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 pinouts.ru
page.
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.
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