ic 7805 heats up

[sudarshan]

hi
i am having avr 40 pin robot controller v2 board from robokits.co.in
i am using two 6 volt 4.5 ah lead acid batteries inseries so it makes 12volt connected to it
the ic 7805 heats up rapidly sometimes it even resets the mcu.
i have added heat sinks and 1E /2w resistor in series with the battery as suggested by robokits still the problem exisits

please help me

[pratheek]

LM7805 is a linear voltage regulator. To regulate the voltage, it converts the excess input voltage into heat. If the temperature rises too much, it shuts down automatically, that may be reason why your MCU resets.

But it shouldn't heat up to the extent that you are experiencing.

Firstly, check for any short circuits in the board.

Secondly, try reducing your input voltage to 7-9V. At 12V the 7805 will be converting more than half of the input voltage into heat.

If the problem still persists, then try soldering a new 7805 onto the board in place of the current 7805.

[pratheek]

Mispost - Sorry ( Administrators please delete this.)

[avinash_g]

7805 is overloaded may be due to short circuit.

[vikas]

What is the current your circuit consuming . 7805 can handle upto 1 A i think.

Are there motors connected to this circuit ? if yes whats there Current Rating ?

[sudarshan]

I agree with pratheek.
when i used low voltages that is Li battery the ic7805 does not heat up.

I think i should reduce the voltage in two steps that is by reducing 12 volt into 9 volt and then into 5 volt.

what do you think

[Ulhas]

You can try LM2940 - 1A Low Dropout Regulator for which nput voltage can be upto 6 volts, just take tap from your 2 series connected batteries. This will also increase the backup time as power disipation in this case will be 1 W in LM2940 which is much lesser than LM7805

It cost Rs 45 at Lamington Road ,Mumbai

[shobhitkukreti]

Is it the 78L05 version or the 7805 ?

[sudarshan]

it is 7805 version

[MohitM]

hi
i am having avr 40 pin robot controller v2 board from robokits.co.in
i am using two 6 volt 4.5 ah lead acid batteries inseries so it makes 12volt connected to it
the ic 7805 heats up rapidly sometimes it even resets the mcu.

Are you connecting any motors to it? Are the motors powered by the on-board LM7805?
The site claims 2A per channel capacity for the motors, but if it uses the output of LM7805 as the source for these motors as well, that 2A/channel is not gonna happen. LM7805 are rated at about 1A capability. At this current level, since your input is 12V, you are generating (12V-5V) * 1A = 7W of heat at the LM7805. You'll need a decent sized heatsink on it.

i have added heat sinks and 1E /2w resistor in series with the battery as suggested by robokits still the problem exisits

Worst kind of band-aid fix! The fellow you talked to doesn't know a lot of electronics engineering. Its basically to drop the voltage (an IR drop). So supposing you are drawing 1A from the LM7805, which means you are drawing 1A from the battery. So a 1 ohm resistor after the battery, will help you drop 1 ohm* 1 A = 1V, before the LM7805. So now, the input is 11V, but you are still generating 6W. The balance 1 W is dissipated at the 1 ohm resistance. Point is, you have now distributed the heat generation to two places. So its basically a fix, and not proper engineering.

You need to tell us whether the motor outputs are being run using LM7805. In which case, the design of the board is flawed and you should return it to the vendor pronto.

Regards,
Mohit.
www.BioZen.co.in