USB Power for RH1 HiMD from external batteries

by John Beale - June 13 2007 (update Nov. 10 2010)

If you cut a USB cable in half and strip back the insulation, shield braid and foil wrap to expose the four internal wires, you will see something like the photo below. The wires are labelled according to the USB notes at the page.
USB cable wiring colors

Some USB devices, for example the Sony HiMD portable minidisc recorder MZ-RH1 do not have a separate DC power connector. Instead they can take 5V power from the USB port (pins 1 & 4) both to run, and to charge the unit's internal battery.  Sony provides a small AC adaptor box AC-S508U which has a USB port (power only) which provides +5V power via USB cable to the RH1 minidisc.  You can also buy external battery packs with a similar USB-power port which provide approximately 5V using four AA batteries. The nominal voltage of a NiMH rechargable AA cell is close to 1.25 V so this works out.

Referencing USB pin 4 as ground, the measured voltages from the Sony AC-S508U supply are:

1.   red: 5.18 V
2. green: 3.75 V  (Z = 82k)
3. white: 3.75 V  (Z = 82k)
4. black: 0.00 V

Note that the USB data pins 2 and 3 are not floating. They are held at 3.75 V with a moderately high impedance of 82 k ohms, and also there is 20k resistance between pin 2 and pin 3.  

As it turns out, the RH1 minidisc will not work with an external 4-AA USB power pack which leaves pins 2 and 3 floating, such as the one I bought for my RH1 ("Mobiledriven USB Battery Extender AA MD9020-0010").  I have read that other battery packs such as the 4xAA Gomadic and the Li-ion Macally IP-A481 do work with the RH1. According to this article for a DIY battery pack, just putting a 20k resistor between pin 2 and pin 3 is sufficient for the RH1 to work with an external supply.  You can see some discussion on this at the Minidisc forums here.

By cutting open a short USB extender cable and adding a 20k resistor between pins 2 and 3, I was able to get my 4xAA battery pack to operate the RH1. All this information is just FYI. If you need an external battery pack, of course it is better to buy one that works in the first place, especially since it costs the same ($20) as the one I had to hack.  By the way, the pack I have is unregulated so it puts out whatever the sum of the four AA batteries happens to be. Fresh alkaline cells for example will exceed 6 V open circuit. I don't know what the voltage damage threshold of the RH1 is. So far I have used only NiMH cells and everything has worked OK.

RH1 operating from external 4xAA pack

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