Measuring the current through a circuit in order to use a fuse>>>

Hi,
I want to know where to measure the current in the following circuit(no. 2):

http://english.cxem.net/infrared/ir7.php

I want to know the inteare current that the circuit can handle in order to put a fuse, thank you all for your help all the time .

Waiting for your answers.

Abeer_engjbr

Maybe you'll consider my question as a dum one , but to be hounest I've thought that I might measure it from the Voltage Regulator side , but the voltage source that is feeding the circuit if I may say is from there, but again the circuit does not work unless an IR signal is sent.

I know it's concepts but I need some guidness.
Thanks.

Hi Abeer_engjbr

To measure current flow, a meter (ammeter) needs to go in series (between) the supply voltage and the circuit.

Assuming you're using a multimeter, the red (+ve) probe should go to the output from the power supply and the black (-ve) probe should go to the circuit to give you a positive current reading.

SnowyDog

Abeer_engjbr said:

Maybe you'll consider my question as a dum one , but to be hounest I've thought that I might measure it from the Voltage Regulator side , but the voltage source that is feeding the circuit if I may say is from there, but again the circuit does not work unless an IR signal is sent.

I know it's concepts but I need some guidness.
Thanks.

Click to expand...

I would measure the current flow into the regulator and put the fuse in-line with the INPUT of the regulator. This ensures that if the regulator fails (e.g. goes short-circuit) the fuse will protect the supply voltage.

You need to operate the circuit to find the operating current, however you don't want to choose a value of fuse to match this. The fuse needs to be large enough to allow for normal switch-on current.

SnowyDog

Thank you,
I know how to connect the digital multimeter, but , what I am asking about is should I measure the current through sending the IR signal ? and where must I meaure it ( at any componet) so that I got a useful information about the current that the circuit could handle?

something else, I am asking because the DMM I can reach is in collage ,and they've told us to be careful not to make a short and damege them.

To get an accurate idea of the approximate maximum current that the circuit is going to take, yes you need to measure it when the circuit is operating, so in your case, when the circuit is sending the IR signal.

As I said before, you should measure the current going into the input of the voltage regulator from your power supply. The meter must be connected between the voltage regulator input and the power supply.

However, don't forget to choose a fuse value that is larger than the current you measure. When you switch the circuit on, the start-up current is usually greater than this.

You won't damage the DMM if you connect it properly. Before trying to measure the current, make sure that the DMM is switched to measure current and that the +ve probe is plugged into the correct socket on the meter for this. Always connect an ammeter in series with the power supply and your circuit, never connect an ammeter directly across the power supply terminals as this will damage it!

SnowyDog

Thank you very much SnowyDog
Abeer_engjbr

Put a temporary jumper between the output of the IR sensor and GND. This will turn on the 555 and simulate full power. On the other hand about 250mA fuse (or better yet a 200mA PTC) will be more than enough for that circuit.
**broken link removed**
A self resetting PTC thermal fuse.