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Topic: MOSFET as switch and power monitor (Read 1 time) previous topic - next topic


Jan 14, 2013, 06:51 pm Last Edit: Jan 14, 2013, 06:52 pm by Docedison Reason: 1
2 quick things. 1 copper wire has a positive temperature co-efficient that is when it gets hot it's resistance increases which means that while it "Looks" like the current is going up it is actually going down because of the increase in resistance of the copper. It's all right to use copper for connecting things as the rules that that into consideration but not to measure current with because of it's 3300 PPM positive temp co..
And 2. There are many "Logic Level" mosfets available, using a driver and a higher voltage Vgs mosfet is kind of redundant.

{Edit RKJ}
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My previous post was somewhat confusing.. So
OTOH there are 2 common kinds of drivers both open collector. An NPN transistor is used to bias a P ch Mosfet on and a PNP transistor is used to bias an N ch Mosfet. Or both or either as required.
There are also hybrid IC's that are used to drive mosfets where there are no sources of voltages greater than 5 to 10 volts as some high power Mosfets require.
The 'common drivers' are trivial to implement and the 'Driver' IC's do require some special handling and this was the reason for my comment on redundancy.

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The need for high-side switching is when controlling another circuit - not necessary for a light or a heater.

For current measuring a hall-effect current sensor is the way to go, there are various types, probably the simplest
if you can handle surface mount components is the ACS710/711/712 family which are available in various current
ranges.  Being fully isolated you can use a hall-sensor to measure current anywhere in the circuit.

There are other through hole hall sensor modules available, there are resistive shunt modules available too.
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Jan 15, 2013, 01:14 pm Last Edit: Jan 15, 2013, 01:21 pm by Chrono251 Reason: 1
Thanks all for the answers!

My idea for hot side switching is related to the fact that the battery won't be operating inside a building, but outside where dew may be present. I understand that nobody will die for touching a wire with a 12V battery (fuses are present anyway), but I would like to make the device as safe as possible. If I use a switch on the hot side, I have the feeling that the risk for short circuits is reduced as the only way to create one is to touch the PCB. The battery doesn't power just the heaters, there is also the telescope mount, the laptop and the camera that get power from there.
If you say that a N-channel is ok, though, I trust that and that's better for me as at home I only have some N-channel MOSFETs (IRF540).

For the schematic, I was thinking something like the one attached to this post (possibly using the FQP47P06). Will it work this way?

Using a 1 mOhm shunt resistor won't waste much power, but that'd give me a max voltage drop of 15mV at 15A. I can connect that to an OPAMP (like I did in my tests on breadboard - not using the heater but a bunch of resistors and a bigger shunt), but I guess that the readings won't be very accurate. I don't need perfect readings, but something that makes sense and doesn't change much with temperature. That's why I was asking if Hall fx devices are valid for this. I also like the idea of having the sensing circuit isolated from the other one (but an optocoupler may be used for this).
Chrono251 - Stefano


That schematic looks good to me, however I would decrease the 10K resistor to 1K. For current measurement, I suggest a Hall current sensor such as http://uk.farnell.com/allegro-microsystems/acs712elctr-20a-t/sensor-current-20a-soic8-712/dp/1329624. Alternatively, use an axial leaded resistor of about 0.02 ohm such as http://uk.farnell.com/vishay-dale/lvr05r0200fe73/resistor-precision-0-02-ohm-1/dp/1108083 and an op amp. The trick to getting good accuracy with a low value resistor is to solder separate sense wires on to the resistor leads, between the resistor body and where you connect the power. The sense wires feed the op amp. That way, the resistance of the connections doesn't affect the accuracy.
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