Digital power switch

championx · Joined: 28 Feb 2006 Posts: 151

Hi! i am working on a 18f252 battery powered project... i use a standard power switch to turn on and off the circuit... but now i want to turn on and off the circuit by pressing a pushbutton... like a cellphone...

I tried a N CHANNEL MOSFET, but when i drive the gate low, the circuit turn off.. but not entirely...

any ideas???

i cant use the SLEEP function because the consumption on sleep mode is too high (300uA)

thanks!

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

With appropriate configuration, the sleep mode current consumption can be as low as a few uA with most PIC devices. This requires, however, to disable oscillators and any additional internal load, e.g. brown-out detection. Also the port pins must be configured in a way that avoids additional current draw, particularly by floating inputs or loaded outputs and pull-up resistors.

An external switch transistor can further reduce the current, if required. It's difficult to understand, why your circuit doesn't operate as intended, without knowing the schematics.

championx · Joined: 28 Feb 2006 Posts: 151

I cant use SLEEP mode because the circuit have a few components that cant be turned off, like LCD, some zener diodes, etc.

I dont have the schematics. But i have succesfully use the sleep function on other proyects, but in this one, i have to make a DIGITAL SWITCH, any ideas?

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

championx · Joined: 28 Feb 2006 Posts: 151

yes, i know, the problem is that the circuit is already made, so i cant modify the hardware, i just can add a digital switch to it.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

As mentioned before by FvM, it is difficult to help with your problem without knowing the schematics. Can you post a picture/drawing? Or describe in words how you connected the mosfet?

Using Google I found many 'power off' schematics, for example http://www.edn.com/archives/1996/120596/25di_03.htm
Does your setup look similar to this one?

championx · Joined: 28 Feb 2006 Posts: 151

hi ckielstra! i can give you a description:

Is a 18F252 at 20mhz
-I2C memory
-I2C RTCC
-LCD 16x2
-ONE ADC
-LDO 5v regulator.
-5 pushbuttons

The switch is between the pic and the 5v regulator. When turned OFF, it cut off the power from the circuit, except the RTCC.

I only replaced the switch for a 2n7000 Nchannel mosfet. But didnt worked...

I will try this circuit, do you know a P channel mosfet like the 2N7000 (to92)???.

thanks!

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

If you switch off power to the VCC pin of a PIC but leave +5V going to any port pin (with a few odd exceptions) the protection diode of the port pin will forward conduct to the VCC bus of the chip, thus powering the PIC at a reduced voltage.

Maybe this is what you mean by the circuit turning off but not entirely.
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championx · Joined: 28 Feb 2006 Posts: 151

Hi SherpaDoug, no, i mean that the mosfet doesnt entirely cut off the power. I dont have any pin connected when vcc is off. The only thing connected is the RTCC bat pin.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

An NMOS switch connected to VCC would need a gate voltage above VCC for suitable operation. I already made a comment about the useful options with NMOS and PMOS transistors, I've nothing to add to.

Another point, I don't fully understand. You're talking about a voltage regulator. What is it's supply current? As far as I know, even said micropower regulators have about 100 uA supply current at no load. This doesn't sound well-thought. So, if you need voltage regulator and a supply switch, a regulator with disable as a LP2985 seems more suitable.

championx · Joined: 28 Feb 2006 Posts: 151

The regulator that i use have a very low current, about 5uA...