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Have almost finished cleaning up my hoard of stuff ( wife calls it junk ) and have sorted all components, except 3 types I can't identify ( newbie knows nothing ).

I think that all 3 are capacitors.

The first is yellow, about 2mm across, 2 pins. no markings. I have a small "chy12" multimeter and if I place the 2 pins in the Cx slot, it displays 23pF. I assume then that this is a 23pF capacitor.

The second is blue, about 3mm across, 2 pins, and has the marking "104" printed on the blue plastic / ceramic?. My multimeter shows the cap value of 86nF.

The third lot are the ones that have me concerned / worried :

about 20mm across, 2 pin, brown, marked "105K 250JS". These are a lot bigger than the other 2 lots, so something is telling me to ask for advice before I stick this into my multimeter and risk damage to the device, component, or more importantly, me.

Also, I see warning signs on the multimeter to first discharge a cap before testing. What is the best / safest way to do this ?


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The crude way is to simply shorten it out with a piece of wire but this could damage the capacitor (high current) - and give you a fright if it is charged -  better to use a resistor (say 100ohms) and place this across the terminals.  However be aware that a high voltage one will could give you a kick so avoid using your fingers to hold the terminals of the resistor
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Many Thanks for the advice jackrae

Have discharged and tested - shows 1uF, so I assume with a bit of tollerance, that its spec is 1,05uF ( from the 105K markings ).

Regards
Dave
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Value codes for passive components frequently use a code where the last digit is the power-of-ten, so 104 is 10 times 10,000 (10 to power of 4), 105 is 10 times 100,000, 221 is 22 times 10.  Resistor colour-code bands are the same with colours substituted for digits (and a final colour band representing the precision).   orange-white-black-brown is 39 times 1 = 39ohm (final brown is precision 1%)

Capacitors often have a letter code after the numbers that specifies the precision, so for instance the 105K is 1uF +/-10% (1uF is 1000000pF, the code represents the value in pF)  The 250JS is probably the voltage but I don't know what the JS means.
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so something is telling me to ask for advice before I stick this into my multimeter and risk damage to the device, component, or more importantly, me.
As this component has been in you junk box for some time, even if it initially was a charged capacitor it will now be discharged and safe. A few days is enough to let any capacitor discharge due to leakage currents. This is the only thing that can go wrong putting a meter across a device.
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It might be helpful to post pictures of the capacitors.

How old are they?  Wet electrolytic-based capacitors dry up over time and become dangerous to use.  e.g., If they have been un-powered for >5 years it might be best to just throw them away.
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become dangerous to use.
Yes but not to test with a meter.
You can easily reform any electrolytic capacitor by charging them up through a 1K resistor and leaving them over night on charge. Then discharge through the 1K before using in the circuit.
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You can easily reform any electrolytic capacitor by charging them up through a 1K resistor and leaving them over night on charge.

Self healing works fine as long as the electrolytic hasn't dried up.  Your method assumes the electrolytic is still in good shape and would work.  (but it only reforms the oxide to the voltage applied and not the full voltage rating of the cap.)

Sitting on a shelf for a couple of months, reformation should (needs to) be done.  Years?  Probably not worth it.
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Years?  Probably not worth it.
It's an old radio hams trick used on many war surplus radios in the 60s. These hadn't been powered up for nearly 20 years and they almost always fully recovered.
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It's an old radio hams trick used on many war surplus radios in the 60s. These hadn't been powered up for nearly 20 years and they almost always fully recovered.

You're talking a different kind of capacitor than me.  I'm talking about consumer-class lytics.  They have a shelf-life of a couple of years and a working-life a little bit longer than that.

I'm not arguing this method won't work.  In fact, cap manufactuers recommend it for any (wet) electrolytic that has been sitting on a shelf for an extended period of time.  However, once the electrolyte's ions are used up, you can't heal the oxide layer anymore.

Caps from the 60s/70s were likely well over-designed and can withstand a long shelf-life.
« Last Edit: June 08, 2011, 09:10:25 pm by James C4S » Logged

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could the components be MOVs ?

Metal Oxide Varistors
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