7812 gets hot. how to reduce the current.

Thread Starter

imraneesa

Joined Dec 18, 2014
227
i tried to connect 7812 to laptop charger 19V 3.5A. and the wire start to smoke. 3.5A is too much for the regulator?
how to limit it? it is not possible to put resistor in series becoz we need so much wattage.
what alternate things i can do.
i have checked the pinouts in datasheet. so i connected them correct.
do so much current passes through regulator even though no load is connected?
 

wayneh

Joined Sep 9, 2010
17,496
It's hard to understand your results. It sounds like the regulator got hot with no load attached?

Did you use input and output capacitors as recommended in the data sheets? The ripple in the supply voltage may have been causing problems for the regulator. Filtering it may help.

The normal 7812 cannot handle 3.5A, even with a large heat sink. The application circuits in the dat sheet will usually describe how to build a higher current circuit.
 

dl324

Joined Mar 30, 2015
16,846
i tried to connect 7812 to laptop charger 19V 3.5A. and the wire start to smoke.
Are you saying that the charger wire started to smoke? Was there a load on the 7812 at the time?
3.5A is too much for the regulator?
Yes. Maximum current tolerated depends on package and heatsink.
how to limit it? it is not possible to put resistor in series becoz we need so much wattage.
Determine what is drawing so much current and go from there.
do so much current passes through regulator even though no load is connected?
Quiescent current for a 7812 should be around 10mA.
 

Roderick Young

Joined Feb 22, 2015
408
What causes the LM7812 to get very hot is when there is a large voltage across it at that current. You have 19 volts in, and 12 volts out, so that means there are (19 - 12 = 7) volts across the regulator. At 3.5 amps, that's (7 * 3.5) = 24.5 watts, the same as a small soldering iron.

If you need 12 volts at 3.5 amps, your best bet is to buy a ready-made adapter with that output on eBay or somewhere. It won't be very expensive. Second choice, if I was doing it, would be to buy a switching regulator on eBay, perhaps based on the LM2596, and put a big heat sink on the actual regulator part. Last time I got one of those modules, it was less than $3 from eBay.
 

Thread Starter

imraneesa

Joined Dec 18, 2014
227
Thank you for all the comments. i went to recheck and found how stupid i was. i connected hot wire to ground and ground to input.

i spoiled the regulator finally. but i learned something. thank you so much everybody.
 

Thread Starter

imraneesa

Joined Dec 18, 2014
227
Are you saying that the charger wire started to smoke? Was there a load on the 7812 at the time?
Yes. Maximum current tolerated depends on package and heatsink.
Determine what is drawing so much current and go from there.
Quiescent current for a 7812 should be around 10mA.
what is quiescent current mean. can you please explain it in easy words? thank you.
 

kubeek

Joined Sep 20, 2005
5,794
Thank you for all the comments. i went to recheck and found how stupid i was. i connected hot wire to ground and ground to input.
i spoiled the regulator finally. but i learned something. thank you so much everybody.
There is no better experience than letting the magic smoke escape. You won´t do that same mistake anytime soon.
 

ian field

Joined Oct 27, 2012
6,536
i tried to connect 7812 to laptop charger 19V 3.5A. and the wire start to smoke. 3.5A is too much for the regulator?
how to limit it? it is not possible to put resistor in series becoz we need so much wattage.
what alternate things i can do.
i have checked the pinouts in datasheet. so i connected them correct.
do so much current passes through regulator even though no load is connected?
The 78xx appnotes describe how to add an external bypass transistor to get more current - its been discussed several time just recently on this forum.

If you're burning up wires, you have a short somewhere.
 

dl324

Joined Mar 30, 2015
16,846
what is quiescent current mean.
I used the terminology used by the manufacturer. Quiescent == inactive or idle. Quiescent current changes depending on the regulator's load. That is important to know if you're inserting a resistance or diode in the ground connection to change the output voltage. See specs attached below.
 

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ian field

Joined Oct 27, 2012
6,536
I used the terminology used by the manufacturer. Quiescent == inactive or idle. Quiescent current changes depending on the regulator's load. That is important to know if you're inserting a resistance or diode in the ground connection to change the output voltage. See specs attached below.
I couldn't find the bit in the manufacturer's literature about the wires started to smoke.
 
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