Woodstock power supply

[jebem]

The HP-21 HP-25 power supply design uses a form of shunt regulation when the external power charger/adapter is in use and the calculator is turned ON.
The battery is the shunt regulator element and it is in parallel with the external power charger and with the calculator DC-DC converter.
The current limiter R1 (8R2) is only inserted in series with the battery when the calculator is tuned OFF and is used to protect the battery.

Shunt regulators are very inefficient, as it must consumes current in parallel with the load to do the regulation and they require a series current limiter of some sort. The load here it is the DC-DC converter.

In this specific basic HP design it requires a current limiter when the calculator is turned ON, otherwise the difference between the transformer secondary voltage and the battery nominal 2.4VDC would simply either destroy the power adapter transformer or kill the battery.

The solution HP found to limit the current in the above conditions was to design the transformer with a very poor regulation by setting a very high secondary winding impedance to drop the excess voltage above the nominal battery voltage.
The dropped voltage is dissipated as heat inside the transformer.

The calculator DC-DC converter alone will consume around 110mA idle and 150mA with all LEDs lit.

So basically the external transformer is under "overload" all the time we use it with the calculator tuned ON.

The HP power charger can deliver a maximum current of 150mA (1.8W at 10VAC under no load, but just 7VAC under a 47Ohm load for 150mA).

So, to make a long story short, replacing the original battery with a capacitor in parallel with a series of 3 or 4 rectifier diodes, will work but I would expect and additional transformer overload that may result in shorter life cycle.
Additionally, because now we have a capacitor instead of a battery, we have an extra ripple current charging the capacitor at each positive half cycle that will also increase the transformer overload. The larger the capacitor capacitance, the larger the ripple current.

That said, I used that modification many times along the years in different types of equipment because it is a easy and fast fix in many situations.

But I admit it is not my first design choice for two reasons:

- Power inefficiency, as explained above.
There is nothing we can easily do to mitigate this.

- Catastrophic failure of one of the rectifier diodes.
Because these diodes are under heavy current load, they can fail by opening the internal PN junction, resulting in a voltage supply overload that can kill the calculator electronics.
We must mitigate this risk by using diodes with enough dissipation power to cope with the current at all times. I would use 1 Amp 100V Si diodes as a absolute minimum, like the 1N4001.

When possible, I would use series regulators as they can be much more efficient and safer but this would either require internal calculator or power adapter modification.