Controlling 12V from 5V
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Posted by Buddy Smith on June 29, 2005, 3:35 pm
Please log in for more thread options Hi, Newbie question...... I have a device I'm trying to turn on/off that requires 12V. I want to control it with a micro that runs at 5V. My question is can this be done with a power MOSFET, or do I need a relay? Assume that the current running through the controlled device is not much, but it will be on mostly 24/7. It will be turned off several times a day for say 5 seconds or so. ttyl, --buddy | |||||||||||||||||||||||||||||||||||||
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Posted by Byron A Jeff on June 29, 2005, 12:17 pm
Please log in for more thread options >Newbie question......
Fire away. >I have a device I'm trying to turn on/off that requires 12V. I want
>to control it with a micro that runs at 5V. Typical. >My question is can this be done with a power MOSFET, or do I need a
>relay? How about a simple NPN transistor? power MOSFETs generally require a gate voltage that exceeds the drain, which will be at 12V. You can use an NPN power transistor as long as the current requirements of your device doesn't exceed the current the micro I/O pin can supply. The hfe for a 2N3055 ranges from 20 to 70 for example. So an I/O pin that can supply 20ma to the base and control a load between 400ma and 1.4A. You could use a darlington arrangement if you need to drive a larger load. Finally if you used a relay, you would still need a transistor relay driver to drive the relay. >
>Assume that the current running through the controlled device is not >much, but it will be on mostly 24/7. How much? > It will be turned off several
>times a day for say 5 seconds or so. Now a relay could be useful here because you could run the circuit through the normally closed contacts. No power would be consumed the majority of the time. Activating the relay would turn the device off. BAJ | |||||||||||||||||||||||||||||||||||||
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Posted by John Fields on June 29, 2005, 2:26 pm
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On Wed, 29 Jun 2005 12:35:36 +0000 (UTC), Buddy Smith >Hi,
> >Newbie question...... > >I have a device I'm trying to turn on/off that requires 12V. I want >to control it with a micro that runs at 5V. > >My question is can this be done with a power MOSFET, or do I need a >relay? --- If you can switch the low side of your load you can use a logic-level MOSFET, like this: +12 | [RL] | D ON>-------G NCH
S
| GND If you have to switch the high side, you could use a P-channel mosfet driven by an NPN bipolar, like this: +12>----------+----------+
| |
[R2] | | S +--------G PCH | D C | ON>---[R1---B NPN [RL]
E |
| | GND>----------+----------+
>Assume that the current running through the controlled device is not
>much, but it will be on mostly 24/7. It will be turned off several >times a day for say 5 seconds or so. --- If you care to post your load current I'll be happy to give you some advice as to which devices you can use. -- John Fields Professional Circuit Designer | |||||||||||||||||||||||||||||||||||||
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Posted by Buddy Smith on June 30, 2005, 1:35 am
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> ---
> If you can switch the low side of your load you can use a logic-level > MOSFET, like this: This was my first thought when the MOSFET didn't work as i'd hoped. The application is a magnetic door lock. It pulls about 208-210 mA at 12V. I have a couple of NPN and PNP MOSFETs laying around and can get anything I need. --buddy | |||||||||||||||||||||||||||||||||||||
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Posted by John Fields on June 29, 2005, 9:16 pm
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On Wed, 29 Jun 2005 22:35:48 +0000 (UTC), Buddy Smith >
>> ---
>> If you can switch the low side of your load you can use a logic-level >> MOSFET, like this: >
>This was my first thought when the MOSFET didn't work as i'd hoped. > >The application is a magnetic door lock. It pulls about 208-210 mA at >12V. > >I have a couple of NPN and PNP MOSFETs laying around and can get >anything I need. --- MOSFET flavors don't include NPN and PNP. :-) Also, don't forget the damper diode across the coil... -- John Fields Professional Circuit Designer | |||||||||||||||||||||||||||||||||||||
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