Prev: Simple hack to get $600 to your home.
Next: Video cards
From: John Larkin on 18 Jun 2010 11:27
On Fri, 18 Jun 2010 22:22:42 +1000, "Phil Allison" <phil_a(a)tpg.com.au>
wrote:

>
>"oo pere oo"
>>
>> 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
>> V�/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 :)
>>
>
>** 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.
>

I'd guess that a resistor can easily absorb E joules, where E = P * T
and P is its power rating and T is its thermal time constant. T is
maybe 10 or more seconds for something like a carbon comp, maybe less
for a wirewound or axial carbon film, a fraction of a second for a
surface-mount thickfilm.

So it would probably be OK for a 2 watt axial resistor to absorb 20
joules. I've seen specs for 5-watt pulse-rated resistors that are
claimed to absorb over 100 joules.

Somebody should destroy some resistors and publish the results. I
exploded some 7.5 ohm 1206 thickfilm resistors. It takes about 60
watts in 1 millisecond. Pulsed, they look like LEDs in the infrared.

John



From: George Herold on 18 Jun 2010 13:05
On Jun 18, 11:27 am, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 18 Jun 2010 22:22:42 +1000, "Phil Allison" <phi...(a)tpg.com.au>
> wrote:
>
>
>
>
>
>
>
> >"oo pere oo"
>
> >> 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
> >> V /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 :)
>
> >** 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.
>
> I'd guess that a resistor can easily absorb E joules, where E = P * T
> and P is its power rating and T is its thermal time constant. T is
> maybe 10 or more seconds for something like a carbon comp, maybe less

How big a carbon comp? I would have guessed something less.. maybe 1
second. Hmm, just wondering if you can really define a thermal time
constant for a resistor. At least for a carbon comp, the heat is
being generated everywhere inside the thing. I think of a thermal
time constant as heating one end of something and asking how long it
takes the whole thing to warm up.

Perhaps it's the heat capacity that determines the maximum energy it
can dissipate. E = C delta T.

George H.

> for a wirewound or axial carbon film, a fraction of a second for a
> surface-mount thickfilm.
>
> So it would probably be OK for a 2 watt axial resistor to absorb 20
> joules. I've seen specs for 5-watt pulse-rated resistors that are
> claimed to absorb over 100 joules.
>
> Somebody should destroy some resistors and publish the results. I
> exploded some 7.5 ohm 1206 thickfilm resistors. It takes about 60
> watts in 1 millisecond. Pulsed, they look like LEDs in the infrared.
>
> John- Hide quoted text -
>
> - Show quoted text -

From: whit3rd on 18 Jun 2010 14:45
On Jun 17, 5:55 pm, "Shaun" <r...(a)nomail.com> wrote:

> 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

Most circuitry with powered capacitors has a bleed resistor to
discharge those capacitors in a few seconds. For safety, one
is advised not to trust the bleed resistor, and various kinds of
probes
up to 'shorting chains' and down to 'connect with ammeter
setting on your VOM' can be used to ensure discharge.

For 15 kV capacitors in one-meter-cube sizes, use a steel
(but not galvanized) chain on a long insulating pole, with
eye protection.
For PC power supply filter capacitors, try that 100 ohm 5W resistor
(use a carbon resistor if you can, because the initial surge
will be more than 5W).
From: amdx on 18 Jun 2010 15:12

"Phil Allison" <phil_a(a)tpg.com.au> wrote in message
news:hvfoel$njb$1(a)news-01.bur.connect.com.au...
>
> "oo pere oo"
>>
>> 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 V�/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 :)
>>
>
> ** All perfectly correct.

> .... Phil

Did we all just witness a lovefest from Phil? :-)
Mike



From: Shaun on 18 Jun 2010 20:12

"oo pere oo" <me(a)somewhere.net> wrote in message
news:hvfct0$k1b$1(a)speranza.aioe.org...
> Shaun wrote:
>> 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.
>>
>> thanks in advance,
>>
>> Shaun
>
> 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
> V�/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
>


Thanks for all your replies.

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


First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4 5
Prev: Simple hack to get $600 to your home.
Next: Video cards