calculation of rms and average voltage for a heating element(resistor)

[shalini1234]

hi,

I want to calculate the rms and average voltage manually for the load heating resistor of 15ohm.
I have given 11v supply, 18w power and current is applied by means of pwm. I should acheive 25%,50%,75% of heating.
So how to calculate the rms and avg voltage for these requests..

Thanks and regards,
Shalini

 

[smijesh]

Here the resistor is powered by Pulsating DC(PWM). So you have to find out the duty cycle.

Average Voltage = duty cycle * peak Voltage
RMS Voltage = sqrt(duty cycle*peak voltage ^2)

For restrictive load there is no reactive power so you can directly calculate the power
Power = Vrms * I rms

 

[shalini1234]

Here the resistor is powered by Pulsating DC(PWM). So you have to find out the duty cycle.

Average Voltage = duty cycle * peak Voltage
RMS Voltage = sqrt(duty cycle*peak voltage ^2)

For restrictive load there is no reactive power so you can directly calculate the power
Power = Vrms * I rms

Thanks for the reply.
but how to calculate Ton and Toff theoritically...and also i have one doubt.. i am giving only dc supply. But what is the reason for calculating RMS value since RMS is concerned only with AC supply.

 

[smijesh]

Q:but how to calculate Ton and Toff theoretically ?

A:You don’t want to calculate, You told it is PWM so it should have on time and off time.
Q: .. i am giving only dc supply. But what is the reason for calculating RMS value since RMS is concerned only with AC supply.
A: If it is a PWM (Pulsating DC) you should calculate the RMS value.

 

[godfreyl]

I want to calculate the rms and average voltage manually for the load heating resistor of 15ohm.
I have given 11v supply, 18w power and current is applied by means of pwm....

This doesn't make sense. Applying 11V across a 15Ω resistor will only result in 8W power dissipation. Where does the "18W" come from?

 

[shalini1234]

This doesn't make sense. Applying 11V across a 15Ω resistor will only result in 8W power dissipation. Where does the "18W" come from?

The maximum voltage that can be given to resistor of 15 ohm in my case is 16.4 v and hence power of 18w. I want to achieve 25%, 50%, 75%, 100% of duty cycle for 11v, 14v and 16v. So i want to calculate the rms and average voltage for these duty cycles at mentioned voltages theoritically. and also need to calculate the activation and deactivation time of heater theoritically..

- - - Updated - - -

This doesn't make sense. Applying 11V across a 15Ω resistor will only result in 8W power dissipation. Where does the "18W" come from?

The maximum voltage that can be given to resistor of 15 ohm in my case is 16.4 v and hence power of 18w. I want to achieve 25%, 50%, 75%, 100% of duty cycle for 11v, 14v and 16v. So i want to calculate the rms and average voltage for these duty cycles at mentioned voltages theoritically. and also need to calculate the activation and deactivation time of heater theoritically..

 

[betwixt]

The proportion of power is the same as the PWM on/off ratio (duty cycle). If you want 25% heating use 25% of the time on and 75% off, for 50% use equal on and off times. The speed at which the cycle repeats is entirely up to you, it is the proportion of the cycle that matters. RMS is not really an appropriate term here. The reason RMS is used is to remove the complication of the average being zero when the polarity reverses but in simple PWM that doesn't happen. You could consider that the RMS value would be that which gave the same heating effect as DC would and work out the figure from that.

Brian.

 

[shalini1234]

The proportion of power is the same as the PWM on/off ratio (duty cycle). If you want 25% heating use 25% of the time on and 75% off, for 50% use equal on and off times. The speed at which the cycle repeats is entirely up to you, it is the proportion of the cycle that matters. RMS is not really an appropriate term here. The reason RMS is used is to remove the complication of the average being zero when the polarity reverses but in simple PWM that doesn't happen. You could consider that the RMS value would be that which gave the same heating effect as DC would and work out the figure from that.

Brian.

But smijesh said we need to calculate rms voltage for pulsating dc. suggest me, whether i have made right calculation in the attached link..

https://obrazki.elektroda.pl/9181437300_1391509117.png

 

[FvM]

But smijesh said we need to calculate rms voltage for pulsating dc.

I don't know if you need to calculate it, but you did it right. I ignore the unrelated,meaningless or erroneous columns like "maximum power" and "activation time".