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Electronics => Beginners => Topic started by: Roverfan on June 15, 2021, 03:50:45 pm

Title: Capacitors - high resistance to ground on both sides
Post by: Roverfan on June 15, 2021, 03:50:45 pm
Been testing dashcam pcb trying to understand how it works. It has the usual small smd capacitors (orange-ish light brown body)

Was testing for shorts to ground on capacitors in circuit but instead found some capacitors which had a high resistance to ground both sides. This is not an area I am familiar with.

First one I found like this had [diode mode figures of] 450 on one side and 44 the other. Some others had very high resistance/open circuit to ground both sides.

Is this indictive of a fault? What shall i think of it?
Title: Re: Capacitors - high resistance to ground on both sides
Post by: tooki on June 15, 2021, 04:12:45 pm
Caps aren’t only used for power supply decoupling, so it’s a totally normal thing to have caps that aren’t connected to ground on either side.

I assume by the numbers you stated that you mean resistance in ohms. If so: please do not invent your own terminology. “Voltage drop” is a term that already exists and has a specific meaning, and it’s measured in volts (and I think it’s highly unlikely there are such high voltages inside a dashcam for a 450V voltage drop). In general, always include the units on values.
Title: Re: Capacitors - high resistance to ground on both sides
Post by: Roverfan on June 15, 2021, 04:23:23 pm
Thank you tooki. Please excuse the newb errors

The figure I quoted was the figure the multimeter read in diode mode. After much searching previously online, the only references i found to what the meter reads in diode mode was being refered as voltage drop.
Title: Re: Capacitors - high resistance to ground on both sides
Post by: james_s on June 15, 2021, 04:50:00 pm
Why are you using diode mode to test capacitors? The diode test range is for testing diodes (and diode-like things such as transistors) and indicates the forward drop of the junction in millivolts. 400 millivolts is 0.4 volts.
Title: Re: Capacitors - high resistance to ground on both sides
Post by: tooki on June 15, 2021, 04:56:15 pm
Thank you tooki. Please excuse the newb errors

The figure I quoted was the figure the multimeter read in diode mode. After much searching previously online, the only references i found to what the meter reads in diode mode was being refered as voltage drop.
No worries, ALL of us were newbies at some point, and this is the beginners forum. (Which is why I also gave some feedback on terminology. Thanks for being receptive to it!)

Yes, the diode drop is indeed a true voltage drop, meaning its unit is volts. Assuming you didn’t overlook a decimal point (in which case it’d be e.g. 0.45V), then that’s in whole millivolts (mV).

So were you simply using diode mode as a continuity tester? In that case, the values don’t tell you too much, if you don’t already understand what you’re looking at.

Here’s a good video on the basics of caps: https://youtu.be/otQGdPLyF3w

And this video is a gorgeous animation of how they actually work (as in, the physics inside them) and some demos
https://youtu.be/f_MZNsEqyQw
Title: Re: Capacitors - high resistance to ground on both sides
Post by: Roverfan on June 15, 2021, 04:59:48 pm
Thank you James, i have spent a few hours previously searching and watching youtube for the 'unit' represented in diode mode. So it is a voltage drop and the unit is milivolts, thank you

I wasnt testing caps as such, but checking for shorts

Title: Re: Capacitors - high resistance to ground on both sides
Post by: james_s on June 15, 2021, 05:03:18 pm
The diode test function is super useful. Typically a diode will have a forward drop of about 700mV for a regular diode or 300mV for a Schottky type. A transistor can be tested like two diodes, one B-C and one B-E. You do have to be careful testing in-circuit though because other components in the circuit can cause a false reading but often you can get a pretty good idea if a diode or transistor is ok.
Title: Re: Capacitors - high resistance to ground on both sides
Post by: Roverfan on June 15, 2021, 05:08:20 pm
Thank hou tooki, i will definitely watch those videos. I am trying to grasp circuit level electronics just as a hobby for general knowledge.

Unfortunately my meter does not show any decimal points however it does show unused zeros like 044, 003. That may be its way to say insert decimal here
Title: Re: Capacitors - high resistance to ground on both sides
Post by: Roverfan on June 15, 2021, 10:32:46 pm
Took the plunge to search for 'inline capacitors' (although the word inline is slightly misleading) - apparently capacitors are used without a direct ground in (logic) circuits where pulsing is required, for (momentary) pulling up or down voltage.


Title: Re: Capacitors - high resistance to ground on both sides
Post by: james_s on June 16, 2021, 02:54:53 am
They're used for all sorts of things, filters are a common one, and DC blocking, such as coupling between audio stages.
Title: Re: Capacitors - high resistance to ground on both sides
Post by: JustMeHere on June 16, 2021, 04:53:10 am
I've seen this arrangement where one micro was resetting another.