Transistor Partially On?

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mastermime
 
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Transistor Partially On?

Post by mastermime »

Hello,

I am building a robot and am using a Beaglebone Black as my SBC. I am currently trying to send HIGH signals through the GPIO pins to MOSFET transistor (IRFZ14PBF-ND 0) which switch off various devices that draw >100Ma.

For some reason, when I send a high signal to the transistor, it seems like the transistor is passing only a really small amount of current. For example, when I send the signal, the fan I have twitches (so there is definitely a current path), but it doesn't even rotate 1 full revolution. I put a meter on the drain and source voltage and got only 2v when it should be 12v!

Here's a schematic of my configuration. Scroll down to the one titled 'Robot Schematic' http://engineeering.weebly.com/schematics.html

Any idea whats going on? Could it be the resistors? I didn't know it was possible to have transistors that were 'half on'. I thought they were on or off, not somewhere in the middle.

Please someone enlighten me

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Renate
 
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Re: Transistor Partially On?

Post by Renate »

BeagleBone Black has 3.3V I/O pins.
https://www.adafruit.com/product/1278

IRFZ14BF MOSFETs have a maximum gate threshold of 4V.
http://www.vishay.com/doc?91289

It would probably be easiest to switch to a lower threshold MOSFET.

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zener
 
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Re: Transistor Partially On?

Post by zener »

Yes. You need a "logic level" mosfet. Sometimes they have a "L" in the PN. For example the logic level version of IRFZ14 is IRLZ14:

http://www.vishay.com/docs/91325/sihlz14.pdf

They sell one here:

https://www.adafruit.com/product/355

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adafruit_support_mike
 
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Re: Transistor Partially On?

Post by adafruit_support_mike »

Renate and Zener have nailed the problem and the solution, but let me add some theory in response to this:
Mastermime wrote:I didn't know it was possible to have transistors that were 'half on'. I thought they were on or off, not somewhere in the middle.
It's very much possible to have transistors "half on". That's where most interesting analog circuits live.

In the technical vocabulary, BJTs are 'transresistance amplifiers' and mosfets are 'transconductance amplifiers'. Those names are kind of intimidating at first, but they boil down to descriptions of what goes in and what comes out.

Any electrical signal can be described in terms of current and voltage, and the easiest way to model the circuit at the point where that signal appears is to pretend it's a resistor. Plug the current and voltage into Ohm's Law to get an equivalent resistance, and then you can do quick, in-your-head estimates of how the circuit will behave if you change something.

Example: a BBB's output pins can deliver a signal at 3.3v and 10mA. That means the pin is roughly equivlaent to a 330 ohm resistor connected to a 3.3v battery. If we connect that to a load whose resistance is also 300 ohms, we can expect the pin voltage to drop by about half (probably not a good idea).

In those kinds of estimates, you choose either a current or a voltage to use as the input, then do the math to see what happens to the other one. If you start with a current and want to get a voltage, V=IR says you multiply the value you have (current) by the resistance. If you start with a voltage and want to get current, we write Ohm's Law as I=V*(1/R), where "1/R" is called the 'conductance'.

Taking that back to transistors and running the options for voltage/current in -vs- voltage/current out, we get:

- current in, current out: current amplifier
- current in, voltage out: looks like a resistance calculation
- voltage in, voltage out: voltage amplifier
- voltage in, current out: looks like a conductance calculation

I used the words "looks like" because for transistors, the input and output appear at different pins. The shorthand for "what happens here makes things change over there" is the prefix 'trans-'. So:

- transresistance: changing the input current changes the output voltage
- transconductance: changing the input voltage changes the output current

BTJs and mosfets both have gradual transfer curves (graphs of the input-vs-output behavior). To use them as switches you decide how much change you want at the output, figure out how much change you need at the input to get that, then apply more input than necessary. That puts things in a state where the amount of current flowing through the transistor is limited by the supply voltage/current, not by the control input.

In this case, a BBB can't supply enough voltage to take that particular mosfet past its "more than I need" point.

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mastermime
 
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Re: Transistor Partially On?

Post by mastermime »

Wow thanks for the excellence responses. I'm glad you guys were able to recognize the problem so quickly.

I have already ordered 10 new MOSFETs with a lower gate threshold voltage so they should come Tuesday and I'll have them swapped out in no time.

Adafruit_Mike- Thanks for the much needed lesson on BJTs and MOSFETs. I've never heard them described like that from the angle you took. I learned a lot.

Thanks everyone!

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