Turn on thread page Beta
 You are Here: Home >< Physics

# Controlling a small current with a large current. watch

1. Could someone explain what's going on here step by step, I can't understand what the text is trying to say.
2. The current that can pass through the LDR isn't enough to run the motor.

However, it is enough to operate a relay switch, which, when closed, will be able to carry enough current to run the motor.

Basically, the current required for the motor to work is more than the LDR can pass.

Not really sure if that helps, but thats what I get from it! We've started doing all this stuff in Physics too, but I did electronics last year so kinda remember some stuff
3. Ha, if youre doing edexcel Physics old specification, I can almost guarantee that you won't need to know about that :P
4. OK, so you are using an LDR to turn on a motor:

So the motor draws a large current from the battery and the LDR is in the same circuit as the motor. When the motor is running, the large current it is drawing will also run through the LDR (current is the same throughout the circuit) and this would burn out the LDR.

Instead, there are two seperate circuits:

1 - This contains the LDR, a small power supply and a relay. When the LDR is 'on', the circuit closes a relay, this is a switch that can be operated by an electrical current.

2 - The motor circuit is connected to the battery using the relay to switch it on.

Simple

P.s. More information can be gound here: http://en.wikipedia.org/wiki/Relay#B..._and_Operation
5. In practise I'd be surprised if the current through an LDR was enough to energise a relay - I'd use a transistor.
6. (Original post by beesbees)
The current that can pass through the LDR isn't enough to run the motor.

However, it is enough to operate a relay switch, which, when closed, will be able to carry enough current to run the motor.

Basically, the current required for the motor to work is more than the LDR can pass.

Not really sure if that helps, but thats what I get from it! We've started doing all this stuff in Physics too, but I did electronics last year so kinda remember some stuff
So wait, what's happening is the current is passing through the LDR and activating the relay coil which then magnetises the contacts and brings them together? This is because we can't let enough current pass through for the motor to work, so essentially we're activating the motor by using a small current?

If I'm wrong here can anyone clear up exactly what the role of the coil is in this example. And more importantly, why they've got an LDR in the diagram?

Turn on thread page Beta
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 19, 2009
Today on TSR

### Should I drop out of uni

...to become a pro gamer?

### University open days

• University of Buckingham
Psychology Taster Tutorial Undergraduate
Fri, 14 Dec '18
• University of Lincoln
Mini Open Day at the Brayford Campus Undergraduate
Wed, 19 Dec '18
• University of East Anglia
UEA Mini Open Day Undergraduate
Fri, 4 Jan '19
Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE