Would this P-CHannel circuit work? - Page 1
Forum section: electronics / beginners

author: lutkeveld
author_rank: Regular Contributor
timestamp: November 26, 2017, 10:48:42 am
content: Hi everybody,I need a circuit which protects against reverse polarity, can pass 5A, works on 5 to 24V and has low (<30uA) quiescent current.I constructed a Multisim model and it seems to work, but I want to make sure before I get the PCB produced.The top part seems to be a well known circuit, I'm more hesitant about the bottom part. I selected the SI2369 P-channel mosfet.I want to switch the 'PSW' node with an arduino (floating=off, low=on). Would this work reliably or do I need to add/remove something?

author: Peabody
author_rank: Super Contributor
timestamp: November 26, 2017, 03:51:56 pm
content: On the protection circuit, I assume the diode is there because the mosfet's maximum GS voltage is 20V and you want to run up to 24V. But if you found a part with a 30V or higher maximum GS, you might not need either D2 or R16. For 9V battery circuits I've done, I've just connected the gate directly to ground, and it has worked fine.I'm not clear about the switch. Are you saying you want Vcc to go as high as 24V? Or is it always going to be Arduino-level Vcc? If Vcc will be more than 4.7V higher than the Arduino Vcc, then you will have a problem of exceeding the maximum voltage applied to the Arduino pin. Actually, I don't think the switch works. Seems like the mosfet will always be on. The gate will be either 4.7V below the source, or grounded through the resistor, but either will turn on the mosfet.Well, others will know better than I, but if Vcc will be above the Arduino's Vcc, then I think you'll need another transistor to isolate the GPIO pin from the high voltage. What if you tied the gate to Vcc through a resistor, and pulled that down through a smaller n-channel mosfet with DS of 30V, controlled by the Arduino pin. Then you wouldn't need the diode at all. But then you would need a pull-down on the n-channel gate so it doesn't float. And the Arduino logic would be floating = off, high = on.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 26, 2017, 03:56:37 pm
content: Hi,The first circuit will provide polarity protection.This was recently discussed in this thread:https://www.eevblog.com/forum/beginners/mosfet-direction/msg1359163/#msg1359163The second circuit may or may not work as a switch.There is a very strong chance that the pin on the Arduino has a diode from the output pin to the uP supply. This could prevent the PSW signal from going high.Think about this:You need Gate _ Source Resistors in both circuits to make sure the MOSFET turns off.Regards,Jay_Diddy_B

author: Peabody
author_rank: Super Contributor
timestamp: November 26, 2017, 07:42:26 pm
content: I guess I would still have the Arduino output pin drive something like a little VN0104 n-channel mosfet which would turn on the p-channel mosfet. The VN0104 has a 40V D/S limit, which should be plenty. But I'm still uncertain about the p-channel and the diode, and how that works all the way from 5V to 24V.

author: Bootalito
author_rank: Unknown
timestamp: November 29, 2017, 03:27:28 am
content: For good reverse polarity protection use an n channel mosfet "backwards" going to ground :http://www.electronicdesign.com/power/reverse-polarity-protection-automotive-designI've used this design in many personal projects and it works great, very very low resistance, etc.Check out my schematic for an 18650 battery internal resistance measurement device which has a red led for when the polarity is reversedhttps://easyeda.com/terryjmyers/InternalResistanceTester_copy-7f3776e936c04983a531854dac8a6e2f

author: Bootalito
author_rank: Unknown
timestamp: November 29, 2017, 03:34:20 am
content: Quote from: Bootalito on November 29, 2017, 03:27:28 amJust a quick note about that schematic. It's incomplete as far as an actual Internal resistance tester. The full "schematic" is just written out in the comments of the code:https://github.com/terryjmyers/InternalResistanceTester/blob/master/IRT.inoBut for the purposes of reverse polarity protection that schematic is fine.