Capacitor Discharge Tool Questions

[Electro Fan]

I'm thinking about making a Capacitor Discharge Tool similar to what is shown in the video below:



Question 1:  With a 50 ohm 50 watt resistor is there any reason to prefer a "Non-Inductive" (less inductive?) resistor?  (Or for some reason other than cost would the Inductive version be preferable for use in a Capacitor Discharge Tool?)

"Inductive"
http://www.mouser.com/Search/m_ProductDetail.aspx?Vishay-Dale%2fRH05050R00FE02%2f&qs=sGAEpiMZZMtlubZbdhIBIOzZS54oCXX%2fGPn392boNXM%3d

"Non-Inductive"
http://www.mouser.com/ProductDetail/Vishay-Dale/NH05050R00FE02/?qs=sGAEpiMZZMtbXrIkmrvidDNaDpN5VXc5imSduYlJpgA%3d

Question 2:  Is there any notable increased safety value in going from a 50 ohm resistor to a 100 ohm resistor - the higher resistance would slow down the discharge rate but would it also improve safety during discharges?  If not, the faster discharge rate of the 50 ohm resistor would seem more convenient.

Thanks

[Ghydda]

No and no.

[Electro Fan]

Thanks

If the answer to Number 2 is no, any reason the resistance spec shouldn't be reduced (to 25 ohms for example) - which would make the discharge time faster (and more convenient)?  Or will reducing the resistance increase the allowed current and effectively increase wattage (and heat), thereby putting more stress on the 50 watt spec?  Net, net:  what determines the "sweet spot" in the specs?  Perhaps it's just a matter ultimately of how many uF you want to discharge safely within a given amount of time?  If so, is there an amount of time (or a curve) which reflects reasonable discharge safety?

Thx again

[DanielS]

A 50W resistor would have a huge amount of specific heat. Unless you are discharging huge capacitors, most will be discharged without much noticeable heating.

If the resistor weighs 100g with a specific heat of 10kJ/K/kg, discharging 1kJ into it would only warm it up by 1 deg.C.

The more important factors to consider would be pulse-withstand current and maximum operating voltage.

[Ian.M]

Check the specs of the resistor carefully.  Most are not rated for pulsed overloads of x10 the normal continuous rating.  A few expensive ones are, for one second.
The highest voltage you are likely to meet in most PSUs is 350V, with high mains input and either a simple rectifier or in North America, a voltage doubler.

(V^2)/R gives you the instantaneous peak power, so to keep it under 500W you need to use a resistor of at least 245 ohms.  The time constant of 270R with 1000uF (which is many times bigger than a typical SMPSU input cap) is 0.27, and, starting from 350V the cap will be touch-safe after 3 time constants, and nearly totally discharged within 1 second.

However, that's one big expensive clumsy resistor, and unless you work on very large PSUs frequently, you don't need it to discharge that quickly.   1.8K 7W at the same x10 peak overload will discharge more typical reservoir caps of up to 200uF in about a second.

Check your discharger resistance before and after use!

[Electro Fan]

Thanks for the info.

This is the resistor I have in mind (50 ohms, 50 watts):

http://www.mouser.com/Search/m_ProductDetail.aspx?Vishay-Dale%2fRH05050R00FE02%2f&qs=sGAEpiMZZMtlubZbdhIBIOzZS54oCXX%2fGPn392boNXM%3d
- I didn't see anything about handling pulse overloads, but maybe I missed something in the specs?

According to calculations:

25 Volts with 10,000 uF should fully discharge in 14 seconds, or 25 Volts with 1,000 uF should fully discharge in 2 seconds - let me know if this doesn't look right.  Thx

(The calculator I used said that 350V with 200uF using a 1800 ohm resistor would nearly fully discharge in 3-4 seconds but would take 10-11 seconds to fully discharge.)

[kripton2035]

I bought this tool some times ago - works fine for that purpose
search for "sparkpen" on any online seller you want

http://www.dx.com/p/repair-tools-sparkpen-capacitor-discharge-pen-for-camera-mobile-phone-home-appliances-129179

[geggi1]

This is a bit similar to what we in the Ham comunity call a "o jesus stick"
Usualy it is made up by an old screwdriver a lenght og copperwire and a lug, all connected together.

Operating procedure.
1. Connect log and wire to the chassis of your tube amp.
2. Turn of the power on your amp and unplug.
3. Put the tip of the screwdriver to the HV capacitor terminals.
4. Discharg the capacitor and say the magic word. "o jesus"
5. Repeat 4. until all capacitors are discharged.
6. Safe to work.