Does anyone have a specific recommendation for a DC power supply that they have found worked well with the BrickPi? From a previous note there was the general recommendation for a regulated power supply 9V or higher able to give 2A, but I’d rather not have to guess. There seems to be a wide variety out there! Is there a list of a few that would work well?
There isn’t a list that I know of, but here are some specific numbers to work with.
The voltage should be regulated, and in the range of 8v to 10.5v. The BrickPi can safely run at 12v, but Lego motors are designed for 9v, so they would run faster and get hotter at 12v. NXT and EV3 motors have built-in thermal protection, so although you could damage them or at least void a warranty on them, they shouldn’t pose any threat to safety even if using them at voltages beyond what they are designed for.
As a general rule, I try to size the power supply so that it’s able to provide at least 1A, plus 1A for each motor that will be used. For example, if you have a robot with three motors, you should use a power supply capable of at least (1A + (1A * 3)) == 4A continuous power.
Under most circumstances 1A will be more than enough for a RPi with WiFi, even if you are also using Ethernet, keyboard, and mouse.
At 9v with 100% power, the stall current could be as high as 2A per motor, but a robot shouldn’t be designed where the motors will be physically stalled at full power. Additionally, even if a motor were to be stalled, the internal thermal protection should trip within a few seconds, reducing the draw to maybe 10%. The higher the supply voltage, the quicker the thermal protection will trip (at which point you need to wait a bit for it to cool down and return to operation). For more details regarding the current draw of Lego motors, please refer to this article.
If 12V is too high, why does the BrickPi come with an 8-AA pack as opposed to a 6-AA pack? also, for some reason (not sure why) but it seems easier to find a 12V regulated power supply at 4A than a 9V for the same thing, but perhaps I’m not looking in the right places…thus my original question for recommendations. I’ll keep looking…
So the idea of using the external power supply is to reduce the use of batteries so that I can use the batteries only when there is a demonstration and not while debugging. If I use a 9 V power supply and then switch to 12 V battery pack won’t the motors run faster? I’d prefer to have the power supply behave just like the battery pack if that’s not going to cause a problem. The number of different power supplies online is pretty bewildering!
12v does work and is safe, it’s just beyond what the Lego motors are designed for. We recommend using our 9.6v rechargeable battery pack, or 8xAA rechargeable cells (9.6v nominal), but it will still work with 8xAA alkaline cells (12v nominal). As long as your application works with 12v, it shouldn’t be a problem. The biggest differences are that the motors will run faster compared to the power percentage, and the motor internal thermal protection will be activated much faster (once you remove power, it will cool down and return to normal within a short time).
12v power supplies are indeed much more prevalent than 9v power supplies, especially at over 1A capacity.
I have found that with the battery pack, if the batteries are at all different (e.g. slightly different levels of use, a little old, or certainly any different brands) then the batteries explode. These motors, and the entire BrickPi seem to draw a lot of current – which is why it took me a while to find a good DC power supply (one that is at least 4A). Never tried with a straight-up 9V battery, but I imagine it couldn’t handle the current load.