Testing multiple output IPSU with dummy load

[Gabriel Sá Pinto]

Hi everyone!
I'm currently working on a new project that consists in developing a dummy load to test a multiple output IPSU, it has 9 rails and their voltage ranges from 1.25V to 5V, the output current should be less than 1A, but for testing purposes let's consider 1.5A.

The idea is to have an active load controlled by a microcontroller that can perform tests to all the rails regarding overvoltage and overcurrent situations and print the results on a PC screen.

I'm having a hard time with the first steps of this project, my knowledge in Power Electronics is not that great so I'm trying to understand how things work and what do I need to do.
I found this programmable electronic load but it might an overkill either in power or cost, so I've been researching some related projects on the internet and I found this one, seemed like it could work.

However there are several doubts that come to mind... How can I test all 9 rails at the same time? How do I know which load to use for each rail and what output current they have?
Note that I didn't design this IPSU, I've just been making some tests with it mostly in software simulation to check efficiency and threshold values.
Also I have a passive dummy load that was given to me to make some consumption tests, it has loads for all the rails but it's built in a way I really don't understand.
This forum doesn't let me upload the schematics for the IPSU but I'll upload an image of the schematic of the passive load.

If someone could help me with the first steps of this project I'd be much appreciated.

Thanks in advance

 

[Reloadron]

For what you want to do and in the interest of simplicity I would give some thought to using an Arduino Mega 2560 board:

The MEGA 2560 is designed for more complex projects. With 54 digital I/O pins, 16 analog inputs and a larger space for your sketch it is the recommended board for 3D printers and robotics projects. This gives your projects plenty of room and opportunities

You will not find a way to easily test all the rails at the same time. Normally you MUX (Multiplex) the channels. Now I am not quite sure what you mean by:

The idea is to have an active load controlled by a micro controller that can perform tests to all the rails regarding overvoltage and overcurrent situations and print the results on a PC screen.

If you want to be able to vary the loads on each rail it can get complicated but if you can work with fixed loads it is not all that bad. For example like your drawing a bunch of fixed resistors as loads. For example for a 1.5 amp load on a 5.0 volt rail you want a resistance of 3.333 Ohms and 1.5 amps at 5 volts equals 7.5 watts so you want a margin. I would use a 2.0 Ohm 10 watt resistor in series with a 2.0 Ohm 10 watt pot to trim the current and get the 3.33 Ohms. Ohmite would be a good brand name to work with, the pots can get costly. The voltage is simply run into an analog input of the Arduino uC (Micro Controller). Switching loads is done using a MOSFET for each channel. The Arduino D/O (Digital Outputs) turn the MOSFETS On/Off. To sense the current since it is low I would think about a canned solution like SparkFun Low Current Sensor Breakout - ACS712. The ACS 712 is a good sensor for 0 to 5 amps. Buying the ready made board makes for a nice solution as amplification and scaling of current is done for you. You can also roll your own but that means making the boards and if you want simple this is the way to go. The current is converted to a voltage proportional to the current and the work is done in the code. That is how I would approach it but there are numerous ways to go about it and numerous I/O modules rather than the Arduino I mentioned. Budget? Oh yes, finally how accurate does this need to be and what resolution? For example a 0 to 5 volt analog input with 10 bit resolution means 0 to 5.0 volts equals 0 to 1023 bits so 5 volts / 1023 bits = 4.88 mV resolution. If you need better things start to increase in cost.

There is more to this but you have an overview. All of the math like Over Under Voltage and Current is done in the code.

Ron

 

[Gabriel Sá Pinto]

For what you want to do and in the interest of simplicity I would give some thought to using an Arduino Mega 2560 board:

I was suggested an MSP430 uC because we already work with those in our company, forgot to mention that. I just need to evaluate if there are enough ADCs and DACs to perform what I intend, I think they're enough.

If you want to be able to vary the loads on each rail it can get complicated but if you can work with fixed loads it is not all that bad. For example like your drawing a bunch of fixed resistors as loads.

I think you misunderstood, what I really want is an active load, that I can change via micro controller interface with the purpose of testing the rails, to see if they shutdown when a certain threshold is reached, fixed or passive loads is what I have right now in the image I uploaded. This project is an update to that passive load, make it active, variable, by changing the gate voltage of a MOSFET probably, and that control being made through a PWM signal emitted by the uC.
I found another link that better illustrates what I'm trying to do. Of course it doesn't need to be as fancy and with cooler fans and all but the idea is close.

To sense the current since it is low I would think about a canned solution like SparkFun Low Current Sensor Breakout - ACS712. The ACS 712 is a good sensor for 0 to 5 amps. Buying the ready made board makes for a nice solution as amplification and scaling of current is done for you. You can also roll your own but that means making the boards and if you want simple this is the way to go. The current is converted to a voltage proportional to the current and the work is done in the code.

I'm going to design a PCB of my own, so no ready made boards. I forgot to mention this isn't a school project, it's a project in the company I work for and its end application is to test an IPSU meant to be used in space, and we require a simple and effective way of testing the supply units.

Oh yes, finally how accurate does this need to be and what resolution? For example a 0 to 5 volt analog input with 10 bit resolution means 0 to 5.0 volts equals 0 to 1023 bits so 5 volts / 1023 bits = 4.88 mV resolution. If you need better things start to increase in cost.

I think that's more than enough yeah.

Having these additional aspects into consideration is there any other advice you could provide?

Thank you for your answer

Gabriel

 

[Reloadron]

Pretty much covered it. Depending on how fancy you wish to get you could dump the data into an Excel or similar spread sheet creating a history file for tested units. What you have in mind sounds fine to me.

Ron

 

[Gabriel Sá Pinto]

Ok then. Thank you once again!
Cheers
Gabriel