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# Why LM7805 get very very hot

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#### tellMeDaBasix

##### Member level 1
lm7805

I have 12V step down transformer. This I connect to LM7805 or LM7809 after diode bridge.

I been using this setup for long long time to power microcontroller and other IC based circuit.

Recently, I connect DSL router that take 5V input to output of LM7805 and both LM7805 and transformer get very very hot.

Why LM7805 and transformer get very very hot when I connect to router but not other IC or microcontroller?

lm7805 circuit

The router naturally takes more current than the IC or the microcontroller circuit, thus heating up the regulator.

lm7805 circuit to get 5volts

pranam77 said:
The router naturally takes more current than the IC or the microcontroller circuit, thus heating up the regulator.

check the current rating for 7805 & the router ???

integrated circuit is very hot

The voltage is high at 12V, while the output is at 5V.
The larger the difference, the more the energy is dissipated as heat.
This observation is quite typical.

If you want lower heat, try a dc-dc converter.

Best Regards,
Siong Boon

### tellMeDaBasix

Points: 2
7805 very hot with 12v

check current consumption of DSL router

lm 7805

Hi, how to check check current consumption of DSL router?

I have got ammeter.

On router nothing written as it is cheap router. Only voltage given as 5v DC.

siongboon said:
The voltage is high at 12V, while the output is at 5V.
The larger the difference, the more the energy is dissipated as heat.
This observation is quite typical.

Yes! So my question is why 7805 get hot only when I connect to router but not IC (voltage drop by 7805 is (12 - 5) V in both cases!) - main reason I ask question.

lm7805 very hot

Dear tellMeDaBasix!
Power 7805 dissipation is
P = Vdrop * I, where Vdrop - voltage that drops across 7805 and is equal 12 - 5 = 7V;
I - current through 7805.
Consequently P = 7 * I.
tellMeDaBasix said:
Hi, how to check check current consumption of DSL router?
Yes! So my question is why 7805 get hot only when I connect to router but not IC (voltage drop by 7805 is (12 - 5) V in both cases!) - main reason I ask question.
See the formula above. Voltage drop is the same but current is not. Measure the current of DSL router and compare the current of IC.

To decrease the power dessipation take any suitable DC/DC converter like mc34063.

### tellMeDaBasix

Points: 2
lm7805 external transistor

haker_fox, EXACTLY kind of answer I was hope!

Should I measure currnt by putting ammeter between posetive power leads between router and 7805?

Can you also show me some tried and tested mc34063 based circuit except one in datasheet alerady please?

How much current mc34063 push?

lm 7805 and a diode

tellMeDaBasix said:
haker_fox, EXACTLY kind of answer I was hope!
Should I measure currnt by putting ammeter between posetive power leads between router and 7805?
Yes, you're right! Or you can put the ammeter in the negative wire.
tellMeDaBasix said:
Can you also show me some tried and tested mc34063 based circuit except one in datasheet alerady please?
I used example circuits from the datasheet. Only from there.
tellMeDaBasix said:
How much current mc34063 push?
1.5 A without an external transistor.

### tellMeDaBasix

Points: 2
very very hot

Can you also show me some tried and tested mc34063 based circuit except one in datasheet alerady please?
I don't know who wrote this,... but it works and it is very handy.
the cooool thing is you get to see how changing a parameter changes the parts.

### tellMeDaBasix

Points: 2
how to connect a lm7805

treitmey said:
Can you also show me some tried and tested mc34063 based circuit except one in datasheet alerady please?
I don't know who wrote this,... but it works and it is very handy.
the cooool thing is you get to see how changing a parameter changes the parts.