Increasing Digital Output Current on USB-6001 with ULN2003A

[connor016]

I am controlling a stepper motor driver and the current required on the digital inputs of the stepper driver is around 11mA (at 5V).  My USB-6001 is not capable of outputting that high of a current.  I have seen people using the ULN2003A to control relays and it looks like it should work for my application.  Will this work and then can I use the 5V output to go to the ULN2003A since it can output 150mA.  To wire up the ULN2003A would I use the 5V output and put the positive on the COM?

[connor016]

Is this correct for wiring?

[crossrulz]

The COM should be grounded.  Since these chips work as a Darlinton Pair, the output will be connected to the GND when you apply a voltage to the input.  But the output will float when the input is off.  So you need to use a pull-up resistor on your output to your 5V.  I would still recommend using an external power supply to make sure you have enough current capability.

[connor016]

I found another post:

http://forums.ni.com/t5/Digital-I-O/NI-USB6008-and-ULN2003A-relay-driver/td-p/2765908

Based on their schematic it looks like the COM was not grounded.  Could you clarify further? Sorry my background is not in electronics.

[crossrulz]

I would follow the advice in that thread.  Anything hardware related that Lynn (johnsold) says is generally fact.

[johnsold]

Thanks, Tim.

Ground is connected to GND (pin 😎 on the ULN2003. The common or COM connection is for the free wheeling diodes and should be connected to the positive power supply for the motor driver.  Depending on the circuit within the motor driver, you may need a pull up resistor. If the driver input circuit is resistive and not inductive, you might not need the common connection at all. There is no power supplied to the ULN2003 in the usual sense of power to a logic chip. Each of the inverter symbols on the diagram represents a Darlington-connected transistor pair and some resistors and maybe a diode.  The free wheeling diodes manage the energy stored in the inductance of a coil so that it does not destroy the transistor when the curretn is shut off.

Lynn

[connor016]

Thanks Lynn!  I will be using a STEP+/- and DIR+/- to simply enable the driver and change directions (there is an onboard oscillator).  I made an updated schematic based on your advice.  I am not sure if I will need a pull up resistor or not though based on the components inside the drive.  

[johnsold · Accepted solution]

As you have it drawn should be fine. It does need the connection to the DAQ +5 V because the controller inputs are optically isolated. That circuit is also consistent with the 11 mA current requirement mentioned in your first post.

The USB-6001 digital lines are software timed so your maximum step rates will not be very high.

Lynn

[connor016]

Thanks!  The software timing is an issue so this stepper controller has an onboard oscillator so I can simply use the STEP to engage to the drive and there is an analog input that is proportional to the velocity.