USB Woodstock charger?

[smp]

Good morning, all!

I have had the luck to purchase a working looks-like-new HP-25. Of course, no charger is coming with it. There are a variety of HP chargers available on TAS, and I also see this USB charger:

https://www.ebay.com/itm/175614292939

Does anyone have any knowledge about this charger? Does it work? Is there any possible danger with using it?

Any thoughts or comments welcome!

smp

[Maximilian Hohmann]

Hello!

(05-30-2023 02:32 PM)smp Wrote:  Any thoughts or comments welcome!

You probably already know the 100+ threads on this forum warning against charging the batteries inside any Woodstock series calculator.

What puzzles me a little are those two statemets in the description of the charging cable:

"Fully Regulated Voltage - just like the HP original charger"
"Fully Regulated Charging Current - just like the HP original charger"

As we all know, both statements are wrong. The HP original charger does not regulate anything and therefore kills calculators. If the seller starts his description with two wrong statements, I would rather be cautious about anything else he claims!

Personally I would not use this charger. I am so used to charge the batteries outside the calculator that I will not change my habit again :-) If you replace the original 500mAh NiCd cells with more modern 2500mAh NiMH cells, your calculator will last five times as long as originally.

Regards
Max

[smp]

Thanks very much, Max!

Since I've never owned a Woodstock before, I am very much aware of the rule to never try to use the calculator with the charger but no batteries installed - But since I've not been paying close attention, I had not yet heard the rule to charge the batteries separately outside the calculator.

Since so many items on TAS say they are for the Woodstock calculators, and I'm seeing a variety of HP part numbers for the chargers, I'll have to look up the proper HP numbers for the charger and an external charging dock so I'll get this right.

Thanks again!

smp

[BruceH]

(05-30-2023 03:00 PM)smp Wrote:  But since I've not been paying close attention, I had not yet heard the rule to charge the batteries separately outside the calculator.

This is because there is always a risk that one or both of your batteries, replacement or otherwise, might go bad without warning. And if that happens when the charger is plugged in then there is a chance that the calculator gets damaged.

Much simpler therefore just to avoid that risk completely.

[smp]

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

[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

[rprosperi]

(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

I'll answer a different way.

Yes, you can use a Woodstock that is not operating (as a calculator) in nearly every case (some units are so badly damaged that even this won't work). BUT, the original Woodstock power circuit is dumb and you will more than likely damage your batteries, or their life, or both.

Bottom line is this: Get new cells (NiCD or NiMH) and a proper smart charger that knows how to charge them properly. It's 100% safe and the batteries will last far longer.

[smp]

Thanks very much, folks! Sorry for the dumb question.

I already have a good "smart" charger from my HP-41 days, so all I'll need to do is modify the Woodstock battery holder (I'll hate to do that) in order to get the batteries out, or obtain an already modified Woodstock battery holder and some batteries.

Does anyone here sell 3D printed Woodstock battery holders?

smp

[rprosperi]

(05-30-2023 09:33 PM)smp Wrote:  Thanks very much, folks! Sorry for the dumb question.

I already have a good "smart" charger from my HP-41 days, so all I'll need to do is modify the Woodstock battery holder (I'll hate to do that) in order to get the batteries out, or obtain an already modified Woodstock battery holder and some batteries.

Does anyone here sell 3D printed Woodstock battery holders?

Actually it's easy and non-invasive.

Holding the pac with the batteries facing up and exposed terminals toward you, cut the bottom (towards you, at terminal end) of the center strip with an XActo knife. Cut the scored, recessed line parallel with the ends of the cells. It's only a 6-7mm wide strip x 1-2mm high. LEAVE THE OTHER END INTACT.

Now you can pry the cells out, one at a time, just moving the now-loose end of the center strip out of the way. The old, original cells probably have the spring at the far end stuck to it, making it a bit fiddly this first time. Once removed, clean the spring with a glass eraser and then DeOxit (to remove all the crud and oxidation) and you can most likely re-use it.

Easy to do.

[smp]

Thanks for this tip, Bob. Yes sounds pretty easy.

smp

[BruceH]

(05-30-2023 09:33 PM)smp Wrote:  Thanks very much, folks! Sorry for the dumb question.

I already have a good "smart" charger from my HP-41 days, so all I'll need to do is modify the Woodstock battery holder (I'll hate to do that) in order to get the batteries out, or obtain an already modified Woodstock battery holder and some batteries.

Does anyone here sell 3D printed Woodstock battery holders?

smp

There are no dumb questions - only dumb answers. (Unless you keep asking the same question!)

You can get a 3D printed replacement here.

I've previously bought their Spice replacement pack and was very satisfied with the product and the service. In fact, now I've realised they have a Woodstock one I think I'm going to get that as well!

The only downside is the colour really - less of a problem for the Spice version.

[smp]

(05-31-2023 10:00 AM)BruceH Wrote:  You can get a 3D printed replacement here.

Thanks very much for the pointer, Bruce!

smp