resistor calculation

[sllimbri]

Suppose that an electromechanical relay has a coil voltage rating of 5 volts, and a coil resistance of 55 Ω. However, you desire to energize this relay with a 9 volt batt. You know that a series-connected resistor might enable the 5-volt relay coil to be powered by the 9-volt supply without damage, but you do not know what size of resistor would be appropriate for the task.
Explain how the resistor's value would be calculated, along with the calculated value in ohms.

Thanks.

 

[rjvh]

Suppose that an electromechanical relay has a coil voltage rating of 5 volts, and a coil resistance of 55 Ω. However, you desire to energize this relay with a 9 volt batt. You know that a series-connected resistor might enable the 5-volt relay coil to be powered by the 9-volt supply without damage, but you do not know what size of resistor would be appropriate for the task.
Explain how the resistor's value would be calculated, along with the calculated value in ohms.

Thanks.

This is a clasic school home work question and all you have to do is knowing Ohms law U=IxR

U=5V

R= 55Ω

Based on that you can calculate I

if you have I than you calculate the Resistor value that you need for the exesive voltage that you don't need for your relay with the same formula

Robert-Jan

 

[Pommie]

You can also do it by propotional values, 5/55 = 4/X but I doubt the teacher would accept this as it is not done in the conventional way.

Mike.

 

[Mikebits]

You can also do it by propotional values, 5/55 = 4/X but I doubt the teacher would accept this as it is not done in the conventional way.

Mike.

I don't know why the teacher would object. Algebra and proportions seems like a good solution. Then again there is that silly concept thingy

 

[sllimbri]

ok, so I connected the batt (regular 9v Square Batt) then kept adding resistors until I got a reading less than 6V from neq batt terminal to last resitor leg. I measured the resistance and it is a total of 6Meg ohms. This does not seem to make any sense. Am i missing something. My calculated value came out to 44ohms. (4/44=5/55)

 

[Pommie]

You have something very wrong. Check all your wiring, your calculated value is correct.

Mike.

 

[Boncuk]

you want 5V (measured between relay coil pins). Measuring at the transistor you won't get the true result.

Supplying a 5V relay from 9V using a transistor you will have to "fry" 3.3V in the resistor because of its forward voltage drop of 0.7V. (9V-5V-0.7V).

The resulting resistance will then be 36.3Ω. (33Ω will do)

"Wer viel mißt, mißt viel Mist." -> literally "Who measures a lot, measures a lot of garbage, too."

Boncuk

 

[eblc1388]

ok, so I connected the batt (regular 9v Square Batt) then kept adding resistors until I got a reading less than 6V from neq batt terminal to last resitor leg. I measured the resistance and it is a total of 6Meg ohms. This does not seem to make any sense. Am i missing something. My calculated value came out to 44ohms. (4/44=5/55)

You CAN NOT use only a resistor and your DVM to get 5V first and then connects to the relay. You must connect the 44 ohm resistor and the relay coil to the 9V and then you can measure the coil voltage. This will give you 5V across the coil.

Without the relay coil connected, you are working with the DVM internal resistance(~10Meg) and the external resistor in series and so you will need 6Meg.