Attempting to drive two devices with PIR

Thread Starter

Brianaala

Joined Feb 15, 2019
62
Well, I know the motor can handle up to 12v but runs strong at 5v and seems to pull (under load) about 1A. This is a random motor I just had in my shop with no markings but it has a gearbox so it was perfect for my prop. The PIR I have no idea what current it puts out bu I'm sure it's really low (like in the sub 100mA range).
 

wayneh

Joined Sep 9, 2010
17,153
Well, I know the motor can handle up to 12v but runs strong at 5v and seems to pull (under load) about 1A. This is a random motor I just had in my shop with no markings but it has a gearbox so it was perfect for my prop. The PIR I have no idea what current it puts out bu I'm sure it's really low (like in the sub 100mA range).
Ok, the MOSFET should be rated to 4A or so. That’s not a problem at all and you may choose one rated to 10A or more. IRL540 is a common part. It’s totally overkill but would be fine. If size or cost are issues, go ahead and look at other options.

If it hasn’t been mentioned already, you should have a reversed bias diode across the poles of the motor. This will absorb any inductive voltage spike when the motor is switched off and protect the MOSFET. Something like 1N400x, where x=1 thru 7.
 
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pyroartist

Joined Oct 9, 2015
98
This is just my observation over years of experimenting. I have built quite a few devices and circuits that use power FETS, usually the IRZ44. They all seem to work fine but at some point they don't. Sometimes they become intermittent. I have rebuilt most of them and replaced the FET with a relay. Others I just gave up on. Relays are cheap and easy to use and seem to last forever. Best of all they are more reliable than FETs and not as fussy to drive.
 

Plamen

Joined Mar 29, 2015
101
Hi folks,
I'm relatively new to creating circuits. I'm trying to drive a very low current (~27mA) Mp3 module and a small DC motor from a PIR running through a TIP31c transistor. The entire circuit is powered from a 5v 2A PC power supply. The PIR outputs 3.3v to switch the base of the TIP31c through a 1K resistor with 5v to the collector. It will power the MP3 but when I attach the motor nothing seems to power up. The DC motor runs on 5v and pulls less than 1A so the TIP31c can easily handle that, however the same circuit with the MP3 removed (trying to run just the motor with the TIP31c and PIR) does not work either...so clearly the addition of the motor is the issue but I'm not sure why, could it be from voltage drop?
Petkan:
Using NPN (BJT) transistor calls for substantial base current. Make it a Darlington by adding at the base a small (say 2N2222) as emitter follower. Certainly you could use MOSFET as well. There are also "High side Drivers" from the likes of Infineon, IR etc
 
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