Charging solution for old calculator, can it work?

[DarkEnergy]

I acquired an old calculator that's unfortunately without a power supply. There's also no way I'll ever find a power supply for it. It's able to run off the wall and battery power which is through 6xAA 1.2V NiCD batteries soldered together creating 7.2V, aka not the easy pop out kind and they're long past their expiration. The original power supply outputs two voltages, 7.2V 225mA DC and 7.0V 45mA AC. The AC output is very confusing and doesn't make sense to me considering the batteries which the calculator can run off of supply DC, why not just output DC only? It plugs into a 3 pin port. My best guess is that it powers the calculator at the same time it's charging the batteries. 

 

My main question is if it makes more sense that the 7.2V DC is for charging the batteries and the 7.0V AC is for powering the calculator (assuming it converts it to DC then in the calculator?). I say this because the battery compartment can be removed and charged separately with the same plug (but using only two pins from the adapter, assuming it uses same adapter), and there's only one diode in that whole compartment, everything else is wires. 

 

I have very limited knowledge with how circuits work so I might have missed something. I'm currently thinking of getting this battery charger and hoping one of the ends has holes that line up and fit. Then I'll get those diy batteries you can solder the metal tabs together. 

 

I've attached a highly technical picture with how I think the charging works, taken from continuity tests.

 

 

[Bombastinator]

1 hour ago, DarkEnergy said:

I acquired an old calculator that's unfortunately without a power supply. There's also no way I'll ever find a power supply for it. It's able to run off the wall and battery power which is through 6xAA 1.2V NiCD batteries soldered together creating 7.2V, aka not the easy pop out kind and they're long past their expiration. The original power supply outputs two voltages, 7.2V 225mA DC and 7.0V 45mA AC. The AC output is very confusing and doesn't make sense to me considering the batteries which the calculator can run off of supply DC, why not just output DC only? It plugs into a 3 pin port. My best guess is that it powers the calculator at the same time it's charging the batteries. 

 

My main question is if it makes more sense that the 7.2V DC is for charging the batteries and the 7.0V AC is for powering the calculator (assuming it converts it to DC then in the calculator?). I say this because the battery compartment can be removed and charged separately with the same plug (but using only two pins from the adapter, assuming it uses same adapter), and there's only one diode in that whole compartment, everything else is wires. 

 

I have very limited knowledge with how circuits work so I might have missed something. I'm currently thinking of getting this battery charger and hoping one of the ends has holes that line up and fit. Then I'll get those diy batteries you can solder the metal tabs together. 

 

I've attached a highly technical picture with how I think the charging works, taken from continuity tests.

 

 

Given the limited amount of information, (the make and model is not mentioned for example) I can only make some guesses.

There may have been originally a printer or screen or something that went along with it. Or depending on the readout type it may need the AC for the display. Iirc some Nixie tubes used AC for example. If you know the voltage and the ampherage though I would look at one of those replacement wall warts like radio shack used to sell (still might, I heard they’re back in a very small online way kinder a new owner and methodology) match the voltage and amperage to the DC and see if it works.  If it doesn’t the AC option may involve old fashioned doorbell transformers or even possibly wrapping something.  Not a project I would relish myself.

 

One thing to remember with recovered stuff though is there is always a reason it was put out of service.  With a calculator it very well could be simply that something better came along, but it could also be broken.

Edited August 7, 2022 by Bombastinator

[DarkEnergy]

50 minutes ago, Bombastinator said:

Given the limited amount of information, (the make and model is not mentioned for example) I can only make some guesses.

There may have been originally a printer or screen or something that went along with it. Or depending on the readout type it may need the AC for the display. Iirc some Nixie tubes used AC for example. If you know the voltage and the ampherage though I would look at one of those replacement wall warts like radio shack used to sell (still might, I heard they’re back in a very small online way kinder a new owner and methodology) match the voltage and amperage to the DC and see if it works.  If it doesn’t the AC option may involve old fashioned doorbell transformers or even possibly wrapping something.  Not a project I would relish myself.

 

One thing to remember with recovered stuff though is there is always a reason it was put out of service.  With a calculator it very well could be simply that something better came along, but it could also be broken.

