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Topic: transistor or MOSFET?? (Read 9 times) previous topic - next topic

KenH

Hello all, I quick question here.  for driving a relay from the Arduino m168 chip - a transistor such as 2N2222 can be used, OR a MOSFET such as the BS170 can be used - is there a real advantage of one over the other?

The relays I'm talking about are small 5vdc to 12 vdc relays with max of <100mA coil current...

Thanks for any insight,

Ken H>


jackrae

Given the simple yes/no question then go for the mosfet.  Transistors, if not fully turned on, can dissipate a fair amount of heat.  Mosfets are either on or off and have extremely low on resistance to dissipate little heat.
jack

KenH

#2
Jul 02, 2010, 07:51 pm Last Edit: Jul 02, 2010, 07:52 pm by KenH Reason: 1
That makes sense - Thank you Jack.

Had I though this thru, knowing the low resistance of the MOSFET vs transistor, I can understand why this is true.

That is why a bipolar transistor can be used as bypass transistor in a regulated power supply - i.e. 18 vdc in, 13.8 vdc out.  A MOSFET would not work at all there.

To energize a relay, where the load is fully ON, or fully OFF all the time, either would work ok, but a MOSFET is best due to low voltage drop across MOSFET and not requiring as much heat sink.

Am I understanding the above correctly?

Thank you.

Ken H.

retrolefty

#3
Jul 02, 2010, 08:06 pm Last Edit: Jul 02, 2010, 08:09 pm by retrolefty Reason: 1
That is why a bipolar transistor can be used as bypass transistor in a regulated power supply - i.e. 18 vdc in, 13.8 vdc out.  A MOSFET would not work at all there.

That is not a true statement. A mosfet can also be used in a bypass application, it's all in how much gate voltage you apply. A mosfet can be used as a linear device as well as a switching device, again it's all in how much bias voltage is applied to the source/gate.
To energize a relay, where the load is fully ON, or fully OFF all the time, either would work ok, but a MOSFET is best due to low voltage drop across MOSFET and not requiring as much heat sink.

Yes MOSFETs (with suitable ratings) are very good for high current switching application, if the appropriate gate voltage is applied. Note that most mosfets won't fully turn on at the Arduino 5volt logic level output pins, as typically they require a +10 volt gate voltage for minimum source/drain resistance. What is required for direct connection from output pins is a LOGIC level mosfet

Am I understanding the above correctly?
Again both bipolar transistors and mosfet transistors can operate as digital switching or linear amplification devices, it all depends on the base biasing condition or source/gate voltage bias applied.

Lefty


KenH

Quote
most mosfets won't fully turn on at the Arduino 5volt logic level output pins


Ahhh, that is why some MOSFETs are listed as TTL logic so they will turn on at 5vdc?  Good point.

Well, as can be seen, nothing in life is "black or white" - usually shades of gray.

Thanks to all for sharing info - this is a GREAT site and I've learned a lot from the knowledge shared.

73 de Ken H>

pwillard

#5
Jul 03, 2010, 02:42 am Last Edit: Jul 03, 2010, 02:48 am by pwillard Reason: 1
Time to be a curmudgeon...

Each has it's place... for sure... but...

I feel strongly that bipolar transistors are easier for newcomers to electronics to deal with.  Unless you are trying to drive a relay yanked out of an Automobile whose coil might draw hundreds of milliamps, your average 2N2222 transistor will easily drive a small relay.   No issue with heat... geez the 2N2222 IS BARELY working hard to drive a simple relay like that.

Read the WIKI on the 2N7000 mosfet and then read the 2N2222. Literally interchangeable and 2N2222 might even be superior.

How often have we seen people misuse/misapply  the IRF510... because they thought a MOSFET is a MOSFET...  or because they thought it was just BETTER to use one and it ends up being a complete waste of (or lack of) engineering if used to drive a  5V relay in a 5 Volt circuit.

Now driving a 12V Relay... where you really are driving the relay with 12 volts...   and the relay draws more than the average hundred or so milliamps... maybe an IRF510... (you need a extra level converter transistor too)  but the relay is supposed to SAVE you from doing the heavy lifting... which is why many draw less than 100 ma  and most are between 100-300ma.   A 2N2222A can cope with up to 800ma and you usually only need to add a 1K resistor at the base.

If it's a 2N7000 NCHANNEL MOSFET or 2N2222 NPN transistor you are going to spend about $0.50 for either one, on average.

If you are using anything in a T0-220 package to drive a relay... you wasted your money.

Logic Level Bipolar transistor...   Um... no. that's not a problem when dealing with standard transistors.

Udo Klein

I had the same issue a while ago. I ordered both types. Then I started working with the NPN driver transistors PDTD113ZT. Turned out that they are more than sufficient for my purposes and cheaper than the mosfets. Since they come with integrated resistors they need no additional parts as well. However these are SMD parts though.

Another issue is that bipolars usually have no ESD issues. So I figured that the (maybe) small loss in efficiency is more than compensated by the pluses: price and hopefully better reliability (with regard to ESD).

BTW: I am running them @~30kHz to drive a small coil for powering a backlight. Peak current is currently 150mA.

Udo
Check out my experiments http://blog.blinkenlight.net

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