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Author Topic: Need Help with Power problem  (Read 2744 times)

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Need Help with Power problem
« on: August 10, 2012, 09:10:43 AM »
Hello Everyone,
can anyone tells me where is the problem with my circuit.
to be specified this is a security door unlock with two boards one is the brain and the other is the controller.
my problem is i have a regulator 7805 it can deliver up to 1A of current, i'm pretty sure that the circuit is not taking more than 500mA and i did put a heat sink to the regulator but it get really hot when in use and the system collapse after few hours and stop working till i pull it out of electricity and wait till it cool
i am attaching the Eagle files and pictures of the circuits diagram.
please i need you help with this.


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Re: Need Help with Power problem
« Reply #1 on: August 10, 2012, 11:28:47 AM »
there is one parameters that people tend to ignore. Linear regulators dissipate the rest power as heat (thats why they are not very efficient). What this means is that if you provide lets say 12 volts and you get 5, then there is an amount of power that has to be dissipated to get your 5 volts. To calculate this, you subtract the 5 from 12 and then multiply by the current drawn. Suppose that you circuit draws 0.5A (500mA), and you power from 12 V:

Vdif = 12 - 5 = 7V

Pdis = 7 x 0.5 = 3.5 Watts

From my experience i can tell you that 3.5 Watts of power dissipated will make the 7805 to fry within one minute. You'll get a bad burn if you touch it.

So, what to do: first measure the current drawn, the MAX current drawn.  Then, use 2W resistors to drop the excess of power. In the previous example, i would try to dissipate the 3.5W of heat into 4 elements, 3 resistors and the 7805. This way, each one would have to dissipate some 1.1 watts. To calculate the resistors, you reverse the P=I^2xR formula like this:

R = P / I2 = > R = 1.1 / 0.52 => R = 4.4 Ohms

So i would put 3 resistors in series before the 7805, each resistor is 5Ohms 2 (or more) watts

Obviously, the 500mA that you mention is not a real value, because as i said, 4 watts are TOO much. So you need to precisely measure your MAX current

Solution 2: Use an SMPS buck regulator... And problem solved... It is not as simple as the 7805, but it will dissipate only a fraction of the power provided

Solution 3: Use a smaller voltage as a main supply. If for example you use 6.5 volts to drop them down to 5, the power dissipation is:

1.5 x 0.5 = 0.75 Watts

A small heatsink is enough to dissipate this power.


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Re: Need Help with Power problem
« Reply #2 on: August 10, 2012, 13:36:54 PM »
i think there are some circuit examples in the datasheet that help to work around high input voltages. give it a shot. or do as kam told you