USB Woodstock charger?

[teenix]

(05-30-2023 03:38 PM)smp Wrote:  Thanks very much, Bruce! Sounds like good advice to me.

There are plenty of Woodstock units available "For Parts or Repair." Is it possible to use an otherwise dead Woodstock as a charging station, or does the charging portion of the circuitry get fried too when the calculator gets fried?

smp

The mains charger is just a simple unregulated transformer which feeds AC voltage into the calculator. When the calculator is switched OFF, this voltage goes through a single rectifier diode and a 2 watt 8.2 ohm resistor to charge the battery. The battery helps to smooth the half wave rectified AC like a big capacitor to give a DC voltage and it also places a load on the transformer which will lower its output voltage to a value that gives the proper charge current as the engineers intended.

When the calculator is switched ON, the 8.2 ohm resistor is shorted so more current is available to charge the battery and also run the calculator.

Therefore, if the battery is not connected or is old and damaged and has a high internal resistance, there is less or no smoothing effect or reduction in the incoming AC which can then place higher than normal voltage on the calculator circuits and damage them.

You could construct a simple battery charger circuit like the original (diode and series 8.2 ohm 2W resistor) and wire it inside the case of a damaged calculator but you would have to get a power transformer with similar characteristics to the original. You could also construct a better charger, there is plenty of info on the web.

It might be easier just to buy a proper NiCad charger and wire the output to the battery connectors in the old case. If it is a good design, it will take care of your batteries.

All said and done though, I still wouldn't use anything to charge the batteries whilst in the calculator.

The person in the ad might be referring to the Classic chargers which do have basic regulation for the battery and powering the calculator while on charge.

cheers

Tony