A question on power suply to UMI-7764

[Dejun]

I used a regulated 5V DC power supply to UMI-7764. I used meter to measure zero-load output, stable 5.02V

When I run the motor, I found the encoder can not feedback correct information (motor often became out of control).

Yesterday I found the problem came from this way: As long as I connect the power to UMI-7764 and also UMI-7764 is connected to NI-7344 motion controller (computer is power on), the actual voltage on the UMI +5V connetor is much less than 5V. Aslo when the motor run, this voltage randomly change and motor became out of control. So I guess the power supply caused the problem.

Then, I used a 12V AC-to-DC adaptor and a potentiameter (change resistor) to make another poweer supply. I found that only when I adjust potentiameter to obtain about 10V DC zero-load voltage will the +5V connector on UMI will 5V power supply. And with this power supply, the motor runed perfect (correct encoder feedback). However, a problem with this is the potentiameter became hoter and hoter very quickly.

So, looks like as long as the UMI-7764 is connected to a powered PCI-7344 controller, it eats away some power supply voltage. Can you tell me why this happen? What kind of power supply shall I use? WHat the amp requirement on the power supply?

Thank you very much for your help.

[Buechsenschuetz]

The UMI manual says that power consumption of the UMI box is 0.2A.

If I remember it correctly the +5V power output of the 7344 boards is specified to supply 0.1A only. I am not sure what will happen if you exceed this current, but in case they use some kind of current limiting on the boards the output voltage will decrease significantly if the maximum current is exceeded.

I would not recommend to use the UMI box with a non-regulated power supply, as you are doing with a 12V supply and a series pot. Actual voltage supplied to the UMI box will depend on the current comsumption which may vary. You should be careful not to supply more than 5.25V to any digital circuitry - overvoltage is the safest way to damage them.

We have been using the UMI box with a home-brew 5V/1A regulated supply device as well as with a regulated lab power supply and never had any problems with voltage drop.

You should check the power requirements of your encoders. Maybe they consume more current than your power supply can deliver. The popular HP/Agilent encoders of the HEDS series, for example, are rated at 70mA power supply. Using four of them, you need approx. 300mA for the encoders only, plus 200mA for the UMI box, so you should use a power supply which is rated at 5V/500mA minimum.

Most wall transformers have non- or poorly regulated output voltage. Benchtop supplies with 5V and 1A regulated and current limited output are widely available. Beware of any power supplies with non-constant, i.e. load-dependent, output voltage.