It's a commodore minuteman 1. It does work, I rigged some batteries together and hooked it up. There's not much information about it online though and the PCB is sandwiched together with about ~10-15 pins so not easy to look at the components.

[Bombastinator]

44 minutes ago, DarkEnergy said:

It's a commodore minuteman 1. It does work, I rigged some batteries together and hooked it up. There's not much information about it online though and the PCB is sandwiched together with about ~10-15 pins so not easy to look at the components.

That isn’t one I’ve heard of, but the make and model should say just about it.  I can’t find much on the internet.  Commodore apparently made calculators before they made computers.  I can tell yo that the name “minuteman” was popular in the late 70’s early 80’s because of the bicentennial and the minuteman nuclear missile, both of which were heavily promoted. Might provide a time frame. This may be some short-lived bandwagon marketing thing.  Unfortunately that was when a lot of display technologies were being developed some of which fell by the wayside so that AC very well could go to the display.  Better part of valor may be to fake up a battery shaped DC thing and run it that way.  Batteries are just DC power of a given ampherage and voltage.  Work that out along with the space you need to fill and the connections you need to make (which may well be some odd wired thing with an early rechargeable) drill a hole in the battery cover for a cord, and run it like that.  It’s just not something I’ve heard of before though that doesn’t mean much. Commodore generally used a code system to name its products.  This breaks that.  It might actually have another name which is a code number but merely one that wasn’t used in marketing and the thing may be better known that way.

Edited August 7, 2022 by Bombastinator

[DarkEnergy]

11 hours ago, Bombastinator said:

That isn’t one I’ve heard of, but the make and model should say just about it.  I can’t find much on the internet.  Commodore apparently made calculators before they made computers.  I can tell yo that the name “minuteman” was popular in the late 70’s early 80’s because of the bicentennial and the minuteman nuclear missile, both of which were heavily promoted. Might provide a time frame. This may be some short-lived bandwagon marketing thing.  Unfortunately that was when a lot of display technologies were being developed some of which fell by the wayside so that AC very well could go to the display.  Better part of valor may be to fake up a battery shaped DC thing and run it that way.  Batteries are just DC power of a given ampherage and voltage.  Work that out along with the space you need to fill and the connections you need to make (which may well be some odd wired thing with an early rechargeable) drill a hole in the battery cover for a cord, and run it like that.  It’s just not something I’ve heard of before though that doesn’t mean much. Commodore generally used a code system to name its products.  This breaks that.  It might actually have another name which is a code number but merely one that wasn’t used in marketing and the thing may be better known that way.

I guess I'm trying to figure out if I can just use this type of charger for charging the batteries since the battery compartment can be detached and charged separately. There's only a single diode in the compartment which I'm trying to figure out if it was used as some sort of half wave rectifier (thus used the AC) or if I can use DC directly (from the amazon link charger) and not worry about the purpose of the diode. It's a charger for batteries so I'm assuming it would know when the batteries are full. My guess when looking at the design is that the diode is to prevent current from flowing from the battery to charger like when the adapter is plugged into the calculator but not plugged into a wall. 

 

Added a couple pictures to show how it's set up. Calculator can be used without battery compartment. The two pins I circled are for hooking up an adapter only, they don't connect to the calculator. The two plastic plugs on the bottom are the ones that connect to the calculator.

 

 

 

[Bombastinator]

I doubt the original batteries will take a charge anymore, but you can try I guess.

[DarkEnergy]

15 minutes ago, Bombastinator said:

I doubt the original batteries will take a charge anymore, but you can try I guess.

I'd replace them first. They've leaked and one of the bridges connecting two of them broke due to corrosion.

[mariushm]

NiCd batteries have a very simple charging mechanism ... if you trickle charge them with a low current, you can just put the voltage on the terminals and the batteries will charge up.

so you have 7.2V 225mA DC and 7.0V 45mA AC.

I suspect when you insert the plug, the battery is disconnected and the 7.2v  DC 225mA is connected directly to power the logic. 

 

As the battery is disconnected, it's reconnected to the 7.2v AC 45 mA  through that diode, which causes only half the AC output to reach the batteries.

