PWM Controlled Variable DC-DC Converter

I am having a harder time than anticipated locating a step down DC-DC converter that I can control with a PWM signal.

This is going to be part of a project that non-enthusiasts may try to replicate/build, so I am ideally looking for something that can be pretty simply wired up instead of building my own converter from scratch. I found this description of a buck converter, which leads me to believe that a buck converter is the correct choice for this application: http://www.learnabout-electronics.org/PSU/psu31.php

The application: I have a small, what is essentially a battery powered radio. The battery is a nominal 7.5V and lasts for about 1 day. As the battery voltage drops at the end of life, some strange things begin to happen. For training, I want to be able to simulate the dying battery stage on demand over the course of 30 minutes to an hour.

The device: The device that takes 7.5VDC with a max input of 8VDC (damage occurs above 8 volts and it's about $250 so I don't want to have any possibility of more than 8V being applied). I would like to use a 7.5-8V power supply, run through a PWM controlled converter that is essentially transparent (or close to it) at 100% PWM, and then can ramp the power down. I don't know what the minimum voltage will be, but I envision it's somewhere between 4-6 volts so as long as the converter can drop the voltage below that level, we are good.

All the variable buck converters I have been able to find are manually set with a pot, which is not what I am looking for. Does anyone here know of something that might work for this application?

What you're describing is a variable dc power supply. Throwing PWM into the mix is Just creating a classic x-y problem situation.

How much current do you need?

Hi avr_fred - I am not sure what the current draw will be right now as I don’t have the actual transmitter to test on. It should be less than 1 amp (that estimate is based off the knowledge that it will run off of 5 C-cell batteries in series for 24+ hours).

What I am looking for is essentially a variable power supply that I can control with PWM from the Arduino.

Something that I did not make clear is that this system will still be run off of a battery, but it will be a larger 12V battery that will allow it to run for several days during training.

If the battery supply is a requirement then yes, a buck regulator is what you need.

Using this buck regulator and this I2C pot to replace the stock 10k trimpot will do what you want.

One last detail, there is an 0805 smd resistor on the 2596 board that would need to be replaced to operate at 12 volts full scale rather than the 40 volt range as shipped. I'll calculate that value if this something you want to pursue.

I seem to learn something new everyday! I didn’t know there was such a thing as an I2C controlled potentiometer. There are a lot of different variable buck converters out there that you adjust through a pot, but I passed them up as I didn’t think about a digitally adjustable pot.

With your tip, I think I can figure this out. Thanks a ton!

Just be careful with your digital pot selection, most (especially the eBay cheap-o's) can only withstand the same voltage on the pot element as is applied to the digital logic side. The Microchip device I linked to is a specific higher voltage withstand pot which would be required for your 12-14 volt input application.