Creating a myDaq Voltage Variable Power Supply

[Ratmslasher]

Hi Everyone,

Just received my NI myDaq the other day and just starting to play around with it.

I was a little disappointed to find out that the Power Supplies were not Variable (i.e. fixed 5V and +/- 15V), but that's what I get for not reading the fine print :).

I figured a Variable Voltage Power Supply would be a fun good first project, but have no idea where to begin. I was hoping you guys could give me some advice/point me in the direction to look.

I plan on using myDaq for primarily breadboarding for EE post-labs.

Since the 5-V supply allows for the most current to be drawn, I figure that would be the first supply to be built.

Can anyone point me in the direction to move towards?

Thanks!

-Ernie

[Steven_K.]

You can actually use the Analog outputs as a variable power supply. It can supply 2mA. You can increase the current by using an ampifier.

[charlesy · Accepted solution]

Hello Ernie,

Depending on the required output voltage and and current, there are a few approaches you can try.

Buffering the Analog Output(s) is the most straightforward method. You could probably get to +/- 12V if you pick an opamp with small enough headroom requirements and add 20% gain. You'd use the +/- 15V rails to power the opamp (be sure to consider rail tolerance), and you should limit your output to the 32 mA spec of the +/-15V rail. The buffer is essentially a linear regulator, so be careful of the power dissipation in the amplifier at high currents.

If you want higher output currents at the lower voltages, consider a switching regulator. There are a variety of topologies depending on the input-to-output voltages. Look for Buck to step down a higher input voltage to a lower output voltage; Boost to step up; Buck-Boost to step up or down. There are others, but these are simplest to start with. You can use myDAQ's analog outputs to inject an error voltage into the regulator's feedback to allow programmatic output voltage control. Take a look at power supply chip vendors' websites (TI, Analog Devices, Linear Tech, National Semiconductor, etc.) for app notes and component recommendations. 

Another approach is digitally-controlled switching regulators. These provide extensive control and read back capabilities through a digital interface to the chip. Google "digital power" for ideas.

If you need even higher power, you'd need to bring in an external supply or battery, but the control could still come from the myDAQ AO or digital interface.

It can be a challenging exercise, but I hope this helps get you started.

Charles Y.

[Ratmslasher]

Hi All,

Thanks for the advice/help. Once I thought about it, I simply built a negative feedback op-amp with a potentiometer to adjust, which gets me about 0-11V, which is all I need for now.

However, I haven't thought so much about it's drawbacks. I will look at the other suggestions when I get off work; I've been wanting to play with some regulators chips. I didn't know that people made power supply chips as well.

Thanks for the advice for this fun little project!

-Ernie