Discharging Capacitors Prior to ESR Meter Probing

[[email protected]]

I got the Dick Smith device and am ready to tackle jobs. What
techniques/hardware are being used to make sure that capacitors are
properly discharged? I will be working mostly on older stereo
equipment. My understanding is that the non-power supply caps will
discharge after the equipment is turned off. For a typical cap to be
found in this application, what would be a good way to survey the board
and discharge the caps? What size and type resistor? I have a bunch
of audio grade sandcast types from partsexpress, would these be good
enough? Should I even bother or are these caps inoffensive?

The power supply caps, which I am guessing are identified by size and
proximity to the power source are a different animal. What should be
done about these?

 

[Leonard Caillouet]

I just use about a 20w 1K ceramic power resistor from a scrap unit with some
leads soldered to it. Probably a little higher value than needed for audio
work and a little lower than optimal for TV HV, but it is a good compromise
that works fine for everything so far.

One thing that I like about the Atlas and Sencore meters over the DSE (I
have them all) is that you don't have to worry about it. Actually, I use
the Atlas more than anything in the field and for protable applications. I
like the size and the fact that you can get cap value readings. I still
like to discharge the large PS caps and CRTs just for safety, but for the
lower capacity and lower voltages I just let the meters designed to do so
discharge them. You can get a dischage update for the DSE, IIRC.

Leonard

 

[[email protected]]

I soldered in two 600 volt diodes back to back across the probes
input connectors on my Dick Smith ESR meter. About 1 or 2 amps
rating would be OK. Connect one diode with the anode toward one
probe connector and the other diode with the anode toward the other
probe connector.
I haven't had any problems in the 3 years I have been using my meter.
The diodes do not seem to affect the meter reading.
Hope this helps.
John

 

[[email protected]]

"I just use about a 20w 1K ceramic power resistor from a scrap unit
with some
leads soldered to it. Probably a little higher value than needed for
audio
work and a little lower than optimal for TV HV, but it is a good
compromise
that works fine for everything so far. "


Like one of these?

http://www.mouser.com/index.cfm?han...uctid=793712&e_categoryid=359&e_pcodeid=58822

 

[[email protected]]

"I soldered in two 600 volt diodes back to back across the probes
input connectors on my Dick Smith ESR meter. About 1 or 2 amps
rating would be OK. Connect one diode with the anode toward one
probe connector and the other diode with the anode toward the other
probe connector.
I haven't had any problems in the 3 years I have been using my meter.
The diodes do not seem to affect the meter reading.
Hope this helps. "

I saw this modification in the documentation that came with the unit.
Let's say I placed the meter with this mod across a filter cap on a
120W audio amplifer without prior discharging, what would happen?

 

[I.F.]

I got the Dick Smith device and am ready to tackle jobs. What
techniques/hardware are being used to make sure that capacitors are
properly discharged? I will be working mostly on older stereo
equipment. My understanding is that the non-power supply caps will
discharge after the equipment is turned off. For a typical cap to be
found in this application, what would be a good way to survey the board
and discharge the caps? What size and type resistor? I have a bunch
of audio grade sandcast types from partsexpress, would these be good
enough? Should I even bother or are these caps inoffensive?

The power supply caps, which I am guessing are identified by size and
proximity to the power source are a different animal. What should be
done about these?

When I used to repair PC monitors it was often necessary to discharge the
SMPSU primary side reservoir cap, which could be charged to as much as 320V!
The device I found best to dump this charge was a NTC thermistor, most
monitors and some TVs have these as inrush surge limiting devices in the
mains input circuit, the ratings vary considerably depending what you
salvage them from, the cold resistance can be anything from a few hundred
Ohms to about 12K, as the discharge current heats the NTC thermistor it's
resistance falls to typically less than 10 Ohms and discharges the cap in a
controlled manner - rather than blowing chunks out of the PCB track like
when shorting it out with a screwdriver.

 

[I.F.]

"I soldered in two 600 volt diodes back to back across the probes
input connectors on my Dick Smith ESR meter. About 1 or 2 amps
rating would be OK. Connect one diode with the anode toward one
probe connector and the other diode with the anode toward the other
probe connector.
I haven't had any problems in the 3 years I have been using my meter.
The diodes do not seem to affect the meter reading.
Hope this helps. "

I saw this modification in the documentation that came with the unit.
Let's say I placed the meter with this mod across a filter cap on a
120W audio amplifer without prior discharging, what would happen?

There's a slight possibility one of the diodes might fail S/C - but it
should protect the ESR meter.

 

[GregS]

"I soldered in two 600 volt diodes back to back across the probes
input connectors on my Dick Smith ESR meter. About 1 or 2 amps
rating would be OK. Connect one diode with the anode toward one
probe connector and the other diode with the anode toward the other
probe connector.
I haven't had any problems in the 3 years I have been using my meter.
The diodes do not seem to affect the meter reading.
Hope this helps. "

I saw this modification in the documentation that came with the unit.
Let's say I placed the meter with this mod across a filter cap on a
120W audio amplifer without prior discharging, what would happen?

I have been reading this, and I guess your talking about two
parallel reversed diodes, not back to back series diodes, right?

greg

 

[John Bachman]

I soldered in two 600 volt diodes back to back across the probes
input connectors on my Dick Smith ESR meter. About 1 or 2 amps
rating would be OK. Connect one diode with the anode toward one
probe connector and the other diode with the anode toward the other
probe connector.
I haven't had any problems in the 3 years I have been using my meter.
The diodes do not seem to affect the meter reading.
Hope this helps.
John

The problem with this solution is that there is no limitation of the
discharge current. Exceed the dv/dt rating of the capacitor being
tested at your own risk. You may create a bad cap instead of just
finding one.

John

 

[Andy Cuffe]

I got the Dick Smith device and am ready to tackle jobs. What
techniques/hardware are being used to make sure that capacitors are
properly discharged?

The power supply caps, which I am guessing are identified by size and
proximity to the power source are a different animal. What should be
done about these?

Just check any caps that might be charged with a volt meter. If they
are charged, use a large resistor to discharge them. Adding back to
back high current diodes to the ESR meter isn't a bad idea to protect
it from the occasional cap you forget to discharge, but don't rely on
the diodes to discharge caps.
Andy Cuffe

[email protected]

 

[[email protected]]

"Just check any caps that might be charged with a volt meter. "

You can do this with any digital multi-meter on any sized cap? No
danger of damaging it, even when reading a power supply cap?