I’m looking to power a raspberry pi zero W off of a 15V DC power source.
After looking through the different step-down convertors for 5V output, I’m a bit concerned at the efficiency graphs for low currents. The graphs for most of the Pololu convertors start very low for low power output. It would appear most of these convertors are intended for 500mA or more.
The pi zero can draw as little as 65mA when idle (according to the web) and will be drawing around 300mA on average for my project.
Am I overthinking these efficiency curves? Most tend to drop down to 50% or less at the <100mA mark. The S9V11F5 looks like it might be suitable, but I thought I’d ask here for some opinions.
Thanks all for your time.
Welcome to the forum!
The S9V11F5 from what I can see would be the most suitable step-down regulator for your application. anything more than that would be overkill for a Raspberry Pi Zero even at full load.
Do be aware that the module can get hot under normal use, so be careful as to the location of the step-down module in relation to the Pi Zero as you do not want to damage your Pi.
Firstly I am not going to buy into whether this converter is suitable for your purpose or not. I don’t know exactly.
i would just like to comment on a couple of things in your post.
Congratulations, quite a lot of first time contributors don’t consider conversion efficiency at all let alone at low currents. Can be most important and can bring you unstuck down the line very easily.
If you are charging a battery there is an efficiency figure for charging as well. 5V??? to 4.2V would be an example for single or parallel cell(s) which you would have to consider if you are charging and using at the same time.
I don’t think you can “overthink” this. As I just stated you can easily come unstuck if you don’t.
Another thing worth considering is that a 3.7V 2000mAhr cell is NOT 2000mAhr at 5V.
The easiest way to do all this sort of calculations is to work in W or Whr. Then you can go right through a system and only do the conversions between W and V and A at the ends.
Example 3.7V @ 2000mAhr is 7400mWhr.
7400mWhr at 5V is 1480mAhr.
This has to be modified for conversion efficiency.
Some converters are as low as 85% so we will use this.
Your current capacity is now 1258mAhr.
So it is quite incorrect to assume the 2000mAhr would carry through to the 5V point. So you can realise just how far out you could be by not considering all these little losses.
I’d like to pass on some of the praise from Bob’s comment - your Google Foo is great!
Depending on your project you might be able to optimise power a bit more, the S9V11F version with the higher pin count features an EN pin, a phenominal tool for optimising power usage in a project.
The only drawback is, when using a Pi you usually loose access to toggling IO when safely shutting down so an external microcontroller is required adding a lot more complexity. Just an idea worth throwing out if you need to save every second of power!