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 turns OFF)	~ 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!