When I connect my VM140 card to my PC via USB I noticed the following behaviour:
I use the DEMO8061 Programm.
I have got a stable input value at the AI.(0…10V)
If my AO is zero the AI value in the DEMO-Program is stable.
When I increase the AO value without any load on the output side everything is fine an the AI value is stable to.
When I connect a load (a “OSRAM 64435 U” Lamp 24 V 20W) to my Output the Input (in the DEMO-Program) starts to vary.
The higher the output becomes the more the A-Input Value varies.
As you can perhaps see, I am not a quite specialist in electronics.
Can anyone tell me whats the problem with my installation?
And witch measures could be taken to avoid such a behaviour?
Thank you!
I believe that like other Velleman boards I am more familiar with, the VM140 doesn’t use a separate, clean reference voltage for the ADC, but the general +5V supply rail. So the accuracy of the ADC depends on the overall electrical noise on the board.
A micro controller doesn’t actually generate any analog output signal. The way these interface boards produce such is by generating a high frequency pulse width modulated signal (PWM) with the micro controller, then using a capacitor, resistor and an operational amplifier to produce the analog signal (variable voltage) from that. If you look at that output voltage closely with an oscilloscope, it has a tiny AC wave on top of it, that is of the frequency of the PWM signal. In the case of the VM140 it should be 15.6 kHz.
What happens when you drive a load with that output is that the OP-AMP will vary its power consumption a tiny bit more in exactly that frequency and thereby introduce more noise to the supply voltage, which negatively affects the precision of the ADC.
One way of avoiding this is to avoid using the OP-AMP on the board to drive your load. Instead you could use the PWM signal directly and drive a Darlington power transistor with that. The lamp would then be connected to its own, external power supply and controlled through the power transistor.
You connect a load of 24V 20W?? That is almost 1 ampere you are trying to get out of the output. This is impossible to do! The output can’t deliver that power.