HP calcualteor power adapter

From: Joe <rigdonj_at_cfl.rr.com>
Date: Sat Jun 1 12:28:51 2002

At 12:31 AM 5/28/02 +0100, you wrote:
>>
>> A little while ago I seem to remember someone in the UK saying they
>> needed a power adapter for one of the original HP calculators -
>> HP35/45/55 etc.
>> I have one if that person would contact me off list....
>
>It wasn't me, even though I have a fair collection of HP calculators
>(including, I think, all but one of the devices to use that PSU).
>However, this seems like a good place to post some information on
>repairing said PSUs.
>
>This unit is one of the few that charges the battery properly. The PSU
>has 3 connections to the calculator. Ground, a constant-voltage (4.2V)
>supply to run the logic and a constant current (55mA) supply to charge
>the NiCd. When the charger is not connected, then the battery is
>connected to the logic supply line. When you plug the charger in, it
>moves a contact in the calculator plug away from the pins and disconnects
>the battery and the calculator.
>
>For models _without card readers_ (35, 45, 55, 70, 80) it's safe to
>connect the charger without a battery pack in the calculator. There is no
>connection between the battery and anything else when the charger is
>plugged in.

   WARNING: this is true for all models EXCEPT the LED C models (25C, 29C, etc). DO NOT try to run one of them without a GOOD battery or you'll burn up the memory chips. FWIW In these the ROM, RAM and constant memory are all on the same ICs so you may destroy more than just the constant memory.



 For models with the card reader (65, 67), you must have a
>good battery pack installed in the calculator before connecting the
>charger. Otherwise the card reader sense amplifier chip sees the full
>voltage of the constant current supply (about 16V) and may fail. This
>chip is, of course, HP custom.


   Yes, However the same IC is used in the 67, 97 and the HP 41 card reader. So there are at least a supply of replacement ICs available if you're willing to sacrifice something.



>
>Anyway, getting back to the charger. There are basically 3 stages to the
>circuitry. An unregulated DC PSU giving about 16V (transformer, 4 diodes
>as a bridge rectifier, 400uF capactitor). A constant voltage regulator
>(resistor + zenner and an emitter follower). And a constant current
>regulator (a couple of transistors and resistors). All discrete
>components, nothing custom.
>
>As to what goes wrong with them :
>
>1) Open-circuits in the output cable, particularly close to the
>calculator connector. Trivial in theory, harder in practice. You have to
>slice open the connector as it's moulded to the cable.


  This is by far the most common problem and they're usually intermitant so that makes them even harder to spot.



>
>2) The 400uF smoothing capacitor goes open-circuit.

   Easy to spot since the output voltage drops under load. The cap is short and replacements are difficult but not impossible to find.


>
>3) Some versions have a fuse between the transformer secondary and the
>rectifier. It's a soldered-in picofuse. Sometimes it fails for no good
>reason.
>
>4) The power transistors (especially the NPN one in the constant-voltage
>circuit) can go open-circuit.
>
>Here's an ASCII-art schematic of the classic charger....
>
> HP82011 (etc) 'classic series' charger
> --------------------------------------
>
> (Unregulated PSU)
>
>-----)||
> )||(----+----->|--+-----+------ +16V
> )||( | | |
> )||( | +-->|--+ === 400uF
>Mains)||( | | ---
> )||( +--)--|<--+ |
> )||( | | |
> )||(-------+--|<--+-----+---+
>-----)|| |
> ---
> ///
>
> (Constant voltage regulator)
>
> c e
> +16V ----+------\ >---------------------- +4.2V Logic Supply
> | \ / (NPN power transistor)
> | -------
> / | b
> \ |
> / 470R |
> \ |
> | |
> +---------+
> |
> ---,
> ^ 4.7V zener
> |
> ---
> ///
>
>
> (Constant current charger)
>
> (PNP power transistor)
> 13R e c
>+16V -----+----\/\/\---+--------> /------------- 55mA constant current
> | | b \ / charger supply
> | ----- -----
> | e / \ c | b
> +----------> \--------+
> (PNP signal transistor) |
> /
> \ 4k7
> /
> \
> |
> ---
> ///
>
>
>-tony
>

     Joe
Received on Sat Jun 01 2002 - 12:28:51 BST