How do you calculate resistor temperature

drax · Feb 16, 2001

Instead of a fixed resistor, a supplier told my company to use a varible resistor without the slider to accomplish the same thing. Just looking at it, there is a bare insulated teflon wire laying directly on the exposed sliding element of the resistor. I know the temp of the wire is rated at 200C but I not sure how the resistor will affect the wire. The 750 ohm resistor is used to disapiate (bleed off) a 550 micro farad capicator in the 125VAC circuit. Any ideas on how to calculate the heat of the resistor?

jbartos · Feb 17, 2001

The capacitor will have the discharge decaying transient current
Ic=(E/R)xexp(-t/RC)=(125/750)xexp[-t/(750x0.00075)], Amps
where t is time in seconds
Differential Heat dH of resistor
dH=cxwxdT in kcal or dH=4183xcxwxdT in joules
where
c is specific heat of resistor material in kcal/kg
w is resistor weight in kg
dT=T-To is temperature differential in °C
where T is resistor temperature
To is ambient temperature
Then thermal - electrical relationship is (Equation 1):
Rx[Ic(t)]**2xdt=[cxwxd(dT)+TcxSxdTxdt]x4183
then integrate it by t to obtain the dT function dependent on t
where
Tc is the thermal conductivity of the resistor indicating heat in kcal of the resistor being dissipated via surface S in meter**2 within one second at a rate 1 °C. Notice that the solution T(t) of Equation 1 shall be below the horizontal line at Th=200 °C, which includes the ambient temperature To in it, for all t.

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