Power supply for microcontroller

[Veketti]

Dear All,

Time after time I run into this problem that what is the best way to power microcontroller projects which need 5VDC. I've used step up converters, LM7805 linear regulator and step down converter. All of these are more or less quite power consuming devices and I'm still looking what would be the best and most effecient solution for battery powering. 3x1.5AA for example in case of LCD display is not good directly as it needs minimum 4.5V. This latest project which uses four I2C temperature sensors need battery powering and circuitry itself draws 5mA. No matter how I put it battery life on continuous operation wouldn't be more than two weeks, but perhaps I can live with that.

Please experts suggest what is the most effecient solution? It doesn't need to be cheap as I'm not planning to mass produce, so component price is not important. I don't want to plug it to wall so AC is not an option. Bottleneck here seems to be the LCD display which needs 4.5-5.5V. Microcontroller and other peripherals (except display) seem to be able to go as low as 3V or so..

 

[KlausST]

Hi,

What Ah has an AA battey? 1Ah?, 2Ah?

With 2Ah and 5mA you may expect 400h of battery lifetime. Easy to calculate: 2Ah / 0.005A = 400h

You say two weeks. This is 14 x 24 h = 336h .... close to the 400h.

*****
Do you see the solution now?
-> the only way is to reduce power consumption.

That's why microcontrollers have built in power reduction modes.
And low power displays. Even for 3V3.

Select low power consuming display with working voltage range 4V .. 5V, or wider.
Program your controller to consume less power.

Klaus

 

[Veketti]

I was using 2Ah batteries as a reference. I was looking from Digikey and were able to find some 3.3V-5V displays, but they're all OLED displays. Nothing wrong with the OLED except they seem to draw 35mA compared to 1.5mA from normal LCD (without backlight). Perhaps I'll make it so that use the OLED display, 3xAA directly power and have button to show values for few seconds and then go to sleep...

 

[KlausST]

Hi,

35mA is not less than 1.5mA...


****
You could power down LCD also.

Looking into LM78M05 datasheet (you could do this, too) says it is drawing 3.2 ...6.0mA of quiescent current.
Here you could save a lot.
--> choose a low power voltage regulator. Like LM295x series..

Klaus

 

[Veketti]

Hello,

Sorry, I meant to say that OLED display would be perfect if it draw same amount of power than conventional lcd, but it doesn't. Only good thing about it is that with it I wouldn't need to use any inefficient voltage regulation or converters.. Just directly use 3xAA batteries.

That LM295x looks very good. It seems to have low dropout voltage (40mV), so if I were to use 4 AA, it would work down to 1.26V / cell. Which is not awfully low, and it looks according to battery discharge tests half depleted battery.. If I understood right?

 

[Veketti]

Hello,

If I may continue with this topic even though this is not directly related to this. I made power saving mode to this device so that after some time Pic goes to sleep. Also I added transistor BC337 that is controlled by one PIC's pin. This transistor will cut the ground from all peripherals (MCP9808 sensors and display). However if the ground is disconnected from the display, circuit seems to short circuit. I'm using character display with 4 bit connection.


Why is it so? Even though I remove the contrast resistors so GND on that display should completely float, it is still shorted.

Thank you in advance, your help is greatly appreciated.

 

[KlausST]

Hi,

I'm not the friend of cutting GND, because it us the referecne to all signal and power.
I'd rather cut VCC.

In any case:
* if you cut GND, then all other signals need to be switched to VCC before.
* if you cut VCC, then all other signals need to be switched to GND before.

And for sure after connecting supply again, you need to wait some time then initialize the display. Read display datasheet for details.

Klaus

 

[Veketti]

Hello,

I don't like cutting the GND either, but I don't know how to cut the VCC. If I were to use transistor with load on emitter side, I've understood that base voltage should be 0.7V higher than collector voltage so it wouldn't work. So my guestion is what is the correct way of cutting the VCC? Any example circuit would be greatly appreciated, so that I understand.

 

[zsolt1]

hi,


ideas ...

 

[Veketti]

zsolt1, by using PNP transistor as you suggested, I was able to cut the VCC. And when I did like KlausST mentioned, pull all display signals to GND before sleep, now display is not drawing any current. I thought that when it goes to sleep it will automatically pull all outputs to gnd, but fortunately I was wrong. Now this thing works like a charm thank you guys. You've been very helpful.

Figures: 0.5mA in sleep mode and 5mA when on.

One thing I'm wondering, should I pull I2C SCL and SDA to VCC or GND when pic goes to sleep?

 

[dinesh_jaddu]

Interface the temperature through I2C Protocol,mostly all controllers have open drain or open collector.circuit so connect the pull-up res across the SCL,SDL of the I2C.

 

[FvM]

Figures: 0.5mA in sleep mode and 5mA when on.

Processor sleep current should be in a low µA range. 0.5 mA might be 7805 quiescent current. It's essential to avoid any floating inputs of processor or other CMOS devices.

 

[mtwieg]

I assume you're using an older MCU like an AVR or PIC which are specced for 5V operation. To get extended battery life you should use a 3V MCU powered directly from the batteries, or with a LDO with very low IQ (many CMOS LDOs have Iq in the uA range). If you are clever with your component selection and use sleep modes correctly, one can reduce power consumption to be so low that the self discharge rate of the batteries becomes dominant.