Hello,
Recently after reading some favorable reviews I decided to get a linear power supply and since this is for hobby, I have chosen KORAD 3005P, paid 200 CAD (i.e not as cheap as it can be found in US), shipped from Amazon.ca
The PSU is not ideal, includes incorrect documentation that has not been updated for quite some time (I mean setting of M1-M5 modes), OCP, OVP are turned off by default, occasional strange behavior requiring power cycling etc - the list goes on and those issues are mostly known to KORAD existing users.
But there is one thing that really baffles me - the way OCP works in this PSU.
I must say I am a big fan of OCP mode (and don't care about OVP) . And since I mostly work with low current devices I always keep the OCP limit low and close the to it realistically consumed/drawn value, i.e. average + 50%-70% which gives quite workable numbers. I.e. if a MCU board consumes 50 mA (max) I would set OCP limit to 130 mA and start working on it being sure that in XX cases out of 100 OCP will save the board if I short something. The time has proved this approach to be correct and saved me and the hardware many times.
Having said that I expected the same behavior from KORAD 3005P and what I see is totally different. I connected a small MCU board normally consuming about 40 mA to the KORAD with OCP limit set to 200 mA and when I turned the output on the PSU immediately turned it off due to the triggered OCP. I increased the OCP limit to 0.4A - no dice, 1A - nope, 1.5A - no bueno, 2.6A - success. If I turn the OCP off the power supply powers the board without any problem, the drawn current is about 40 mA.
What is going on? Turns out a capacitor (4700 uF) installed on the + 5V rail of MCU board starts charging and trips OCP but this is not right.
Eventually, I decided to test PSU using the following simplified test setup (using capacitive load to simulate the small MCU based board):
0.1 ohm is a current shunt resistor allowing us to see the drawn current during output voltage raise time using blue trace (the channel 3 of SDS1204X-E), while the yellow trace (the channel 1) will allow us to monitor the output voltage.
How do I find the OCP limit to be able to power the load with enabled OCP? I will turn the OCP off and measure the current on the shunt. Please pay attention to the initial current spike and max output voltage (10.24 while PSU output is set to 10V, but btw it was not the worst case, I will show worse cases later)
If the OCP limit is set to less that 2.5A and OCP is on then the PSU will constantly trip overcurrent protection and I find it very strange.
My conclusion regarding the work of OCP in this particular PSU is this: if during the output voltage raise time the drawn current exceeds the OCP limit then OCP will be engaged. It seems to be wrong due to multiple reasons and one of them is because some loads (not necessarily capacitive ones) may drawn more current then underpowered (i.e. the output voltage did not reach the nominal value yet).
I have a different PSU (DPS 5005) with OCP and OVP support and the OCP is implemented differently - during the output voltage raise it will LIMIT the current to the OCP set limit and when the output voltage reaches its programmed value then the OCP will start monitoring the drawn current and turn the output off in case if the limit is exceeded.
This is how it works when OCP limit is set to something like 50 mA - it has no problem with starting up with OCP enabled.
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At this point I have two questions:
a) Could someone who has experience with PSU from known brands tell me how OCP behaves in their PSU during voltage raise time?
b) Could someone who has KORAD 3005 make a test by simply connecting some sort of capacitive load (a 1000 uF capacitor would be enough) and check the behavior of their unit? May be I simply have a defected PSU and should send it back for replacement.