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Topic: Is 51 degrees (C) (124 F) too hot for the voltage regulator? (Read 1 time) previous topic - next topic

Nick Gammon

I am running my RFID system from a 12V wall-pack. It needs 12V to reliably unlock the door (taken from Vin). However the voltage regulator seems quite hot. I measured it at 51 C with my meter. Does this seem unacceptably high? This was when the ambient temperature was around 22 C, and it can get a lot hotter, particularly where the processor is installed (say, 45 C ambient).

Hmm - I measured the plug-pack output. It actually is outputting 17V under light load. I'm substituting one that is closer to 12V, hopefully that helps.


Nope, that is expected. 51C is not very stressful for an electronic component. I only get worried when the temp starts to get above 90C-100C. If 17V is coming in and 5V is going out, all that excess voltage is going to have to be dissipated as heat, and it gets worse as current goes up.

An input voltage closer to 12V will definitely help, as will a properly-mounted heatsink.

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Nick Gammon

Sorry for not being more specific. It is a Uno.

It has connected to it an ID-12 RFID reader which draws a nominal 30 mA according to the datasheet.

I haven't bothered with any power-saving code, as it is connected to the mains (via the plugpack).

Nick Gammon

After plugging it into a supply which delivers just over 12V the temperature has dropped to 44 C.


I'm not aware of any commonly used regulators that have issues with 51c.
If you want it cooler you can glue a heatsink onto it, but IMO it's just fine.

If you touch it and it burns you instantly (or very quickly), that's too hot.


Have you read this?

What is important is the junction temperature, that is the temperature of the silicon inside the regulator chip. While you want it as cool as possible for reliability as long as this is below the maximum allowable then it is fine. For every ten degrees hotter something gets it's life halfs, approximately. Running a lower voltage into it will reduce the amount of power you have to burn off, depending on the regulator it only needs to be about 1.5V above the regulator's output.
Heat sinks will help but only to a certain extent.


@Nick Gammon

Can you measure the current using a DVM. I know, it mean to "cut" or created on open and place in serie the DMV in current measurment mode from  output of the regulator ? if possible...  a 7805  is about 1.5 A max <-- I maybe wrong... If you measure close to 500 mA and above... You may use a bypass transistor ( a TO-220 type or TO-3 type <-- That is beefy !! ) I made one PSU using a 7805 with a bypass transistor to power my XM radio for my car ( use a TIP32 - 12 V in - 5 V out ) But man it is WARM !!!

Sorry that I sound "overkill" here.

Nick Gammon

I measured the input to the Uno (that is the current from the plug-pack to the input power socket) as being 75 mA. I know this isn't what you asked for, but that would tend to indicate that the voltage regulator is not drawing more than that.


@ Nick Gammon

That good enought. Let calculate the Power. Let see P=V * I. You find I is 75 mA. V is Vout - Vin. So what is Vin and Vout ?

Nick Gammon

Vin is 11.46 V. Vout (the 5V rail) is 5.00 V.

Does that 75mA include the 30mA going to your RFID reader?  75mA seems very modest to account for such a high temperature.

Yes it does.

Bear in mind that after installing the "improved" plug-pack (that actually delivers just over the claimed 12V) the temperature at the voltage regulator is now 44C (just reconfirmed that). The ambient temperature today is 22C. So the regulator is 22C above ambient.


So you have 12 - 5 = 7V across the regulator and with 75mA you have 7 * 0.075 = 0.525 Watts of power being generated.

So 22C above ambient is not unreasonable.

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