I recently built a couple of Bench Power Supplies using salvaged parts and inexpensive kits and modules from the Chinese electronics suppliers. I Blogged about these builds here on element-14.
and
https://www.element14.com/community/people/jw0752/blog/2017/02/11/oh-no-not-another-power-supply
It was suggested by michaelkellett that I should do a comparison test to see how well the power supplies perform against each other. I have also added in a comparison to a relatively inexpensive commercial bench supply that is my primary supply.
Here are the three supplies that we will be comparing in this experiment. I will refer to them throughout this blog as the Commercial Supply ( Mastech HY 3005F-3), Linear Supply, and Switching Supply.
The Commercial Supply is a 0 to 30V 5 Amp power supply.
This is the Linear Supply as described in the Blog "Oh No! Not Another #@&* Power Supply" and has a range of 0 to 27 volts with a max 3 Amp current.
Here is the Switching Supply as described in the Blog "Using the Coarse + Fine Control Circuit in My New Bench Supply" Which has a voltage output range of 1.2 Volts to 26 Volts with a 5 Amp max current.
The experiment will look at the ripple of each supply with no load and with full load. Each supply will also be tested and compared to see how it reacts to a load as well as how it reacts to the removal of a load. The test parameters will be 10 volts from channel one of each supply. The load will be an automotive brake light which has a beginning resistance of approximately 0.8 Ohm and an operating resistance of approximately 5.7 Ohms. The low beginning resistance of the bulb will serve to highlight the ability of the power supply to respond to a high current load.
The test instrument is a Rigol DS 1102E DSO.
Here is a layout of the test rig wiring.
I have set up the rig so that both power supplies are switched to their loads simultaneously. One of the weak spots of this experiment is my assumption that the two load bulbs that I will be using are identical. This may not be true in the strictest sense but they will be close enough for the tolerance of this experiment.
Our first test will be to look at supply ripple without and with the load bulbs in the circuit. As a convention when displaying comparisons I will display the Commercial Supply first followed by the Linear Supply and then the Switching Supply. First however let's look at the base line noise on the scope. This is 32 mV PP with the scope probe shorted to its ground wire.
The No Load Trace of the three power supplies is as follows:
The Commercial Supply has 56 mV PP, the Linear Supply has 60 mV PP and the Switching Supply has 40 mV.
1.75 Amp load on each supply gave the following results:
The Commercial Supply has 80 mV PP, the Linear Supply has 700 mV, and the Switching Supply has 640 mV. The characteristic of the switching supply scan is however showing a much more regular pattern which is likely a vestige of the switching frequency.
Next I tested the supply for how they would react to being switched onto the test load.
The Commercial reacts with a 6 volt instantaneous drop that over corrects slightly and stabalizes after 350 ms, The Linear Supply also dips slightly less than 6 volts and also over corrects with a stabilization reached in about 450 ms, and the switching supply reacts with a 3 volt drop and recovers without an over correction to a stable level in about 100 mS.
The last test that I ran was to look at the situation when the load was removed from the supply.
The Commercial supply has the lease reaction with only a 400 mV PP disruption while the Linear had a 960 mV deflection and the Switching had an 800 mV deflection.
My conclusion is that the Commercial at 10 times the cost of the other supplies was the best performer with respect to ripple on the voltage under all test conditions. The surprise for me was that the Switching Supply actually out performed the Linear Supply in my opinion. This was not expected as I had always assumed that the switching supplies would be worse. The obvious switch noise on the Switching Supplies output may be more of a concern than the apparently random noise on the Linear supplies output but I do not know enough to make a proper evaluation of this. I have not gotten to the stage of precision in my experiments so far where I have had any problems . I want to thank Michael for suggesting this experiment as it has given me some potentially valuable insights into the performance of these three supplies.
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