MOSFET control of M2-ATX

**markmakeitso** · 11-23-2009, 01:35 PM

Good day all,
I'm working on a datalogging project with several hybrid vehicles using mp3Car systems with M2-ATX powersupplies. After a drivecycle we sometimes need to keep the PC alive for several minutes, which we do by turning on a keep alive circuit to the signal wire. We have several methods of turning on the PC using the signal wire, each through a forward bias diode:
-vehicle ignition
-keep alive circuit
-vehicle plugged into AC outlet for charging
-bypass switch

Unfortunately we've had the PC hang up several times, draining the 12V battery. These vehicles are driven by state employees who aren't involved with the testing other than operating the vehicle, so manually switching the PCs on and off isn't an option.

In an attempt to avoid 12V discharge we're developing an inline automatic switch using a PIC. Ideally it will monitor voltage and turn off the PC power when voltage gets down to 12V. We're using a P16NF06 MOSFET since it should be directly controllable by the PIC's TTL output.

http://www.alldatasheet.com/view.jsp?Searchword=P16NF06

No luck getting powerup on the bench however. Power in and signal are both high, ground is good, but I don't get bootup with either 12 or 5V gate voltage. Bypassing the MOSFET gets a bootup, and swapping the PC for a lightbulb gets proper operation. We have a 4.6K resistor from gate to ground to discharge the gate wire and we're currently switching the +12V wire. I'm not an EE or anything, but I've dug through my share of datasheets. Anyone have some insight?

Thanks
Mark

**Curiosity** · 11-23-2009, 09:00 PM

That won't work, sorry.

Being the MOSFET is N-channel and on the high side, the gate threshold is drain + 5V. It would work directly with the PIC if you were switching negative. You could add an NPN transistor between gate and source to switch it between source and ground, plus add a 1K resistor between the gate and NPN emitter for good measure.

Always remember, the PIC has a 5.1V zener diode on every pin. When used as output, switched on it will pull down to 5.1, when off it will pull to gorund. So if you're touching that gate directly to 12V with the PIC connected, it's most likely going to smoke it.

Easier solution as I use these in my products: A PSU IGN input draws very little current. A simple darlington optocoupler works great. The input requires a 470 ohm resistor to drop V just like an LED. The collector connects to 12V, emitter to PSU IGN. Or you can use a darlington transistor, or set up 2 as a darlington.

**markmakeitso** · 11-23-2009, 10:40 PM

Hi Curiosity, thanks for the response.
I played around with the setup for most of the day today, and had some progress. I should have explained that I'm not currently using the PIC in the circuit, just a 5V regulator, the P16, and a standard 12V battery. I was simply triggering the gate either from +12V or from the regulated output, the latter of which would presumably be similar to PIC output.
I could get the MOSFET to work when using a 12V gate voltage and a different load (a PC fan and a 12V bulb in parallel) but couldn't get the PC to start using the same setup. Voltages for supply and ignition seemed high enough to start however. When switching the positive side I always had a forward drop across the MOSFET, about 3.5V I believe, leaving 8 and change across the load. After your suggestions I retried ground switching and it seems to work much better, no forward drop so the full 12V is across load and the regulated +5 works for the gate. More careful reading of the datasheet would have shown that threshold voltage is measured G-S, not G-ground. Live and learn I guess.

I appreciate the help, several steps closer to a fix now.

**Curiosity** · 11-23-2009, 11:17 PM

Oh yes, MOSFETs can be a load of fun. That MOSFET has a pretty low RDSon and can switch 16A. It might require something as a load, where as the remote line is just sensing and probably consumes 0.005A at most. You could use a resistor to pull more current to ground and get that power flowing.

Not sure about the forward voltage. Here's something fun to try. If say the MOSFET RDSon is 8 ohms, use a load of 8 ohms and connect the gate to drain. I'm not sure about a logic level MOSFET but normally you'll see half the voltage on the drain.

**markmakeitso** · 11-23-2009, 11:53 PM

The MOSFET controls the power wires, not the signal itself, that probably wasn't clear in my earlier posts. It's a last defense against killing the 12V battery. Yes, with 16A capacity it would be grossly oversized for governing the signal wire.

**Curiosity** · 11-24-2009, 12:34 AM

It would be easier to just put a timer on your keep-alive, or possibly some setting on the M2-ATX. Or if you get fed up with the MOSFET, try a P-channel. It's much easier to deal with on the hot/high side to interface with a PIC. Just reverse your levels.