Why 100mA Sinking Driver measures 21mA

[naseeam]

A controller has 100mA Sinking Driver. I connected one terminal of 500 Ohm resistor to the output of this driver and other terminal of resistor is connected to +12V DC power supply. When output of this driver is Logic Low, current flows from power supply, through 500 Ohm resistor, into the driver. I measured this current to be 21mA.
How does this circuit work? Does 100mA spec. mean that this driver can sink 100mA max.? What resistor value do I need to tie to +12V DC power supply to sink more than 21mA? What's the resistance at the output of the driver? Does this resistance change for different resistors connected to DC power supply?

Thanks.

 

[on1aag]

Hi Naseeam,

When a driver can source or sink 100 mA means that it can
maximum source or sink 100 mA, and that current depends
on the resistance of the load and the supply voltage. (Ohms law)
You should take the necessary precautions that this current
is never exceeded. In this specific case the theoretical minimum
value of the load resistance should be limited to 120 ohm.

on1aag.

 

[Hero999]

The actual current drawn will depend on ohms law.

At 12V and 500hm: you'd expect 24mA to flow but you're a little short at 21mA which is because the resistor is probably at the higher end of the tollerence range and there is some voltage loss in the driver.

 

[chemelec]

The actual current drawn will depend on ohms law.

At 12V and 500hm: you'd expect 24mA to flow but you're a little short at 21mA which is because the resistor is probably at the higher end of the tollerence range and there is some voltage loss in the driver.

Actually even if you use 12 volts, you won't have 12 Volts across the resistor.
The Sink Circuit will have a Small Voltage Drop, of about .8 Volts, maybe even MORE.
But with the .8 volts, That would make the output current of 22.4 mA.
The resistor tollerence could be the rest of the error.

 

[naseeam]

You mentioned 0.8V or more voltage drop across the sinking driver. Is this voltage drop constant for any resistor tied to +12V DC. For any resistor I mean, any resistor that would not violate the 100mA spec.

 

[audioguru]

You forgot to attach your driver's datasheet so we can't look up its max voltage loss at 100mA. The datasheet will show a curve for typical voltage loss at different currents. The higher the current then the higher the voltage loss. Usually, the lower the current the lower the voltage loss.

 

[chemelec]

You mentioned 0.8V or more voltage drop across the sinking driver. Is this voltage drop constant for any resistor tied to +12V DC. For any resistor I mean, any resistor that would not violate the 100mA spec.

Well You Mentioned its "A controller has 100mA Sinking Driver".

Without Actually seeing the Circuit I can Only Guess.
But Usually they use an NPN Transistor for Sinking the Current. So depending on the exact parameters of the transistor, I would say it should be a Fairly Constant Voltage Drop.

To Find Out, JUST MEASURE IT.
Place a Voltmeter between your connection of the Resistor/Sinking Driver and Battery Negative.
Than try some other Resistor Values and repeat the test.

 

[philba]

the voltage drop is dependent on the device itself and the current through the device. some times data sheets will show curves (Vdrop vs I). I always use .7V when calculating it for bipolar transistors (unless the data sheet says different). For MOSFETs, though, it's a resistance but is more complex to determine.

Like it was said earlier - measure it. Use Ohm's Law to calculate current or voltage drop.

 

[Hero999]

The Sink Circuit will have a Small Voltage Drop

That's what I meant by "there will be some voltage loss in the driver"

 

[naseeam]

Thank you Everyone for providing many excellent answers.