# LDO gets hot when using a relay module?

#### dorke

Jun 20, 2015
2,342
I would suggest you use a power source of 4.5-4.8V and not the 9v battery with the LDO.
You can do that with AAA batterys 4*1.2v or 3*1.5V .
"Switch" S as before.
you may need to recalculate the series resistor to the LED.

#### Tha fios agaibh

Aug 11, 2014
2,252
Sorry, I didn't catch that at first.

So we know coil needs 85ma to operate.
Say for instance a 9v source was used directly, would we use a 36 ohm dropping resistor?
36r+70r (coil)=106ohms X .085=9v

Or would it be 59 ohms calculated for the coil, making it a 47 ohm resistor?
Sorry, I don't want to hijack the conversation.

#### davenn

Moderator
Sep 5, 2009
14,263
I would suggest you use a power source of 4.5-4.8V and not the 9v battery with the LDO.
You can do that with AAA batterys 4*1.2v or 3*1.5V .
"Switch" S as before.
you may need to recalculate the series resistor to the LED.

View attachment 23395

of course that still isn't going to work, as there is no voltage to the S pin to turn on the transistor

you still need to take a probe wire from the 4.5V to the S pin that can be tapped on and off to activate the relay

cheers
Dave

#### dorke

Jun 20, 2015
2,342
of course that still isn't going to work, as there is no voltage to the S pin to turn on the transistor

you still need to take a probe wire from the 4.5V to the S pin that can be tapped on and off to activate the relay

View attachment 23396

cheers
Dave

Dave you are correct.

I did write in post # 21: "Switch" S as before
The op did that already in post #1 (that isn't the problem he faced):
"When the signal (S) pin is low, the module and LED are off.
When S is high, the module and LED are both on."

#### dorke

Jun 20, 2015
2,342
So,How "hot" does the LDO get?
to answer this we need to know which part is it.
Pd(LDO)=4*85mA=0.34W
Thermal resistance of a TO-220 packages case-to-ambient is about 50°C/W to 60°C/W.

Tcase over ambient temp. =0.34*60=20°C
so for a 25°C room we get 25+20= 45°C case temp.
this can't be related to as truly "hot".
it doesn't pose any problem to the LDO and no heat-sink is actually required.

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