I need to plug a micro controller in device which seems to run a PSU with a common VCC and two "grounds"

A common 0V a -3.3V and a -5V. I want to add a 3.3V powered microcontroler (The easy part) and drive a 5V relay using the microcontroller.

I tried to drive the relay using a mosfet but since the relay is 5V powered and as far as I understand there will be a 1.7V Vgs between the -5V and the -3.3V (GND ref for the microcontroller) and this will be enough to switch on the mosfet.

So I thought about this: http://www.circuits.io/circuits/2632

Will this work ?

  • \$\begingroup\$ Power supplies can be negative. Saying there are two grounds with a single Vcc is incorrect. What you probably have are (V, V+1.7, and V+5), of which if you're certain V+5 is common, then the other voltages are -5 and -3.3. That sounds like a rather strange combination to me, though. I'd check the polarity of your multimeter probes and such, or post the specs to the PSU or wherever you're getting those numbers from. \$\endgroup\$ – Chintalagiri Shashank Mar 8 '13 at 12:02
  • \$\begingroup\$ Are you sure that you are measuring the polarity correctly? I ask because this is an unusual power supply configuration and mistaken polarity is one possible explanation. \$\endgroup\$ – W5VO Mar 8 '13 at 15:26
  • \$\begingroup\$ I'm sure there's a common ground (0V / GND), a -3.3V output and a -5.0V output \$\endgroup\$ – user19911 Mar 8 '13 at 16:47

From attempting to understand the power rail description in the question, this is an assumed situation / solution... See if this schematic works for you.

The dotted-line box indicates your microcontroller, note the power rail connections. Output drive logic needs to be inverted (high to turn off).

The MOSFET indicated is a P-Channel MOSFET with a Vgs(th) of -2.5 Volts or so, used as a high-side switch.

The MOSFET selected must go firmly on with a gate voltage of -3.3 Volts - refer to the relevant graph in the datasheet.

There is no current limiting resistor indicated in the coil drive path, add one according to the current rating of the coil and the DC resistance of the coil, from the relay's datasheet.


simulate this circuit – Schematic created using CircuitLab

Add concerns or issues in comments, for an updated schematic addressing those.

  • \$\begingroup\$ I designed this in eagle: i.imgur.com/8Euxn0D.png I hope i didn't made any mistakes. Is this OK ? Thanks for your help. \$\endgroup\$ – user19911 Mar 8 '13 at 16:44

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