Hello I put together the DC to PWM module and works great using a floating source. Except if I use a computer ATX power supply as a source or a DAQ to control the input voltage the circuit becomes useless. The problem is when the Gnd of the K8004 is referenced to earth ground. As soon as earth ground is in the circuit, the output constantly saturates high no matter the input voltage. Is there a way to use a referenced source??? Am I alone with this issue? Thanks for the help!
Power supply ground and control voltage ground should be at same level for circuit to work correctly.
Please measure with multimeter between both grounds. Most likely there is some kind of ghost voltage due to capacitive coupling in the switch mode supply.
Hello, first off thanks for the quick response. Since I tried many things and configurations I will layout the less confusing and simple one.
Setup #1 If I power the board using 12v from a floating source(com not tied to earth/chassis ground) and I use the onboard Vref as the control voltage. I arrange the pots so that at Vin=0v PWM=15%. And Vin=5v PWM=75%. Works PREFECT!
Setup #2 If I then take the board and connect 12v from an ATX power supply (com=earth/chassis ground), again using onbard Vref as the control voltage. Vin=0v PWM=100% and Vin=5v PWM=100%. No PWM!
As per your question since I used theonboard Vref so the power supply ground and control voltage ground have a differential of 0v, as these two points tracks are shorted on the PCB and verified with a multimeter. I also noticed as in setup #1, if configure it to be a common ground(ground the com to chassis/earth) I get the same effect as Setup #2. It really seems not to like earth/Chassis ground! I would like to use a NI-USB 6009 to control the voltage, the problem with that itself is that the analog out is referenced to ground and would stir trouble. Any ideas? Thanks
Sorry, at this time it is unclear to us what the exact cause of troubles is, but it is clear that it is related to the SMPSU.
Is there excessive noise on the output of the SMPSU?
Maybe the K8004 is a ‘weird’ load for the SMPSU, since it is not lineair, maybe the feedback circuit of the SMPSU gets upset?
It could also be related to the fact that the SMPSU is directly coupled to the AC grid via filter caps…
Two things to note. I do not think it has anything to to with the switching power supply but have earth ground common. I used my isolated power supply and it works great. As a test I ran a wire from com to ground with this supply and as soon as the circuit sees the earth ground, the output saturates high.
As another test, with the computer ATX supply, I cut the ground prong (120AC) so it only has hot and neutral. With this case the PWM works, except the signal is very noisy when low.
By accident I might have ran into something that you might be able to explain and something I can use. While looking at different points with the oscilloscope I noticed that the signal between Load (-) and GND to be perfect and clean with the exception of the duty cycle being the inverse (as Vin increases, duty cycle decreases).
Sorry, at this time we don’t seem to be able to help.
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