Controlling current that enters circuit

yoboy · Feb 27, 2010

Hi all,

My friend needs opinions and suggestions on how he can control current that enters into his circuit. He is involved in a project that required him to read Car Battery Voltage using an ADC module from PIC Microcontroller. The voltage is as high as 12V or so and the associated current is in the range of 13A. While he has no problem down-ranging the 12V to 5V to be read by his PIC, he still has no clue on how the current be limited. He is affraid that such large current could do permanent damage to his circuit.

To add to the problem, the current would overshoot when the car engine starts, so it would go way over 13A. So, what can he do to protect his system from such large currents and at the same time still be able to read voltage from his car battery using ADC module of his PIC Microcontroller?

Any suggestion is welcomed.

Torben · Feb 27, 2010

yoboy said:
Hi all,

My friend needs opinions and suggestions on how he can control current that enters into his circuit. He is involved in a project that required him to read Car Battery Voltage using an ADC module from PIC Microcontroller. The voltage is as high as 12V or so and the associated current is in the range of 13A. While he has no problem down-ranging the 12V to 5V to be read by his PIC, he still has no clue on how the current be limited. He is affraid that such large current could do permanent damage to his circuit.

To add to the problem, the current would overshoot when the car engine starts, so it would go way over 13A. So, what can he do to protect his system from such large currents and at the same time still be able to read voltage from his car battery using ADC module of his PIC Microcontroller?

Any suggestion is welcomed.

The circuit will only draw the current it needs. A power source "pushes" voltage into a load (i.e. the circuit), but the load "pulls" the current it needs from the power source. If the circuit is properly designed and constructed, it will not draw more than it needs. In the event of component failure or accidental short it *could* draw much more current, so fusing it is a good idea in order to avoid damaging the battery or starting a fire.

You will want to protect against voltage spikes on the supply, though. Check out this thread: https://www.electro-tech-online.com/threads/power-for-automotive-circuits.103137/

Torben

Hero999 · Feb 28, 2010

The voltage meter should be able to measure up to 15V, as the PIC will probably be powered from 5V a potential divider with a ratio of 1:3 is required; Google is your friend.

The PIC should be powered using a voltage regulator such as the LM78L05 or LM7805 (the circuit is on the datasheet and can be found using Google) which will convert 12V to 5V. There is no need for over current protection as the regulator IC has it built-in and the resistors in the potential divider will limit the current if a short occurs.

ericgibbs · Feb 28, 2010

Hero999 said:
The voltage meter should be able to measure up to 15V, as the PIC will probably be powered from 5V a potential divider with a ratio of 1:3 is required; Google is your friend.

The PIC should be powered using a voltage regulator such as the LM78L05 or LM7805 (the circuit is on the datasheet and can be found using Google) which will convert 12V to 5V. There is no need for over current protection as the regulator IC has it built-in and the resistors in the potential divider will limit the current if a short occurs.

hi hero,
The OP is proposing to measure the 12Vdc of the battery by using a potential divider from 12V to 5V, he has solved this problem himself.

He is asking how to measure the current in the cars circuit, I assume by using a 'shunt' and to be sure that excess current drawn during car starting the car dosn't damage the input to the PIC.

He is not asking how to power the PIC.??

Hero999 · Feb 28, 2010

He did not make it clear that he wants to measure the current, he said he wants to control the current to the circuit which doesn't make any sense.

A sense resistor or hall effect sensor can be used to measure current but I can assure you that the current flowing to and from a car battery will be grater than 13A. You also might need to measure the current in both directions as it will be reversed when the alternator charges the battery.

ericgibbs · Feb 28, 2010

Hero999 said:
He did not make it clear that he wants to measure the current, he said he wants to control the current to the circuit which doesn't make any sense.

A sense resistor or hall effect sensor can be used to measure current but I can assure you that the current flowing to and from a car battery will be grater than 13A. You also might need to measure the current in both directions as it will be reversed when the alternator charges the battery.

he already knows this.

To add to the problem, the current would overshoot when the car engine starts, so it would go way over 13A. So, what can he do to protect his system from such large currents and at the same time still be able to read voltage from his car battery using ADC module of his PIC Microcontroller?

Any suggestion is welcomed.

Hero999 · Feb 28, 2010

Don't tell me he's asked this in another thread?

I give up.

Controlling current that enters circuit

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Controlling current that enters circuit

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