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I'm trying to use a MOSFET (BS250P datasheet) to switch a circuit on and off while the MCU is asleep. It's a P-channel MOSFET that closes the circuit when the gate is pulled low. The most power hungry module in my circuit is a HC-05 bluetooth module. The specs on the website I bought it at rates it at 3.3V - 6V and a max current draw of 50mA when pairing and around 9mA when connected. Hooking it up to an Arduino 3.3V source confirms that it works at that voltage and pairs it up with another identical module just fine.

Now when I put that same 3.3V source through the MOSFET with a 10k pull up resistor and connects the HC-05 to the drain, the module does not power up. I measured the voltage on the drain and when nothing is connected it's the same as the source. After connecting the HC-05 it's about 2.4V (maybe this is a clue). Even if I run the power through a step up converter to 4.10V it does not power up through the MOSFET. If I increase the voltage to 5V it all works fine. I would ideally run the circuit at 3.6V in order to preserve current for the rest of the modules, and not having to use a voltage regulator for the RX of the HC-05.

I assume the issue is with the MOSFET properties, but as far as I can read from the datasheet it can sustain 230mA current. It has a VGS(th) of 3.6V, but since I'm always pulling the MOSFET low in this scenario that shouldn't be an issue? Measuring the resistance when the gate is pulled low indeed gives just a few ohms as the datasheet suggests.

So basically, what am I missing here? :-) Thank you for any help with this!

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That's not a good choice of MOSFET I'm afraid.

The on resistance is quoted as 14Ω when the gate is at -10V - however, you are only able to get -5V, so it will be considerably worse.

That massive on resistance will be causing a massive voltage drop, which is what you are experiencing.

You should find a MOSFET with an on resistance that is in the milli-ohm range when the gate is between 0V and -5V.

  • Hi again Majenko and thank you! I know it's not allowed to ask for shopping items etc so I won't, but these would be available with pins as well, not only surface mounted? – miccet Jun 21 '18 at 20:59
  • You may need to shop around a little for a through-hole one. I only ever use SMD devices unless I have a really good reason not to, so I don't know what is available in larget packages. – Majenko Jun 21 '18 at 21:20
  • The problem with through-hole is they tend to be either high on resistance (i.e., low current capacity) for logic switching, in a small (TO-92) case, or low on resistance for high current use, and hence in a (relatively) massive TO-220 case. For instance the NDP6020P has the resistance specs you want but is aimed at higher currents, so is chunky. – Majenko Jun 21 '18 at 21:26
  • The LP0701N3 has higher resistance, though considerably lower than your current one, but in a smaller case. – Majenko Jun 21 '18 at 21:27
  • Thanks for the links! I found the NDP6020P in my go to shop for electronics. Apart from being clunky, even in a battery scenario it would actually perform better in terms of battery life with less resistance than the LP0701N3? – miccet Jun 21 '18 at 22:47

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