# Calculating current, voltage drop & resistance

1. May 29, 2012

### SummersLCFC

Hi i built a circuit for a project and im having some trouble working out important factors of the circuit. My circuit is powered by a 12V power supply, its wired in series, it contains 85 LED's that are connected with a 1kΩ resistor across 3 off the LED's. This is then connected to a PIR sensor and a 12V relay because the PIR is n/c contact.

I need some help on working out the current, the voltage drop and the total resistance of the circuit.

Thanks tom

2. May 29, 2012

### Staff: Mentor

So from the 12V battery you have a 1kΩ resistor in series with 3 series LEDs, then this current returns to the other terminal of the battery? Is that the arrangement? Can you measure the voltage across just one LED when it conducts, here in this circuit?

3. May 29, 2012

### SummersLCFC

There are 85 LED's in the circuit, so it starts with 3 LED's connected up with 1 resistor across them, then another 3 LED's and so on, so that makes 29.

4. May 29, 2012

### Staff: Mentor

Perhaps I should ask the important question first: is it working well as you planned?