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sudarshan
05-30-2010, 05:12 PM
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
05-30-2010, 05:29 PM
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
05-30-2010, 05:30 PM
Mispost - Sorry ( Administrators please delete this.)

avinash_g
05-30-2010, 05:53 PM
7805 is overloaded may be due to short circuit.

vikas
05-30-2010, 06:43 PM
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
05-30-2010, 08:02 PM
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
05-30-2010, 09:20 PM
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
05-30-2010, 09:46 PM
Is it the 78L05 version or the 7805 ?

sudarshan
05-30-2010, 10:37 PM
it is 7805 version

MohitM
05-31-2010, 12:05 PM
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 (http://www.BioZen.co.in)

sudarshan
05-31-2010, 01:06 PM
the motors run only on the supply of 7805.
can i use some higher value resitance
what is the exact fix for the problem sholud i connect 5-7 ohms resistor .

MohitM
05-31-2010, 04:11 PM
the motors run only on the supply of 7805.
Tough luck then! Get your self a PC SMPS. Identify the +5V and 0V. Remove the 7805. Connect the +5V to where the output of 7805 pin would have been and 0V to the PCB ground.
In any case, the design of the board needs serious rework. Get yourself another board from people who know what they are doing.


can i use some higher value resitance
what is the exact fix for the problem sholud i connect 5-7 ohms resistor .
You could but supposing you are drawing 2A, then 5 ohm resistor will be dissipating 10W. You will have to get a block of ice and securely mount the resistor to it. Just kidding... 5 ohm resistor @ 2A will drop 10V LM7805 sees only 2V at its input. So your board won't even start, or will shut down as soon as the motors kick in.

Ulhas
05-31-2010, 09:44 PM
Are you connecting any motors to it?
Actually it maters only when motor is rated for 5 Volt which is not normal case and if it is rated for 12 V then it is drawing current from power supply and not from the 7805. Hence that i don't think will be the reason for making 7805 hot

check that have you mistakenly connected 12V motors to 7805 output 5V?

sudarshan
05-31-2010, 10:03 PM
it is a board from robokits so i cant change any thing. when the motors are runing it consume around 2 amps .when the motors are not running it draw around 0.3A only but the Ic 7805 heats up steadly. when motors run it heats up rapidly. but this does not happen when 7.4 Li battery is connected.

MohitM
06-01-2010, 04:04 PM
How hot does the IC feel when you touch it?

pratheek
06-02-2010, 06:07 PM
Did you get any circuit diagrams for the board?

It will really help solve your problem.