Low power relay

[Guest]

Hi,

I want to control the live side of a battery supply (3.9vDC) using a PIC, running from the same battery (3.9vDC) is there a nice simple simple answer to my problem like a low voltage relay ( 1.5v coil ) which I can simply connect directly to the PIC pin?

I have found some relays like this but I cannot find a supplier (RS / Farnell / Digikey) I am in the UK.

So does anyone have a solution where I can get the parts in UK


Regards

[SherpaDoug]

I assume you can't use a P channel FET because 3.9V is not enough gate drive to turn the FET on hard enough for your load. In that case I would use a "voltage doubler" circuit to get more gate drive for a FET. It adds some parts to the FET circuit but it is still probably cheaper than a relay.

For the voltage doubler you drive a PIC pin with a square wave when you want the FET to turn ON. The PIC pin drives a network of capacitors and diodes to generate a DC voltage nearly twice the square wave voltage. There are "voltage trippler" circuits if you need even more gate drive voltage, but efficientcy starts to suffer as the multiple is increased.

Here is a suitable schematic:
http://www.reconnsworld.com/power_voltdoubler.html
where the PIC takes the place of U1 and you want to reverse the diodes to get a negative voltage for a P-FET.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[Geust]

I only have one pin to work with so I'm not sure if your solution will work. I was looking at this device.

http://www.europe.omron.com/Images/en/165_19405.pdf

But I was unsure if I can control the posative supply and not the negative side as shown on the PDF, don't anyone knows if I can?

[SherpaDoug]

My suggestion only takes one PIC pin. That pin is programmed for a square wave which drives the voltage doubler, which in turn drives the FET gate. The square wave frequency could be anywhere from a megahertz to as low as 10Hz.
The Omron parts should do the trick also if the output rating of 2A Max and 0.12 ohms and the price is OK. Since it is optoisolated it can switch either supply rail.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[PCM programmer]

What you want is a high-side load switch, with a logic level control input.

One solution would be the Micrel MIC2505, which is basically a logic level
P-channel MOSFET with some added features. Here is the data sheet:
http://www.micrel.com/_PDF/mic2505-06.pdf
I don't know if this is for a "one off" design, and I don't know how much
you want to spend on it, but Digikey has them: http://www.digikey.com

I have used a IRF7306 P-Channel MOSFET as a high-side switch
in a 5v PIC circuit. I haven't done a circuit for a lower Vdd voltage,
though perhaps IRF7404 would be a candidate.

[JimB]

Take a look at an International Rectifier, IRF6433. This P Channel MOSFET only takes a little over a volt on the gate to get it down to around 30 mOhm on resistance and can easily pass 1 Amp. If you need more current there are larger versions, all switchable within the 3.9V range. You only need one or two resistors to set the gate voltage and turn it on with the PIC pin pulling the gate down.

[mcafzap]

In the past I have used small latching relays from Farnell in the UK, NAIS TF2-L2-5V Being latching, they are extremely low power. Although they required a 1mS pulse at 5V I found I could use them at lower voltages by increasing the pulse width and you may just get away with this at 3.9 volts. The trouble is they are cited as being no longer manufactured.
For non-isolated outputs I have used Zetex transistors (ZXT13P12DE6) some of which have a Vbe of only 14mV when saturated.
Farnell part no: 9525408

HTH

Steve