Anth's Computer Cave Tutorials

Make a Multi-Channel Power Supply With 7800ct Serial Regulators

I have been getting lots done using the variable power supply we built last week. Everything is so much easier when you have fine control over power. But what about when you need two different specific voltages at once?

Today I will share a way to achieve this using nothing more than a few components salvaged from an old television.

7800ct Serial Regulators

The range of 7800ct serial voltage regulators
The range of 7800ct serial voltage regulators. Picture: Anthony Hartup

The 7800ct range of serial regulators work like magic and they are simple to use. You will find them in old CRT televisions and some computer monitors. The ones I am using are the 7805ct for 5V, the 7808ct for 8V and the 7812ct for 12V power.

Pinouts for 7800ct serial voltage regulators
The pinouts for 7800ct serial voltage regulators. Picture: Anthony Hartup

The pinouts for these chips could not be simpler. Connect your power supply to Vin, GND to GND, and Vout is your regulated voltage ready to go to your load. The GND from your load connects back to the GND on your power supply.

A wiring diagram for a 7805ct regulator
One of my extra-simple diagrams showing the 7805ct connected to a 5V motor.

The image below shows the regulator receiving 7V from the power supply and outputting a steady 5V. In my tests if you adjust the input anywhere up to 16V the output still remains at 5V.

A 7805ct in action
A 7805ct regulator producing 5V. Picture: Anthony Hartup

And now to take things a little further

If you've visited the Cave before you will know I tend to go overboard with things, and today is no exception. Why would I have just one voltage when I can have four? Here is my creation.

A power supply with four different  simultaneous voltages
My four-channel power supply. Picture: Anthony Hartup

The power for this contraption comes from a 16V printer brick connected to the breadboard. This in turn goes to Vin on a 7805ct regulator, a 7808ct regulator and a 7812ct regulator.

A power supply with four different  simultaneous voltages
My four-channel power supply. Picture: Anthony Hartup

Below is a logical diagram showing the actual connections.

A wirin diagrsm power supply with four different  simultaneous voltages
Wiring diagram for a four-channel power supply. Picture: Anthony Hartup

You can see the 16V output in reality comes directly from the power brick. The GND out is a common GND for all the different outputs.


Volt meters showing three different voltages
Volt meters showing three different simultaneous voltages. Picture: Anthony Hartup

Here you can see three channels, 8V, 12V and 5V, all running simultaneously.


Real world performance.

So how does this setup perform doing real work? So far it seems promising.

The main test I have tried is running one of the 24V stepper motors on my new robot, AAIMIv2. The 12V rail of this power supply does this quite well. The 7812ct regulator gets warm, but not overly so. I think this is about the maximum safe load for these chips.

I tried it with two stepper motors connected and it heated up in thirty seconds, despite only one of the motors being actually driven at the time. Just having the second motor controller connected and powered seemed to tip the balance. This is still impressive, though, considering the stepper motor is rated at twice the voltage of the 7812ct regulator. These chips are easy to find, I can just use one for each motor.

What I have not comprehensively tested yet is how the channels cooperate while each is under load. The power brick I am using only pushes about 750ma so there aren't many amps to fight over. I think with a higher current brick each channel could easily do real work at the same time. A few well-placed capacitors would help, too.

Adjusting voltages

Although primarily used as a set-voltage regulator, you can adjust the voltage to something other than the default value. I will continue this article when I have tested that further.

I will mention something I discovered, though. You can step these regulators up, as well as down. I place a potentiometer in serial with the return GND wire from the 5V input while running a DC motor the other day and attempted to slow the motor by turning the knob. Instead of revving down, the motor revved up as I turned, and I watch the voltage rise. I stopped turning when the volt meter showed 13.5V and the motor was flying. The 7805ct was getting hot by then.

I have not manage to repeat this yet, but I have a couple of ideas about how it may have worked.

Stay tuned, I'll update this article when I have more to share.

Cheers

Anth

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About the Author

Anthony Hartup, creator of AAIMI, Estimcad and Anth's Computer Cave.
Anthony Hartup runs the AAIMI Project, a Python machine-interface platform.
He also codes for the Estimcad Project and ControlCadGUI.