Temperature effect circuit

[ibwev]

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?

 

[Misterbenn]

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.

 

[ibwev]

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.

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

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

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

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

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

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

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.

 

[Misterbenn]

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==

 

[Misterbenn]

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.

 

[ibwev]

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

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?

 

[Misterbenn]

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

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?

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?

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.