My latest experiments revolve around using the Chinese linear power supply board that has been the heart of many of my bench supply builds but rather than powering it with the tradition transformer and bridge arrangement I want to power it with a switch mode power supply.
Here is a schematic of the linear power supply board as supplied from the vendor:
My enthusiasm often gets ahead of my eyes and my brain so in typical form I assembled a board and left off the bridge diodes. I then hooked up a 28 volt power supply and checked to see if it would work. It did not! Digging out my schematics with annotated voltages I quickly realized that the important negative 5 volt supply rail was missing. A look at the schematic revealed that the negative 5 volt rail was generated by the AC input line being fed by way of R2 and C2 to a voltage inverter circuit made up of D5, D6, and C3. This voltage is then regulated by R3 and D7. Subsequent experiments showed that this negative 5 volt rail supplies about 8 mA of current to the negative supply pins (4) of the TL081 Op Amps at U2 and U3.
I was left with the problem of supplying the negative 5 volts from the expected 28 volt output of the SMPS that I hoped to use to drive this board.
I have been using Recom switching regulators quite often in my builds so I went to their site and started looking for a device that would take 28 volts input and give me a negative 5 volts out. Here is a data sheet for the the device, a RH 2405D, which most closely matched my needs:
https://datasheet.octopart.com/RH-2405D-H6-Recom-Power-datasheet-68303594.pdf
The input rating on this converter is 24 Volts +/- 10% which would not get me quite to the 28 Volts that I expected from the SMPS. This problem was easily solved as the Chinese linear power supply board comes with a LM7824 that is to be used to power a heat sink cooling fan. I will use the fan supply to power the RH-2405 pending a successful testing of the RH-2405.
Once the RH-2405 arrived I set to checking it out and bread boarding it into a test circuit.:
Here are the questions that I wanted to answer with my experiment:
Will it work as advertised?
Are the +/- 5 volt outputs and their common isolated as advertised?
How will a load affect the operation and voltage of the outputs?
What will the ripple on the output look like?
The experiment showed that the isolation of the outputs was good and able to be connected as needed to produce a negative 5 volt rail for the Chinese PS board. I hooked up a load and ran it up to 21 mA which is over double what is expected in normal operation if it were used on the linear board. The ripple on the output looked to be around 150 mV. Here are photos of the test equipment as it monitored the experiment.
All the tests went satisfactorily and I am now prepared to depopulate one of the Chinese Linear board of the original components that generated the negative 5 volt rail and hack the RH-2405 converter onto the board in their place. In preparation for this the original schematic was also depopulated and redrawn with the 24 volt regulator and the RH-2405. Here is the modified schematic:
Sounds like a good project for tomorrow night.
John
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