Resistor problems

[Noodles141]

Ok so i a whole mess of stuff to create a mono-stable with and independent trigger but this is irrelevant as the probelm lies with a capacitor and the resistor. focus on this or just picture a simple circuit with a resistor in series with a capacitor . the basic problem is discharging the capacitor with a resistor to limit the current. What i want to know is do you i even need this resistor to limit the current. If this is possible then yay! if not heres my problem at the moment(dont want to try without resistor since i only have one capacitor at the moment and dont want to waste it); I want the capacitor to discharge very fast so how would i do this, I tried using a 100 Ω resistor and it nearly burnt out, heated up enought to burn my finger a little. how could i use a resistor with a small resistance without it burning out or should i stick with a higher resistance?(i know this basic electronics but i am a first year and still getting used to the basics)

 

[MikeMl]

Your description is insufficient to figure out your problem. Post a schematic diagram of your circuit with the problem resistor clearly identified.

 

[unclejed613]

what is the voltage on the capacitor when you discharge it? if it's 100V, then to discharge the cap, you need to pick a resistance that gives a current within the capability of the resistor. so for a 100V starting voltage, and a 1 watt resistor (just for example) you would choose a resistance that would keep the heat dissipation below a watt. 1 watt would be 10mA at 100V, so you would want to use a 10k or larger resistance to keep the dissipation at or below 1 watt. the voltage across the resistor will only be 100V initially and will drop off at a rate determined by the time constant of the cap and the resistor. if you're using a 1/4W resistor, you want to increase the resistance proportionally, but be aware this also increases the time constant.

so the things you want to calculate are:
initial current through the resistor I=E/R
required wattage of the discharge resistor P=EI
if the wattage is higher than the wattage rating of available resistors, solve for the minimum R using R=E^2/P

the time constant is 0.67RC which brings you to 33% of the original voltage, so keep the resistor connected for at least 5 time constants.

 

[Noodles141]

thanks this help me understand it a bit more. But i figured it out. I'm sorry i cant post a diagram if you needed one Mike i don't have the time right now but if you wont i will spend a little time tomorrow putting one together and uploading it. Basically I needed the resistor as it was helping drive the current in the right direction for the mono-stables trigger then leaking the current when the trigger was lifted but i wanted to do it so the trigger didn't need to be lifted. Couldn't find a way of doing it without lifting the trigger so changed my setup and it will work now with an almost instant discharge.

 

[ChrisP58]

A picture is worth a thousand words. And when it comes to circuits, schematics are pictures on steroids.