This is the third blog post of my RoadTest review of the AIM-TTI QPX750SP power supply: AIM-TTI Bench Power Supply
Previous blog posts:
AIM-TTI QPX750SP RoadTest | A Non-Conventional Power Supply Review
AIM-TTI QPX750SP RoadTest | Passive Load Experiments
It the last blog post, we compared a bunch of different power supplies, using static loads.
This time I will continue the road test with experiments involving dynamic events.
1. Introducution
Bench Power Supplies usually let us to set an Output Voltage (V) and a Current Limit (I).
Based on these values, the power supply will work in one of 3 main operating modes:
- Constant Voltage (CV) mode - usually the default mode - the output voltage is kept at the set value, as long the current consumption is bellow the configured limit
- Constant Current (CC) mode - activated when the current consumption exec configured limit - the power supply reduces the output voltage, in order to stabilize the current consumption at the configured limit
- Constant Power (CP) mode - similar to CC mode (if present) - the power supply limits the power consumption to a configured limit, or to a maximum value the power supply can safely handle
Additionally bench supplies can have different protection mechanisms:
- Over Voltage Protection (OVP) - turns off the output if the voltage at the output terminals exceeds a given limit
- Over Current Protection (OCP) - turns off the output if the output current exceeds a given limit
- Over Power Protection (OPP) - (if present) turns off the output if the output power a given limit
The settings are usually set up by some kind of user interface.
This is what it looks like on the AIM-TTI QPX750SP:
and the RD Tech DPH5005:
2. Dynamic Load Setup
In order to test some features of the power supplies, I needed some kind of dynamic load.
Initially, I wanted to build a DIY Electronic load, but I decided to postpone this project, as it may take too long to build.
Instead, I came up with this simple Relay Module + Arduino based setup:
The setup consists of two resistive loads connected in parallel: a 36 Ω static load and a 4.2 Ω load switched by a relay module.
The relay module is controlled by an Arduino MKRFOX 1200, programmed to switch the load ON / OFF every 10 seconds:
- when the relay is OFF there is a single 36 Ω present
- when the relay is ON the combined load is 36 Ω + 4.2 Ω = 3.75 Ω
Two oscilloscope probes are connected to the output of the power supply, and the control signal of the relay.
This time, I also included my newly arrived "cheapo" Hantek CC-65 AC / DC Current Probe 
Using this setup I conducted a set of experiments to observe the behaviour of the two power supply at different dynamic events.
As the experiments I designed are mostly for bench supplies, I will compare the AIM-TTI QPX750SP against just the other bench style power supply I have the RD Tech DPH5005.
3. Load Regulation Behavior
In the first experiment, I wanted to see how the two power supplies handle changing loads.
For this, I used the above described setup and I set the oscilloscope to trigger on the rising / falling edges of the relay control signal.
I measured the time it take of the output voltage to stabilize, and as well any over / undershot.
This is what I got for the Aim-TTI QPX750SP:
| Aim-TTI QPX750SP | Time to Stabilize (ms) | Under / Overshot (mV) | Waveform | ||
36Ω ⇒ 3.75Ω | ~ 3 ms | - 570 mV |   | ||
| 3.75Ω ⇒ 36Ω | ~ 2.5 ms | + 480 mV |   | ||
and the RD Tech DPH5005:
RD Tech DPH5005 | Time to Stabilize (ms) | Under / Overshot (mV) | Waveform | ||
| 36Ω ⇒ 3.75Ω | ~ 1.5 ms | - 570 mV |   | ||
| 3.75Ω ⇒ 36Ω | ~ 0.25 ms | + 390 mV |   | ||
4. Current Limiting Behavior
Next, I wanted to see how the power supplies changes from Constant Voltage (CV) to Constant Current (CC) modes, and vice-versa.
To do this, I set the current limit to 2.0 A, on both of the power supplies. The 3.75 Ω combined load is supposed to take around 3.2 A @ 12V, so it should be current limited.
These are the results for the Aim-TTI QPX750SP:
| Aim-TTI QPX750SP | Time to Stabilize (ms) | Under/Overshot (mV) | Waveform | ||
| 36Ω ⇒ 3.75Ω (current limit turns ON) | 28.75 ms | - 2.02 V |  | ||
| 3.75Ω ⇒ 36Ω (current limit turns OFF) | 10.05 ms | + 1.44 V |  | ||
and the RD Tech DPH5005:
RD Tech DPH5005 | Time to Stabilize (ms) | Under/Overshot (mV) | Waveform | ||
| 36Ω ⇒ 3.75Ω (current limit turns ON) | 8.58 ms | none |  | ||
3.75Ω ⇒ 36Ω (current limit | ~ 1.9 ms | + 160 mV |  | ||
5. Short Circuit Protection
Bench power supplies are supposed to be able to handle short-circuits (accidental or deliberate).
To test short-circuits I replaced the 4.2 Ω switched load with an 30 A automotive fuse simulating a short-circuit.
Then, I measured how fast the two power supplies react to the short-circuit event.
This is what it looked like for the Aim-TTI QPX750SP:
| Aim-TTI QPX750SP | Time to React (ms) | Waveform | |
| 36Ω ⇒ 0Ω (short circuit) | 0.720 ms |   | |
and the RD Tech DPH5005:
RD Tech DPH5005 | Time to React (ms) | Waveform | |
| 36Ω ⇒ 0Ω (short circuit) | 0.250 ms |   | |
6. Conclusions
Interestingly, the cheap RD Tech DPH5005 power supply was able to handle most of the tested events faster, compared to the Aim-TTI QPX750SP.
Also, with the particular test setup I used, there are fewer over / under-shots and other artifacts produced by the RD Tech DPH5005.
On the other hand RD Tech DPH5005, produces a noisier output. This is what observed at the static load tests too.
Although, being "faster" looks to be a good for a thing for a power supply, it may not be the best in all cases. I'm not an expert, so would avoid drawing any deeper conclusions here 
.
Next time, I will take a look a some of the miscellaneous features of the Aim-TTI QPX750SP power supply.
Hope you enjoyed this update!