Temperature effect circuit

The attached circuit works great indoors at approximately 22 degrees Celsius; however, when I operate the circuit outside at approximately -4 degrees Celsius, the pic appears to continually reset. When I apply heat via a hair dryer to the LM317M voltage regulator, the circuit works as expected. I am sure heat from the hair dryer was warming other parts of the circuit as well. I don't know enough about electronics to interpret the datasheet of the voltage regulator. I need the circuit to be able to operate between -18 degrees Celsius to 41 degrees Celsius. Is the voltage regulator likely the source of my problem or is there something else I need to investigate?

Hello,

If you can please get an oscilloscope trace of the regulator output and also the PIC reset pin. That will let us know what's going on.

My initial thoughts:
You're dropping quite a large voltage over that regulator so you'll have quite high losses. With that device you'll be getting about 11 degC temperature rise at minimum power! so I expect you could have problems at higher loads / ambient temperatures. I'd consider using a switching regulator such as TPS84250.

I also noticed from the regulator datasheet:
- Operating Junction Temperature Range 0 - 125 degC; so this device is not designed for sub-zero temperatures
- Input-Output Voltage Differential 40V maximum; so if your driving your pic with 5V and the input on your diagram is correctly marked up at 50V then you're asking for to big of a voltage differential
- minimum load current 3.5mA; with a PIC you could easily be below the minimum load current if you go into sleep mode. You might need a load resistor but you'll increase losses.

Misterbenn said:

If you can please get an oscilloscope trace of the regulator output and also the PIC reset pin. That will let us know what's going on.

Click to expand...

I'm just a hobby electronics enthusiast and don't have access to an oscilloscope.

Misterbenn said:

I'd consider using a switching regulator such as TPS84250..

Click to expand...

I have never worked with surface mount regulators. Would there be anything wrong with using 7805SR-C?

Misterbenn said:

- Operating Junction Temperature Range 0 - 125 degC; so this device is not designed for sub-zero temperatures.

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I missed that subscript in the datasheet. Thanks for pointing that out.

Misterbenn said:

-
- minimum load current 3.5mA; with a PIC you could easily be below the minimum load current if you go into sleep mode..

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I did not know some regulators require a minimum load. I do not see where the 7805SR-C requires a minimum load. My pic will frequently go into sleep mode. Is it safe to use the 7805SR-C?

The power source is from a 24 volt regulator. The voltage at the capacitor on the above circuit is about 36V. The 50V I put on the above circuit diagram is the component's rating.

As your capacitor voltage is 36V I presume that your regulator output is 24Vrms?

36V is dead on the device maximum rating which I'm not very comfortable with.
How about a RECOM device, R-78C5.0-1.0, **broken link removed**
https://uk.mouser.com/ProductDetail...35UZlRG0Y3t3Bqiw1ZfajD60ukaz%2bmV39O09DlV3A==

If your regulator isn't the problem then your circuit still wont work though, but I think its worth changing it just for a better design.

You will also want to put filter capacitors across the Micro's supply pins, with as short a tracks as possible. And I'd also add a capacitor to the reset line just to make sure noise isn't causing a reset.

Misterbenn said:

How about a RECOM device, R-78C5.0-1.0

Click to expand...

1) If I use "C2" (see attachment), what size capacitor should it be?

2) My max current is 174 mA. The case thermal impendance is 70 degree C per a watt. Will the RECOM device need a heat sink?

3) Since the "no load current input" is 1 mA and resistance = voltage/current , should I place a 5 k-ohm resistor (5V/.001A) between Ground and V-out of the RECOM device?

1) If I use "C2" (see attachment), what size capacitor should it be?

Click to expand...

Anything in the 10s of uF range will be fine - whatever you have available. Low ESR resistors would be best.

2) My max current is 174 mA. The case thermal impendance is 70 degree C per a watt. Will the RECOM device need a heat sink?

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You won't need a heatsink, these devices are very efficient and are rated for full load at 68 degC ambient temperature. See derating-graph on page 1.

3) Since the "no load current input" is 1 mA and resistance = voltage/current , should I place a 5 k-ohm resistor (5V/.001A) between Ground and V-out of the RECOM device?

Click to expand...

Actually you want to look at the output current, not the input. On page 1 of the datasheet it gives the minimum output current at 0mA*, the '*' is quite important!
From page 2 of the datasheet "
No load operation will not damage these devices, however they may not meet all specifications. A minimum load of 10mA is recommended."

So you may not need a load resistor - however if you want to be sure the device keeps a safe working voltage then load it with 500 Ohms. That's only 50mW so not a huge power dissipation - it's not as if your working off a battery here anyway. Alternatively if you don't like the idea of just wasting power you could always put a power LED on your circuit.