# Calculate RL where's thersaurus an LDR in the circuit

#### Lewismm99

Joined Jan 21, 2016
4

#### ericgibbs

Joined Jan 29, 2010
15,338
hi 99,
You have not been given sufficient information about the components in that circuit to enable you to calculate the exact values.
The LDR as you may know is a Light Dependant Resistor, the LED operating current is not specified.

Is this Homework.?
E

#### Lewismm99

Joined Jan 21, 2016
4
hi 99,
You have not been given sufficient information about the components in that circuit to enable you to calculate the exact values.
The LDR as you may know is a Light Dependant Resistor, the LED operating current is not specified.

Is this Homework.?
E
Nope it's an assignment for my college work!

#### ericgibbs

Joined Jan 29, 2010
15,338
hi,
OK, Lets assume that the LED is a general purpose type, say 25mA maximum current.
Its usual to under run this maximum current in order to extend the life of the LED, so assume 20mA, also assume a 2V drop across the LED.
Also assume that the voltage drop across the transitor [ collector to emitter ] is 0V.

Can you calculate the value of Rload that allows a current of 20mA to flow.?
E

#### Lewismm99

Joined Jan 21, 2016
4
hi,
OK, Lets assume that the LED is a general purpose type, say 25mA maximum current.
Its usual to under run this maximum current in order to extend the life of the LED, so assume 20mA, also assume a 2V drop across the LED.
Also assume that the voltage drop across the transitor [ collector to emitter ] is 0V.

Can you calculate the value of Rload that allows a current of 20mA to flow.?
E
I should be a to, I'm in college now I'll check with my tutor sometime soon and I'll let you know, thanks for the help!

#### Dodgydave

Joined Jun 22, 2012
10,233
Try a 470 ohms resistor for starters..

#### Lewismm99

Joined Jan 21, 2016
4
Try a 470 ohms resistor for starters..
Do we know it's a 470? Or am just trying 470?

#### Dodgydave

Joined Jun 22, 2012
10,233
470 ohms will give approx 21mA led current, without knowing what the spec of the transistor and led, your guessing.