Need help identifying elec component

Thread Starter

Elektrishun

Joined Dec 27, 2016
58
I am trying to troubleshoot a failed Dukane Nurse Call system circuit. I removed the following component from the board. Based on the description printed on the board it should be a polarized capacitor (pics are terrible but if you notice the image with the red outline which indicates where the component was removed from there is a "+" symbol on the lefthand side and "C9" on the righthand side). When measured with a DMM it shows 23 microfarads. I tried using the markings on the component to search for a datasheet but I can't find anything. The markings are as follows, reading top to bottom, left to right:
- M + (looks like a Motorola symbol but not conclusive and the plus sign is quite faded)
- 226K
- 6V +
- 8533



Any help identifying this type of pointy eneded :) capacitor would be greatly appreciated.
 

Thread Starter

Elektrishun

Joined Dec 27, 2016
58
Yes, that makes sense.

What do you make of the "6V +"? I do not have a long history with electronics but for some reason this cap looks very unique and I wonder if there's more to it than meets the eye?
 

Thread Starter

Elektrishun

Joined Dec 27, 2016
58
Does the "6V +" refer to it's voltage rating? If it does why would it be used in a circuit that is powered with 12V DC?
 

Thread Starter

Elektrishun

Joined Dec 27, 2016
58
6V means it is rated for use up to 6 Volts.

The "+" mark indicates the end that is supposed to be connected to the pad marked "+" on the printed circuit board -this part is polarized.
Thank-you.

As I already mentioned before reading your post - why only 6V in a 12V circuit? It seems more often that I see higher rated voltages on capacitors in circuits powered with a fraction of their rated voltage.
 

wobbinc

Joined Aug 29, 2016
4
This is because at that particular point in the circuit the designer would never expect a 6v rated capacitor to 'see' anything more than 6v.

Capacitors are often used for coupling of AC signals (for example speech) between two parts of a circuit whilst maintaining a degree of DC isolation. The capacitor might only ever have a maximum of a fraction of a volt across it's terminals.

HTH

<edit> Looking at the position of the capacitor and its proximity to 74LS TTL chips, I'd hazard a guess that this cap is sitting across the 5v supply rail to provide decoupling. </edit>
 

Thread Starter

Elektrishun

Joined Dec 27, 2016
58
Thank-you, wobbinc.

....man, I feel like all of my questions had obvious answers but nonetheless.... :)

Ok, so I'm looking at another capacitor on the same board, obviously a different type (aluminum electrolytic capacitor), it's rated for 50V. I know I'm probably comparing apples to oranges, but why the difference in rated voltage? I'm stuck on this idea that if you have a 12V DC powered circuit then you should go bigger with your cap's rated voltage or otherwise increase the risk of a potential failure. I'm not trying to second guess the board's design but rather understand if it is common or uncommon to see capacitors with voltage ratings lower than the rated power for the circuit? Maybe a higher rated cap of that type would be too big for the board?
 

wobbinc

Joined Aug 29, 2016
4
Absolutely :) - it's very common.

A circuit will have at least one power supply 'rail'. There will normally be capacitors connected from this rail with the other end tied to the negative supply, or more commonly known as the 0v rail. As a rule of thumb, the designer will pick a capacitor whose rated voltage is between 20 to 50% higher than the maximum voltage that would normally be expected on the rail.

Suppose you had a board with a 24v dc supply. You might see capacitors rated at around 35v on the supply input. That supply might then go into a voltage regulator (google 7812 for an example) the provides a lower, stabilised supply of 12v. You'd normally see the designer place capacitors rated at around 16v across the 12v rail.

Of course, in a fault situation it's possible that a capacitor might be over stressed, but this isn't that common. Capacitors - especially aluminium electrolytic types - mainly fail because of age / local heat sources / use in a circuit where they are subject to high voltage surges.

HTH
 

RichardO

Joined May 4, 2013
2,271
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