I am inclined to use the P channel solution because it will not break my ground connection. I might want to keep the option to connect my project to a breadboard for some experimenting and create a common ground. However: it is hard to find a suitable P channel mosfet. In a non-smd package they are ~6 euros, way too much. I found a D-PAK mosfet for which I can create a breakout board, but that would mean a lot more hassle. Finding suitable n channel mosfets for veroboard is no problem, at decent prices.
Question 1: is it still possible to create a common ground while using the N channel protection ?
Question 2: which method would you advice ?
Thanks in advance for you knowledge and expertise,
Hi, I have not had experience with these circuits, but I would use the P-Ch version agreeing with your reasoning that you would like to keep a continuity with gnd.
This would be essential if you have other equipment connected to the load that use gnd referencing.
Tom.. 8)
In principle you could use an n-channel mosfet to switch the positive side, however you would then need to provide a floating gate drive supply, resulting in substantial extra complexity.
That seems like a good find ! It's priced euro 1.55 in holland without taxes, so it comes to 1.88 total, that might be why I missed it. BUT: buying quantities will make it 1.30 a piece, and I'm willing to invest in easy solutions.
What does it need to do ?
input voltage: 5.0 Volts
minimum current: 0.05 Amps
current occuring quite often, continuous: 0.5 Amps
peak current (pulses of 2 milliseconds): 1.0 (very very perhaps 1.5 Amps)
acceptable voltage drop: 0.05 Volts (the capacitors on the line should be able to keep my Arduino working)
So it switches at -4 V, that will work, Imax is large enough (31 Amps), Rdson = 0.060 Ohms will yield
Vdrop = I * R = 1.0 * 0.06 = 0.06 Volts for duration of 2 ms max
Power dissipation: P = I^2 * R = 1.0^2 * 0.06 = 0.06 Watts = 60 mWatts < 110 Watts
My only concern would be the voltage drop, especially if I would reach the 1.5 Amps (for a short pulse), any ideas / experience ?
There are plenty of N channel MOSFETs with low enough on resistance values that will produce a voltage drop less than 0.05V, MUCH less than 0.05V at 1.5A. Power cables around 1-2m could easily cause a bigger ground shift than a good N FET.
Go on Farnell, Mouser, DigiKey or whatever you use - and sort the N FETs by on resistance, and select based upon your voltage requirements etc.
EDIT - Just had a look on Mouser, selected N Channel, Through Hole and arranged by on resistance. There are plenty of through hole devices around the £1.5-2 with on resistance values of ~2mOhm.
V = IR = 1.5A * 0.002Ohms = 0.003V.
However, what voltage do you have available as many of these devices will require a gate-source voltage of 10V.
CaptainJack:
That seems like a good find ! It's priced euro 1.55 in holland without taxes, so it comes to 1.88 total, that might be why I missed it. BUT: buying quantities will make it 1.30 a piece, and I'm willing to invest in easy solutions.
What does it need to do ?
input voltage: 5.0 Volts
minimum current: 0.05 Amps
current occuring quite often, continuous: 0.5 Amps
peak current (pulses of 2 milliseconds): 1.0 (very very perhaps 1.5 Amps)
acceptable voltage drop: 0.05 Volts (the capacitors on the line should be able to keep my Arduino working)
So it switches at -4 V, that will work, Imax is large enough (31 Amps), Rdson = 0.060 Ohms will yield
Vdrop = I * R = 1.0 * 0.06 = 0.06 Volts for duration of 2 ms max
Power dissipation: P = I^2 * R = 1.0^2 * 0.06 = 0.06 Watts = 60 mWatts < 110 Watts
That mosfet I suggested is not a logic level mosfet, so its Rds(on) is quoted at Vgs=10V. If your supply is only 5V then you need a logic level mosfet. If the acceptable voltage drop is 0.05V @ 1A, then you need Rds(on) <= 0.05ohms @ Vgs = -5V. I suggest this mosfet http://nl.farnell.com/taiwan-semiconductor/tsm2311cx/mosfet-p-ch-20v-4a-sot23/dp/1864586, mounted on an SOT23-to-DIL adapter if you are not making a pcb. You can get these adapters from Sparkfun (US), Proto-pic (UK), Futurelec, Amazon and eBay to name but a few suppliers.
@jtw11 & dc42:
Ah right ! Yes I have only 5 Volts supply voltage, so only 5 Volts for Vgs, so I do need logic level switching mosfets.
@jtw11: I prefer the P channel solution because it doesn't break the common ground; since you recommend the N channel (general lower rdson, easier to come by (?) ), would you prefer the N channel method ? The site I got the schematic from hints that for military circuits they use N-channel for protection...
@dc42: the main part of the board will be made on veroboard (by students), but I have a special team for etching breakout boards, so if an smd is the only way to go, we can do it.
So I wonder: why can't I get those nice P channel smd mosfets in a TO-package ?
If you can tolerate low tech solutions, why not go for a 5volt relay with a series diode in its coil feed line. Use relay contacts (rated at 1A minimum) to feed your circuit; voltage drop will be circa 0v. If polarity is incorrect, relay will not energise so supply to your circuit will be held open.
As far as I can tell the relays (at farnell) are quite expensive (I checked under "relay"), but it can be very easy to miss out on some elements at farnell because I do not always know in what category to look. Since most sites suggested mosfet protection I started to look into that, but I will keep your suggestion in mind if that is not working out.
Thanks for the replies. "Quite expensive"... most components I select are < 1 euro, if possible. If a 60 cents mosfet will do the job, I prefer that over a more expensive relay.
Final question: the zener diode mentioned in the first schematics is 9.1 Volts. I might not understand enough about the zener diode's workings, but do I need to adjust this value because my battery voltage will be 5.0 Volts ?
Apart from that I think I have enough information to make a decision
CaptainJack:
Final question: the zener diode mentioned in the first schematics is 9.1 Volts. I might not understand enough about the zener diode's workings, but do I need to adjust this value because my battery voltage will be 5.0 Volts ?
You don't need the zener diode at all if the maximum positive input voltage that the circuit needs to handle is comfortably below the max Vgs rating of the mosfet. Otherwise, pick a zener diode with a voltage comfortably below the Vgs rating of the mosfet, but not lower than the Vgs at which the lowest Rds(on) is quoted.
I need to verify that my design for reverse polarity protection with P MOSFET is correct
I have space limitation on my board so i want to use a P MOSFET without the need to have a zener diode and a resistor.
The input voltage to my board is 12Vdc (+-0.5V)
The load current is 3A max
I also need as much as possible voltage drop because the board will be used to control dc motors that are rated 12vdc and are using the input voltage of the board.
I want to know if the Vishay SQ2319ES is suitable for my design
You can use that circuit without the resistor and zener diode if you can guarantee that there will never be transients on the 12V input that take it higher than the Vgs rating of the mosfet (20V for the one you have chosen).
The mosfet you have chosen is IMO not suitable for 3A load current because it is likely to overheat at that current. You need a mosfet with either lower Rds(on), or higher heat dissipation capacity (for which you will need a mosfet in a larger case style such as SOT223). Or two of those in parallel.
