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Forum: Analog Circuits Make capacitor discharging tool


von Lernend B. (Company: KAI Electro Semiconductor Inc.) (lernend05)


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Hello all,
I am in the process of "building" a capacitor discharging tool according 
to Rules of Discharging a Capacitor(see attachment), I could use some 
resistors that I pulled out of other circuits. If not, I'll buy one at 
my local electronic components store.
Would a 10K/2W be enough for discharging (occasionnaly) the filter caps 
in a 5f1? I also have a spare 220k/2W.

Is there anyone hava ideas of it? Thank you in advance.

von Tom (Guest)


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As nobody knows what a 5f1 is: 16uF at about 360V, unless you are using 
bigger caps for less hum.

von Erich H. (Guest)


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I think he means this: 
https://www.tube-town.net/cms/?DIY/Vintage-Serie/5F1
In fact it has no Bleeder Resistors for Anode voltage.

As a rough estimate 10kOhm at 35 uF (in total) have am time constant of 
350 ms.
I. E. After 2  time constants ( 700 ms) the voltage will be drop to 
harmless 50 V.

But the total Power Dissipation of the resistor will be 13.5 W at 367 V!

von Lothar M. (lkmiller) (Moderator)


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Lernend B. wrote:
> Would a 10K/2W be enough for discharging (occasionnaly) the filter caps
> in a 5f1? I also have a spare 220k/2W.
Tom wrote:
> As nobody knows what a 5f1 is: 16uF at about 360V, unless you are using
> bigger caps for less hum.
Then its easy to calculate the time constant which is needed to come to 
70.7% of the initial value: t = RC = 10k*16uF = 160ms.
So starting at 360V you will have 58V after 160ms and 41V after 320ms 
and 28V after 480ms. And 28V wont hurt you anymore.
Of course you could calculate this by using the usual discharge formula.

And also the resistor wont get too hot, because it is only a energy of E 
= 1/2*(U²*C) = 1Ws stored in the capacitor, thats not enough to heat up 
a usual 2W resistor.


And the maximum inital power at the resistor is P = U²/R = 13W for a few 
ms. Even a usual 2W resistor will be able to handle this temporary 
overload. Of course best would be a wirewound or a massive resistor.

> to Rules of Discharging a Capacitor(see attachment)
This is a real funny peace of document. For me the whole information 
therein is t=RC.

: Edited by Moderator
von Erich H. (Guest)


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Ok, now I understand.
The resistor shall not be permanently in the circuit but only a tool 
when the Amp is beeing repaired you want to connect it to the caps and 
discharge it.
Then 2 W should be ok.

When I was a Kid I took always a screwdriver and Bang

von Andrew T. (marsufant)


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Erich H. wrote:
> Then 2 W should be ok.
>

Recommend wire wound, to take the high pulse load of up to 14 Watts.

> When I was a Kid I took always a screwdriver and Bang

some of us learned to take more care of their equipment, tools and ears.
Some of us still enjoy the BANG  ,-)

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