start writing on HP PSUs

hp_psu/notes.typ

@@ -0,0 +1,27 @@
+#import "@preview/acrostiche:0.3.1": *
+
+
+#init-acronyms((
+  "PSU": ("Power Supply Unit","Power Supplu Units"),
+  "PCB": ("Printed Circuit Board",),
+))
+
+= Exploration of the retrofitting of HP Power Supplies to EET
+
+The objective is the build a proof-of-concept for using an EET capture card on the 12V rail of a server #acr("PSU") without changing the form factor.
+The targets are as many HP server-grade #acr("PSU") as possible.
+At the time of writing this note, we have access to three #acrpl("PSU"):
+ - // fill with infos
+
+
+= Current Share ISHARE pin
+Because the server #acrpl("PSU") are tightly integrated, it is very difficult to tap into their 12V rail.
+The difficulty comes from the fact that the current gets generated directly on the #acr("PCB") and is not carried over a wire that we can cut.
+Cutting a trace of tampering with the #acr("PCB") is completely out of the question.
+
+Fortunately, server #acr("PSU") have a feature called ~Current Sharing~ that enables using multiple sources to provide more current to a system.
+The simplest implementation would be to connect multiple #acr("PSU") in parallel to the same 12V rail.
+This solution is presented to be non-robust by many sources although they all recomend to use an external system that share the load between the sources, which is exactly what this solution is doing.
+In any case, the server #acr("PSU") often include an analog output that is proportional to their current and enables an external circuit to adjust their voltage based on their current to spread the load.
+This analog output can also serve as an information source for remote monitoring system.
+The actual reason why this output is there remain somewhat hidden but the important fact is that the #acrpl("PSU") provides us with their current information, which is all we nned.