So, half the time, there's no power going into the batteries, giving them time to "cool down" and the other half the time, the batteries get a "pulse" of up to 45mA of current to top up.

The original batteries were probably in the range of 500-600mAh, which makes 45mA just right, basically charging them at 1/10 of their mAh makes it fairly safe.

 

Modern NiCd batteries reach 1000mAh, for example if you want new batteries you would get something like this, 6$ for a pack of 4 1000mAh NiCd batteries : ULN4AASL-1000 Ultralast | Battery Products | DigiKey Marketplace

Or get 10 for $19, lower 700mAh batteries, but made by Panasonic : https://www.digikey.com/en/products/detail/panasonic-bsg/N-700AAC/1202958

 

If you don't care about keeping it as original as possible, you could replace the 6 AA batteries with a lithium battery and a step-up regulator to boost 3v...4.2v of the battery to 7.2v the calculator expects. 

Then, change the barrel jack connector of the calculator to a barrel jack connector that accepts 5v in, and use a basic single cell lithium battery charger to top up the battery.

 

Example of charger board  ( either use the usb connector or solder two wires in the DC in - and + pads, from the barrel jack connector) : https://www.ebay.com/itm/113789436590

Example of lithium battery (3.7v nominal , 850mAh, 48mm x 30mm - the AA battery is 50mm x 10mm so you have at least 50mm x 60mm space ) : https://www.digikey.com/en/products/detail/tinycircuits/ASR00036/9808768

example of step-up regulator (turn the potentiometer to reach 7.2v) : https://www.ebay.com/itm/284319989858

 

 

 

[DarkEnergy]

5 hours ago, mariushm said:

-snip-

The lithium battery with a step-up regulator is an interesting idea, It would certainly be the cheaper option. Since I'd need to remove the battery pack half of the calculator to charge it anyway (since there's nothing to depress the metal spring in the original charging port), I might use double sided tape to stick the flat lithium battery to the bottom and stick the step up converter on top of the battery as there should be room.

Is there a way to set it up to only need to plug the usb in to charge while leaving the rest wired up or would I need to get some jst connectors and when the battery needs charging, unplug it from the step up converter and plug it into the charger module and visa versa. I've never done circuit design. 

[mariushm]

I would not use connectors, if the battery comes with such connector I would cut it off and solder the wires directly to the charging board. 

 

It seems there's enough space where those 3 pins are that you could even install a usb type A connector there, but a micro USB or a usb type C connector would make more sense, as you can then use phone chargers or just plain usb type a - type c / microUSB cables to charge the calculator. 

 

If I had that calculator, I would look into heating up those 3 thick pins until they can be pulled out of the plastic and then maybe enlarge the hole (join the 3 pin holes together) to make it possible to have a USB connector in that area, where the 3 thick pins currently are. You could use hot glue or other techniques to lock the connector into place there. 

 

For example   microUSB connector on a tiny PCB which has mounting holes : https://www.ebay.com/itm/322763194358

You could literally make a channel just thin and wide enough to slide the pcb from the outside and then drill two holes in the plastic and use a couple screws to lock the pcb to the inside plastic, then maybe even use some hot glue to lock it in place 

 

Here's examples of usb type-c with tiny circuit boards to solder the wires to : https://www.ebay.com/itm/275173990222

This would be harder to lock in place in that compartment, I'm thinking lots of hot glue as the only possibility.

 

You could also use a barrel jack connector, but usb would make more sense because it kind of limits you to 5v. Cheap lithium charger chips can only handle up to around 5.5v-6v, so by using a USB connector you're sort of ensuring you're not gonna go over 5v.  With a barrel jack connector, you or someone could accidentally plug a 7.5v or 9v or 12v adapter into it, and you'd blow the charger chip. 

If this was a commercial project and barrel jack connector is a must, then I would have to add an extra linear regulator or a step-down dc-dc converter to convert whatever comes in, down to ~5v, to protect the charger chip, or I'd have to use a more expensive charger chip which has wide input voltage range.

 

 

Now last but not least... something I didn't mention previously.

The charger board I linked to above is just plain charger, it charges the battery by monitoring how much current goes into it and how the voltage gradually goes up to around 4.25v as the battery is charged. 

