuits now that worked only with a polar cap. Is there something about a polar cap that would allow a circuit to function where a non-polar cap wouldn't?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2014, 03:09:14 am
content: Example?If you're referring to bipolar electrolytics, the additional ESR might cause problems.If you're referring to film caps, the greatly reduced ESR might cause problems, particularly in connection with other low-ESR caps (ceramics, film) and concerning switching supplies (the ESR is a factor in control loop stability).Tim

author: Legion
author_rank: Frequent Contributor
timestamp: March 29, 2014, 03:21:00 am
content: Quote from: T3sl4co1l on March 29, 2014, 03:09:14 amHere's the circuit I was working on today:R1—10MR2—220kC1—1 ?F electrolytic capC2—0.1 ?F film capD1—1N4001 power diodeD2—1N4001 power diodeDZ—1N1N4733 5.1 Zener diodePB—Normally open PBLED—5 mm roundIC1—4046 CMOSWhen I used a 1uF film cap in place of an electrolytic for C1 it wouldn't oscillate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2014, 03:59:08 am
content: LED should have at least 1k in series.Pushbutton should have at least 100 ohm in series to limit charging current. Charging spike probably screws up the power supply??I don't get why pin 3 is connected to pin 4, yet pin 14 and the rest of the phase detector circuitry goes unused.Also don't see what the zener is for, D2 could be the zener (reversed) and it should have much the same effect. Maybe the reverse leakage of a large (1W, 1N47xx series) device gives a better decay profile? Temp sensitive of course.I take it, this is just a "chirp" blinker (i.e., it starts fast, then gets exponentially slower and slower)? In what way does it not work with a film cap?Tim

author: Legion
author_rank: Frequent Contributor
timestamp: March 29, 2014, 04:05:34 am
content: Quote from: T3sl4co1l on March 29, 2014, 03:59:08 amYeah. Proper behavior is the LED is normally on. When the button is pushed the LED flashes really quickly and then slows down and stops. If I use a film cap for C1 it doesn't flash. It just stays on. Pushing the button has no effect.

author: Legion
author_rank: Frequent Contributor
timestamp: March 29, 2014, 08:53:37 pm
content: So would the ESR difference between the caps cause the circuit not to oscillate?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2014, 09:52:05 pm
content: No, or it shouldn't. But a dubious circuit like this (plus whatever it's connected to?) could perhaps exhibit that effect.In other words, the cause and effect may well be correctly attributed, but the preferred solution goes well beyond simply reversing the cause.Tim

author: Legion
author_rank: Frequent Contributor
timestamp: March 29, 2014, 10:14:47 pm
content: Quote from: T3sl4co1l on March 29, 2014, 09:52:05 pmIn this case it's connected to nothing. It's just on a breadboard. It's being used strictly as a VCO. R2 and C2 set the frequency range. R1 and C1 set how long it takes the oscillator to wind down. The input is on pin 9. Power supply is a 9V battery. The zener is just there to drop ~5V. The input on pin 9 starts at 3V and drains.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2014, 12:53:10 am
content: Well, it's not powered by air, is it?Tim

author: Legion
author_rank: Frequent Contributor
timestamp: March 30, 2014, 01:01:56 am
content: Quote from: T3sl4co1l on March 30, 2014, 12:53:10 amPower supply is a 9V battery. When I said it wasn't connected to anything, I meant there isn't another circuit block. The only output is the flashing LED.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2014, 09:27:43 am
content: Ok, so a big, low ESR cap will pretty well be capable of shorting that out momentarily, but won't cause ringing or overshoot. (Whereas, if it were a bench supply or something, on long wires, you could potentially get damaging transients.)Whether that's enough to cause it to freeze or lock up or something, I don't know.Tim