Several weeks ago I purchased a couple of Modules that were advertised as able to provide 1 Hz to 150 kHz signal with a 0% to 100% duty cycle control.
The modules were able to be powered with 3 volts to 30 volts and would output a signal of this same magnitude. The output was rated at 5mA to 30mA depending on the supply voltage. The accuracy of the device at 2% for the Frequency and 1% for the Duty Cycle is acceptable for the type of experiments that I do in my lab.
I played with the modules for a while and then I didn’t think too much more of them until a couple days ago when I needed to run some experiments on a few servo motors. I found the module easy to work with and helpful in my experiment so I decided to see if I could design it into a new piece of lab equipment for the shop.
After some thought I decided to build a variable DC power supply very similar to the ones I have put together before. The power supply would provide the foundation for the new piece of equipment and the PWM module would provide new PWM features for special applications and the convenience of having the module hard mounted and powered.
I wanted to have the following outputs on the device:
- DC 0 to 27 volts from the power supply
- 5 volt signal output from the PWM module
- A MOSFET controlled output that would link to the signal from the PWM module and allow a Load attached to the DC output of the power supply to be pulled to ground for a low impedance inverted output.
- An earth ground / chassis ground terminal which is useful in some applications.
A block diagram of the proposed build was drawn out.
A schematic was also roughed out but rather than post the preliminary schematic, here is the finished schematic. I have put the power supply board, the volt amp meter, and the PWM module in as black boxes.
Fortunately I already had a suitable enclosure for the unit with a front panel that had been cut for a dual bench power supply. The meter hole in the front panel for the right side would need to be enlarged for the PWM module but otherwise only minor modifications would be needed to the front panel. Here is what the inside of the chassis looked like at the beginning of the build.
And here is the preliminary front panel from the back side:
When the unit was ready for testing I could not get the power supply to work properly. It seems I had switched the plugs for the voltage control and the current control. It was also noted that higher power output using the MOSFET caused the V/A meter’s screen to flash and not display the voltage of the DC output. A couple of 1000uF capacitors solved this problem. One was placed across the DC output and the other was put on the power supply to the meter module. When I wire a build I always plan for routing and cabling of the wire after the unit is tested and debugged.
Here are pictures of the finished unit:
Since my scope probe is between ground and the drain of the MOSFET the duty cycle is inverted. You can see the unit is showing 12.3 Volts at 50 Hz and 16.6% Duty cycle while the scope is telling us that we have 12.8 volts at 49.9 Hz and a calculated duty cycle of 16.9%. These numbers are in good agreement for the level of precision that I need. Sorry about the reflections on the oscilloscope screen. I am still too lazy to get a screen print and transfer it. Fortunately I usually work with my shirt on.
The simplicity of the overall project, the modules, and the fact that the case was all prepped made this build go very quickly with a start to finish time of two days.
I will be getting in some more servo motors for testing later this week and I am looking forward to giving the unit a proper test.
If you go looking for the PWM Modules on line be aware that they come in two versions. One version has a push button that allows the output to be turned on and off. The other version is a dollar or so cheaper but lacks this feature.
Thanks for reading my blog.
John
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