These kinds of basic chargers can get.. let's say "confused"... if you also take out power from the battery while the battery is charging, basically they won't damage the battery or anything really bad but it's not good for the battery over long term. So in general, with such basic chargers, it's not recommended to also use the device while charging the battery.

 

There are smarter, slightly more expensive charging chips which can act also act as a switch between battery and dc input. When input is detected, the chip disconnects the battery from the output and starts charging it, and at the same time is has a built in voltage regulator to send power to the output directly.  When input is disconnected, the chip connects the battery to the output. 

 

For example, here's such a smarter chip with board : https://www.ebay.com/itm/142668705028

but note that you don't need that super big capacitor... it's recommended to have a capacitor soldered to the circuit board in the footprint, but it can be a small 10uF 10v+ rated electrolytic which can be laid flat on the board and use 5mm or so of height.

 

So in the picture  below  PWR and GND are your input power  (going to Vbus and GND in usb connector) ,  BATT and GND goes to the lithium battery,  LOAD and GND goes to the step-up / boost dc-dc converter to produce that 7.2v DC the calculator needs.

 

 

 

 

 

[DarkEnergy]

11 hours ago, mariushm said:

For example, here's such a smarter chip with board : https://www.ebay.com/itm/142668705028

but note that you don't need that super big capacitor... it's recommended to have a capacitor soldered to the circuit board in the footprint, but it can be a small 10uF 10v+ rated electrolytic which can be laid flat on the board and use 5mm or so of height.

 

So in the picture  below  PWR and GND are your input power  (going to Vbus and GND in usb connector) ,  BATT and GND goes to the lithium battery,  LOAD and GND goes to the step-up / boost dc-dc converter to produce that 7.2v DC the calculator needs.

 

 

 

 

 

Yeah, I was wondering where the dc-dc converter would go in the original charging board you listed. I was looking at this board (below) that looks like the same idea as the one you just listed but it's all on one board. ebay link.

Or would that still not be considered a smart type board? I've got some 10uF 25V caps around somewhere. The calculator wouldn't be connected when charging though so it should be a non-issue. 

 

I don't want to mess with the pins at the moment, maybe a future project. 

Thanks for the reply. 

 

 

[mariushm]

No, the TP4056 chip doesn't have the  switch input to output and charge battery separately as the one I linked to does.

The extra chips in the picture in your last post are protection mosfets for barebones / raw batteries like a 18650 cell.

 

In the pouch batteries like the one I linked to in a previous post - https://www.digikey.com/en/products/detail/tinycircuits/ASR00036/9808768 - similar protection chips are hidden under the yellow kapton tape ... click link and check picture.

 

The board I linked to last uses the MCP73871 from Microchip which has an ideal diode and some other components inside the chip which allow the power to flow from input to output or from battery to output when dc in is disconnected ... so at the output you get either the input voltage (around 5v) or the battery voltage (3v..4.2v for a lithium cell)

 

See page 3 in datasheet, top left corner and top right corner : https://ww1.microchip.com/downloads/en/DeviceDoc/MCP73871-Data-Sheet-20002090E.pdf

 

They call this feature system load sharing.

This chip is around $1.7 if you buy at least 100, here's proof : https://www.digikey.com/en/products/detail/microchip-technology/MCP73871-2CCI-ML/1680971

 

Or you can buy a pack of 10 charging boards with that basic TP4056 and usb connector and those protection parts for the battery for $5.5 ... so the chip itself is probably under 50 cents.

 

 

[DarkEnergy]

21 hours ago, mariushm said:

-snip-

I think I found the 1-trick pony I need for this to work as cheaply and simply as possible. This board combines the basic TP4056 and dc step-up converter with a more compact potentiometer. I don't need load sharing features as there will never be a load connected while the battery is charging due to how I have to charge the battery. The battery will never be used and charged at the same time. I'm also going with this 1200mAh Adafruit battery (with protection) because it's a higher capacity, thinner, still fits with plenty of room, and by default the TP4056 board is set up with a 1.2k Rprog resistor which is a current setting meant for a 1000mAh battery. With an 850mAh battery I'd have to use a different resistor to prevent damage, and with a 1200mAh battery it will just charge a bit slower which is fine and probably for the best.