# Capacitor discharge probes

Discussion in 'Electronic Basics' started by Shaun, Jun 18, 2010.

1. ### ShaunGuest

Hello,

I recently bought a Blue ESR meter and assembled it and it is working well.
In the instructions it mentions making a jig with a 100 ohm 5 watt resistor
attached to some probes for discharging capacitors before testing them with
the ESR meter.

How does one calculate the required resistor wattage for different capacitor
values. Say I wanted to discharge a 63 volt cap at 500 microfarads, would 5
watts be enough. Right now I've made a probe jig with four 100 ohm 3 watt
resistors for a total of 100 ohms at 12 watts.

Shaun

2. ### Tom BiasiGuest

Hi Shaun,

I could say a few words here but Sam can provide you with much more info.
See here: http://www.repairfaq.org/sam/captest.htm

Tom

3. ### ShaunGuest

Thanks for the link, there is alot of information there but not what I was

How do you calculate resistor power rating required to discharge a capacitor
of x volts and y microfarads?

Shaun

4. ### Phil AllisonGuest

"Shaun"
** You are labouring under a mis-apprehension.

Any resistor connected across a capacitors will EVENTUALLY discharge it.

The only things to worry abut are how long will it take and might the
resistor become damaged by too much current .

A 100 ohm, 5W *wire wound* resistor will discharge almost any electro ( to a
safe voltage) in no more than a few seconds - however a very large, high
voltage electro might have enough energy to destroy it.

..... Phil

5. ### Phil AllisonGuest

The vast majority of capacitors will be discharged by the time you get
your test probes ready to use. I've been doing ESR stuff several years
and only once did I damage a meter by sending power into a Capacitor
Wizard. It was a small value (around 10 ohms) resistor that fried and
only took a few minutes to replace. Or do you just want formulas?

** The " Blue ESR Meter " will be damaged by voltages more than about 50
volts DC

http://members.ozemail.com.au/~bobpar/esrmeter.htm

...... Phil

6. ### oo pere ooGuest

A 1K resistor will discharge a 1000uF capacitor in 5s.
A 100R resistor achieves the same in 0.5s.
Time constant is tau=R·C and in 4 or 5 tau you have discharged your
capacitor.

The power absorbed by the resistor at the moment of closing the circuit
is V²/R. For V=63V and R=100R, this is roughly 40W. At t=0.1s voltage is
just 23V and instantaneous power absorbed 5W. Your jig will certainly
survive such short spikes. If you take 1K, a single resistor will
achieve the same at the cost of some extra time

Pere

7. ### Phil AllisonGuest

"oo pere oo"
** All perfectly correct.

However, a physically small resistor can only absorb so much energy in a
short space of time before the conductor inside MELTS !!

The energy stored in a capacitor is give by the formula:

E = 0.5C x V squared.

Taking a practical, worst case example of a 1000uF cap charged to 400
volts - the stored energy is 80 joules, most of which is dumped in the
first 0.2 seconds.

Depends very much on the construction of the particular resistor whether it
can absorb such a large energy spike or not.

The best type is carbon composition, then wire wound and last of all
deposited film resistors.

..... Phil

8. ### oo pere ooGuest

Just made a quick search on some Farnell 100R resistors. It is
interesting to see that a 1W CCR resistor such as CCR1100RKTB (Tyco) has
a pulse limiting power of 600W for single 0.1s pulses.

Pere

9. ### amdxGuest

Did we all just witness a lovefest from Phil?
Mike

10. ### ShaunGuest

So I guess the important factor is it's pulse power rating for a resistor,
although I haven't seen any resistors with this spec before. If I work out
the math now that I know what to do thanks to you guys, P = 1/10 V^2/R
for 5 tau. What confuses me is that C cancels out, so it wouldn't matter
what value of C I'm discharging, the only thing that changes is the amount
of time it takes to discharge.

Does anyone know a rule of thumb for pulse power verses continuous power for
a given time frame for a resistor??

thanks again,

Shaun