# Arduino Pro Mini Voltage regulator

#### DuzMontana

Joined Mar 25, 2015
24
I've runned into some bad regulators on a batch of Arduino Pro Mini from China, or so I think.

Project draws a peak of 90mA at startup with 70mA dedicated to bluetooth connection. The noname regulator on that (c08s) has to be an equivalent to the Micrel MIC5205 which is rated at 150mA.

I've had 5% failure on 30 units due to burned up regulator.

Do you know this regulator?

Can you run regulators at 60% capacity over a long period?

#### dannyf

Joined Sep 13, 2015
2,197
Calculate the power dissipation on that little regulator and compare it to the data sheet.

It will become crystal clear to you what the problem is and what the solution is too.

#### DuzMontana

Joined Mar 25, 2015
24
Very instructive to do these calculation, but I'm still missing something,

For what I can tell from the specification sheet of the micrel, the maximum power dissipation is 455mW at Imax 150mA with a 2.5mA maximum ground current. I cannot find the datasheet for the c08s regulator I have on the board.

Given these constraints at 90mA max current, I get a maximum input voltage between 9.78-10.1V depending on ground current (not sure how I can measure that). Max ground current is 2.5mA.
Code:
455mW = (Vin - 5.0V)*90mA + Ignd*Vin
905mW = (90+Ignd)Vin
Vin = 905/(90+Ignd)
I'm powering that circuit with 6AA which should imput no more than 9V.

Solving the Pd equation with Iout 90mA, Vout 5, Vin 9V and Ignd 2.5mA I get Pd = 383mW which is roughly 85% of the maximum power dissipation.

What kind of margin should we aim for usually?

#### dannyf

Joined Sep 13, 2015
2,197
What kind of margin should we aim for usually?
If you cannot put your fingers on it it is too hot.

I would say that 100c junction temp is my upper limit and maybe 80c case being comfortable, given fluctuations in the ambient.

Those things are likely 200cpw thermally. So (100 - 25) / 200 or about 400mw is my limit. Lower if you think it can be outdoors. Reserve more for knock off parts.

I think yours are very close to that.

If I were you, I would use a separate regulator for the Bluetooth module. 90ma through a puny and regulator is just not humane.