11 May
2002
11 May
'02
3:49 p.m.
Tony,
That's a good guess but it doesn't have one. It does have a three position
manual switch on the side (DC-Off-AC). I pulled the thing apart today and it has some
very strange circuitry inside. I figured out enough of it to be able to connect power to
it and make it work but I don't understand all of the circuit.
In additon to the two power sources that Don memtioned you can also get an external
battery pack for it.
I think a lot of the power circuitry in controlled by the switch and the wiring in the
various power units. FWIW I talked to a guy today that has about six of the EL-8s (and six
AC power supplies) and none of his PSs work. It turns out that they have some kind of
hybrid circuit in them that seems to be very failure prone.
Joe
At 09:42 PM 5/11/02 +0100, Tony wrote:
Joe, the 3-pin
receptacle on the calculator accepts a cord from either
of two external power units. One being a unit that works off of 12vdc -
cigarette lighter - and the second works off of 110vac. The second unit
is capable of 8.7vdc @ 150ma as well as 9.6vdc @ 230ma. There are a
couple of charging modes as well as a charge while running on external
power and my suspicion is that this accounts for the 3 pins on the
connector. I will see if I can get something more definitive for you.
One totally wild guess (I've never seen an EL-8)....
It's possible that the 3 pin connector incorporates an internal switch
contact that shorts 2 of the pins together when the charger/PSU is not
connected. It could then be wired like this :
switch contact
-----------
V V
+------------o o--------------------- +ve to rest of machine
| o
------- |
----- |
------- |
----- |
------- |
----- |
| |
+-----------------+------------------------- 0V
When the charger is not connected, the battery pack is connected to the
machine and powers it. When the charger is connected, the battery pack is
disconnected. The charger is in 2 parts. A constant current supply
(conencted to the left hand pin in the diagram) to charge the battery and
a constant voltage supply (connected to the right hand pin) to run the
machine.
This is (of course) how the HP 'classic series' and the HP67 are wired.
But I've seen it used elsewhere too.
-tony