Drive LED from low current open collector output

Thread Starter

ilium007

Joined Aug 24, 2013
80
Hi - I have a Hall effect sensor that features an open collector output but only capable of sinking 1mA of current. I want to drive an LED when the sensor is activated ie. output low. The output is pulled high via a 10k resistor to 5v when no magnetic field is present and pulled to ground via the collector output when triggered.

To drive an LED I figure I require a PNP BJT or FET (FDN340P or similar ). I’ve been reading Art of Electronics by Horowitz but I’m still stumped. As a BJT is current driven I’m not sure a BJT will be able to source enough current via the 1mA collector of the sensor to saturate. I have no idea how to choose a FET for this application. Does it definitely require a PNP ?

The other option, more cumbersome and totally overkill, is to use a SMD Attiny MCU but I’d rather try to understand the discrete electronics path before falling back to what I know and use the MCU.
 
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WBahn

Joined Mar 31, 2012
25,918
Is the only thing that the sensor output has to do is make the LED light up?

Is the pullup resistor removable or changeable with a different resistor?

How much current does your LED need?

What's the forward voltage of the LED?
 

Thread Starter

ilium007

Joined Aug 24, 2013
80
It’s only prototyping at this stage, I haven’t yet picked out a SMD LED to use but regardless of forward voltage it will only need 5-20mA to indicate.

At this stage I’m most interested in the methods to choose a FET to use in this and similar 3.3v - 5v projects.
 

Bordodynov

Joined May 20, 2015
2,627
You should look for the maximum threshold voltage to be sufficiently less than the power supply voltage. You should also look at the channel resistance. For LEDs, I recommend the FDV302P transistor (threshold voltage not more than 1.5V and channel resistance 5 - 10 Ohm). This transistor has a small input capacitance and a protective Zener diode.holl.png
 

Thread Starter

ilium007

Joined Aug 24, 2013
80
OK - so what do I need to look out for in the datasheet ? Im a bit confused by Gth Max being -1.5v. This particular application is 5v, does this mean that I can only use a gate voltage of max 1.5v below 5v to trigger the MOSFET ? Then I looked at the Id to Vgs graph and it doesn't show values up to the max Id of 120mA.

I am obviously looking at it all wrong, where do I start interpreting datasheets for particular applications? I imagine I am most interested in Max Rds(on), Vgth and max Id

Also, does the FDV302P has a complementary N channel FET ? The FDV301N looks like a match: https://media.digikey.com/pdf/Data Sheets/Fairchild PDFs/FDV301N.pdf

When I go searching the suppliers a lot of FETs have Rds(on) in the milliOhms but these are rated in the Ohms range, why the massive difference ?

FDV302P:

Screen Shot 2019-04-14 at 5.20.58 pm.png
 
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Bordodynov

Joined May 20, 2015
2,627
Transistors vary in application areas. These are currents and power and voltage. Low-power is not so much. It is clear that you do not need to use a 100 Amp or 1000 volt transistor. The more powerful the transistor, the greater the leakage and the larger the input capacitance. Overkill is a bad practice. The fdv301n and fdv302p transistors have approximately the same chip areas and therefore approximately the same input capacitances. Due to the smaller hole mobility compared to electrons, fdv302p has about three times more resistance. Inside the crystal, a zener diode is about 9 V. It follows from the datasheet that the gate-source limit voltage is 8 V. If you want to get a symmetrical transfer characteristic, then usefdv301n and fdv304p.
All of these transistors work well at 5 volts. Much greater impulse currents are permissible, but most importantly do not use these "big" currents continuously. Watch for power dissipation. When the voltage at the gate is 5 V, the resistance of the transistor will be less than when the voltage at the gate is 3 V. Ie when working in key mode, the greater the voltage at the gate, the better!
 

Thread Starter

ilium007

Joined Aug 24, 2013
80
Thanks but I am trying to understand the theory of MOSFET selection and implementation rather than just copying a circuit so that next time I can choose a FET by myself.

I'm trawling through datsheets and using the search feature on DigiKey after all the YouTube videos I have watched say that I should be looking for low Rds(on) FETs. For example, I have found this one - FDN302P https://www.onsemi.com/pub/Collateral/FDN302P-D.PDF it has an Rds(on) as low as 55mOhm vs the 13Ohm on the FDV302P. What distinguishes these two devices ? The FDN302P has a Vgs of -1.0v so I assume this is also a logic level FET ?

And there are 1 mA LED emitters available
I didn't think of just using a lower current LED !!! Thats why I'm just a pilot and not an electrical engineer ! Still.. I need to understand how to select and use MOSFETs.
 
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Thread Starter

ilium007

Joined Aug 24, 2013
80

danadak

Joined Mar 10, 2018
4,057
If I look at spec it looks saturated at 1 mA. So I wonder if you
could get 2 mA out of it and still be in saturation so it does not
dissipate much power. Look for < .5V at output. That way you
could get a little brighter LED to work with it.

That would operate it out of spec, but may still work fine. Not a recommended
practice, but if its not human safety application and its a throw away try it.

Regards, Dana.
 

Thread Starter

ilium007

Joined Aug 24, 2013
80
If I look at spec it looks saturated at 1 mA. So I wounder if you
could get 2 mA out of it and still be in saturation so it does not
dissipate much power.

That would operate it out of spec, but may still work fine. Not a recommended
practice, but if its not human safety application and its a throw away try it.

Regards, Dana.
This Hall Effect sensor will be used to prevent a 3d printer axis from crashing at its end stop. I don't really want it to be failing !
 

Bordodynov

Joined May 20, 2015
2,627
When turning on the LED, it is always necessary to limit the current at the operating level. The easiest way to do this with a resistor. Let the supply voltage be 5 V, and the required current - 10 mA. The voltage drop for a red LED is about 2 V, and for a white one is ~ 3 V. It is easy to calculate the series resistance. Subtracting from the total supply voltage the voltage drop across the LED and applying Ohm's law, we get: (5V-2V) / 10mA = 300Ω. This is the total resistance of the resistor and transistor. For a small influence of the variation of the transistor parameters, it is advisable to choose such that its resistance is no more than 10% of the total resistance (Ron <30Ohm). But there is a good proverb: “That, too, is not great!”
 

Thread Starter

ilium007

Joined Aug 24, 2013
80
I have designed the circuit, could someone please have a quick look before I send the PCB off for manufacture ? I just want to confirm the 1k R1 resistor to limit current on the A3213 Vout pin as its only rated to 1mA as previously stated. I don't know if I need this as evidently a MOSFET is voltage / charge driven and not current driven. I don't understand how charge can flow to charge the gate and not have any current flow.

I've never created a PCB before so I'm a bit nervous !

Screen Shot 2019-04-16 at 8.43.21 pm.png

Screen Shot 2019-04-16 at 8.42.58 pm.png
 

danadak

Joined Mar 10, 2018
4,057
That should be fine. Althoug in schematic you have source and drain
interchanged. Flip vertically the symbol in your schematic.


Regards, Dana.
 
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