Driving transistor from current sinking device

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
Hi
This is a really basic question but for some reason it is confusing me.
I have a chip, a magnetic sensor, an AFL006 - datasheet here
https://www.nve.com/Downloads/AFLxxx-xx_Data_Sheet.pdf
As it says, it's a current sinking device, i.e. when it's on, current flows into it. I want to use it to drive a little buzzer motor, which will draw about 30mA at 1.5V. How do I connect a transistor so that it will switch on the motor when the AFL006 is on?
Many thanks
 

Reloadron

Joined Jan 15, 2015
7,447
Looking at the data sheet and yes, the 006 is a sinking version. what I don't see and am missing in the data sheet is how much current can the unit sink? I see you mention 30 mA for the motor. Where am I missing the current in the data sheet?

Ron
 

Reloadron

Joined Jan 15, 2015
7,447
hi Ron,
all I can see is 100uA as the minimum Output current on that d/s.
E
Yeah, normally you see a max current somewhere and I couldn't find it. Beats me. :)

I figured if the max current was about 50 mA then it could drive the motor which is 30 mA but I guess a 50 mA max would be asking a lot. So that considered I am taking your suggestion and running with it.

Ron
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
Well interestingly enough, experimentation shows that at 2.8V it will drive the motor directly, drawing about 45mA, although at 1.5V it drew about 25mA and the motor wouldn't start, which I guess is more down the motor's foibles. I don't know how long the ic would supply 45mA happily for - I guess I could contact the manufacturer to ask. Since I felt lucky, I didn't put a diode in parallel with the motor which might have made a difference(?)
 

ericgibbs

Joined Jan 29, 2010
18,641
hi rj,
On Starting or in a Stalled condition the motor can draw upto 5 to 10 times its normal running current, also I would fit a diode.
E
BTW: I don't know the cost of the sensor, but a few pence spent on a transistor would be a good idea.
 

Reloadron

Joined Jan 15, 2015
7,447
You may want to give the first few pages of this a read. They do a good job explaining why a snubber diode. I would use one. You also may want to add a small capacitor across the motor terminals. Finally I agree with Eric in that if the little motor experiences a locked rotor or stall you could toast your sensor. Clint Eastwood: "do you feel lucky"? :)

Ron
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
Yes I know - did it in physics at school a few decades ago - but these motors are so tiny and I've used them quite a bit without diodes that I took a chance. OTOH these sensors are quite expensive so I won't do it again!
Thanks for the PNP advice
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
I had a confusing email exchange with the manufacturers who pointed me towards the 100uA in the datasheet. I said: but this is listed as the minimum not the maximum. They replied: "the output is guaranteed to sink at least 100 μA so you can design around that."
So it appears they won't specify a maximum!
 

BBee

Joined Dec 6, 2018
35
My usual approach is to isolate the triggering circuit by provision of an output stage. This avoids possible bad loading of the triggering circuit and enabling easy sorting of higher switching loads without recouse to precise mathematics. Normally I use fets for switching but the advice of a PnP transistor is probably far easier here.

Tracy
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
I don't have any PNP transistors (I don't think), only some NPN ones (2N3904) and N channel mosfets. But I can anything not too exotic quickly so suggestions welcome
 

ElectricSpidey

Joined Dec 2, 2017
2,756
With only 1mA guaranteed from the sensor you will need a Darlington, if you use a transistor. At least based on the ten to one rule to saturate a standard transistor.
 

BBee

Joined Dec 6, 2018
35
You would need to invert the output so if you don't use a PnP transistor an NpN invertor stage would be needed. Maybe use a 2N3904 to drive one of your Mosfets (low out to high on collector hence switches on the fet). It would need a pull up resistor on the 'Out' of the IC such that the current on it is limited to less than 100uA. The collector current of the 2N3904 could be higher (too high a value of collector resistor may affect switching time). That way a Darlington should not be needed (assuming the fet is up to the job).

Tracy
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
With only 1mA guaranteed from the sensor you will need a Darlington, if you use a transistor. At least based on the ten to one rule to saturate a standard transistor.
Yes but I’m still pretty confused about this max/min thing. I showed that actually it was capable of producing 50mA for at least a short time so I don’t think the 100uA means much
 
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