Laser machine capacitor has no polarity marking? - Page 1
Forum section: electronics / projects

author: zigunov
author_rank: Contributor
timestamp: February 09, 2024, 05:33:55 pm
content: Hi,We got this capacitor from Amazon for a laser project we are working on (one of those tattoo laser removal machines, we want to overclock it) and after the capacitor arrived we just don't see any polarity markings on it. See the picture, there's nothing indicating with side is the negative pole. I'm pretty sure these capacitors are electrolytic.How would I go about finding the polarity? I tried all methods I found online (multimeter in ohms, capacitance setting) on a known electrolytic capacitor with polarity marked, none of the methods are reliable. Is there a more scientific way of finding out the polarity?Thanks!

author: RoGeorge
author_rank: Super Contributor
timestamp: February 09, 2024, 06:39:01 pm
content: Polarized capacitors have distinctive markings for +/- in some form or another somewhere. Are you sure that is a polarized capacitor?If yes, try to measure its leakage current. A reversed polarized should have a higher leakage current. Can be measured with a DMM that has high input impedance (1G\$\Omega\$ or higher) for DC volts in series with the capacitor and a power supply.Not all DMMs have the high impedance mode for DC volts. Expensive ones usually can be set for G\$\Omega\$ impedance at measuring DC volts (usually the high impedance is available only for mV DC range, up to a 1Vdc top, or so. Read the DMM usual manual if you are not sure about your DMM high impedance mode. If yours does high impedance DC volts, then use the DMM as a uA-meter by adding a DC source (or a battery) in series with the capacitor:- set the DMM on mV DC, manual range, gigaohm impedance- discharge the capacitor- connect all 3 in series, the DNN, series with the capacitor under test, series with the battery- read the mV DC on the DMM (the sign doesn't matter)- reverse the capacitor and read again the mV (the sign doesn't matter)- you should see fewer mV when C is correctly polarizedThink of the DMM as if it were a G\$\Omega\$ charging resistor for the capacitor. The "charging" current will be smaller when the capacitor is polarized correctly (less leakage) and higher when the capacitor is polarized in reverse (because of its higher leakage in revers), so the DMM will show many mV when C is incorrect placed, and fewer mV when C is correctly connected.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2024, 07:07:54 pm
content: The capacitor looks quite large in volume. So it could be a film capacitor as well. It looks a bit similar to capacitors used with 1 phase induction motors. The start capacitors for short time use are usually nonpolar electrolytic and the moder run capacitors are usually PP film types.For a laser to be used with a rather high pulse rate a electrolytic capacitor may not be the right choice. The relatively high loss factor may lead to overheating. For the slightly more depending case of an EDM it even matter which type of film capacitor (need really low ESR and low loss, like PP capacitors with thick enough foil).For testing if it is an electrolytic or film type, one could check the dielectric absorbtion: electrolytic shows quite some votlage coming back after a short discharge. Mylar film is already quite a bit better and PP film even more. It would be only old style paper+oil that could be hard to tell apart from electrolytic.

author: moffy
author_rank: Super Contributor
timestamp: February 10, 2024, 11:55:35 pm
content: I agree with Kleinstein it could well be polypropylene, which has a high pulse rating. If you want to check whether it is electrolytic put a 10k resistor in series with it and a 10v supply, the reverse way will drop much more across the 10k than the other, if polypropylene then you will have almost no voltage drop across the 10k either way.

author: zigunov
author_rank: Contributor
timestamp: February 23, 2024, 05:16:11 pm
content: Hi, everyone; thank you so much for your very nice and detailed replies.I got myself a large type C fire extinguisher and some safety gear and tried connecting it to the circuit in both directions.None of the directions made it explode or even get warm to touch. That tells me the direction doesn't matter, so it must be a film capacitor.Hopefully this info plus the post replies help others in the future ^.^

author: CaptDon
author_rank: Super Contributor
timestamp: February 25, 2024, 05:14:56 pm
content: Sweet!!! Kind of like exploding an engine at 9000 RPM with pieces flying around like bullets to see if it will run at 4000 RPM.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 25, 2024, 06:35:49 pm
content: That is definitely some kind of film capacitor. It can be 'motor run' or 'motor start' film capacitor (possibly, a PP film).

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 25, 2024, 06:40:06 pm
content: Quote from: zigunov on February 09, 2024, 05:33:55 pmI don't know how scientific it is, but I would just googled its type since it clearly has its marking 'CBB...' (I'm not a Google, but as I know 'CBB' is a film cap series).

author: gamalot
author_rank: Super Contributor
timestamp: February 25, 2024, 07:02:53 pm
content: Quote from: Vovk_Z on February 25, 2024, 06:40:06 pmCBB means polypropylene film capacitor in China.