If I'll get an old HP calculator that has no working charger anymore, can I in that case use a new NON-HP charger that has the right voltage?
chargers for HP calculators...
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01-09-2002, 03:17 PM
01-09-2002, 03:34 PM
I have dealt with a place called International Calculator website www.internationalcalculator.com. They sell no longer made HP calculators (including the HP41C/CV/CX) and accessories such as battery chargers. I have purchased a working HP48SX, cards for it, and soft calculator cases from its manager, Don O'Rourke. He was very helpful. Also try Ebay www.ebay.com. I joined Ebay about two months ago and bid on a calculator book and later on an HP calculator, both of which I now have. Good luck!
01-09-2002, 05:01 PM
Volts is volts... if you can get the proper voltage (mayby with a sutible current limiter) on the proper pins then it will work. This is easier said than done for some machines. Ebay or the HPMUSEUM classified ads are your best bets.
01-09-2002, 05:58 PM
Not easily. The output voltage is not the whole story by any means, and HP chargers rarely give the specified voltage anyway.
01-10-2002, 11:58 AM
Hello everyone, Just some thoughts prompted by Tony. I don't have access to any of these old calc's but I would imagine that it would be possible for anyone familiar with electronics to fathom the system used. If the charger/power supply is doing the rectification and regulation, then a similar unit could be built. If a working system is available for 'probing' then that would be even better. If the charger/power supply has no obvious rectification or regulation, then these must be on the calc's circuit board and again it would be possible to identify them. Also look for some form of cooling fin.
Go carefully with that multimeter... David
01-10-2002, 02:03 PM
Many of the HP chargers have been 'fathomed out' as you put it. The circuitry is often very simple, but it does depend on the characteristics of things like the transformer.
01-10-2002, 03:42 PM
Tony, regarding the simpler AC adapters, you have enlightened me in earlier threads about the current limiting function of the resistance of the transformer secondary winding. If one were trying to substitute another transformer with the same open-circuit output voltage, and added an additional resistance in series so that the total resistance was the same as the original transformer secondary, could similar performance and protection of the calculator be expected? Also, I understand that a transformer must be considered to transform resistance (or impedance) as well as voltage and current. This suggests the resistance or impedance of the primary winding would also act to limit current. Would adding a resistance in series with the primary as well as the secondary allow a transformer with the same open-circuit output voltage behave like another transformer (assuming the substitute had less resistance in both windings than the original)? Can this be generalized so that, for instance, a table of transformer winding DC resistances could be published which could be used to calculate series resistances to add to the measured resistance of a substitute transformer's windings? In any case, I think the biggest problem with rigging up a substitute AC adapter is the varied and non-standard connectors HP used.
01-10-2002, 04:41 PM
My guess is that you could use a normal transformer with a suitable resistor in series. I'd put it in series with the low-voltage secondary winding, but it should be possible to use one in series
01-10-2002, 06:23 PM
I just read your earlier post re: HP67 charger. That's putting a 3421A to good use! I've got a 3421A with the 10 channel mux. I know I can use two of the channels as external switches. I've planned to do something similar to what you describe to experiment with rechargable batteries: charge them while monitoring voltage and current, then connect a load and monitor voltage and current over time to determine charge capacity. If you use the mux actuator channels to switch load currents, what precautions do you use to protect the relays on the mux board? I know they are rated 2A max but I think I want to use external relays so the mux will only have to switch the relay coil current. Recently I got a digital IO board for the 3421A (haven't tested it yet) which will probably be better to control the loads with external relays.
01-10-2002, 06:35 PM
I didn't use the 'Actuator channels' to switch loads -- like you I was worried about damaging the relays. My 3421 has a 10 channel mux card and a digital I/O card in it.
01-10-2002, 08:43 PM
Thanks for the tips! I like the idea of measuring the resistors instead of using precision ones, although I might use one low resistance precision resistor in series with the variable load to monitor the current with another voltmeter channel. I'm curious to know if you have anything to say about the 3421A voltmeter drifting up (I started another thread about this).
01-11-2002, 06:33 PM
If you know the value of a resistor (which we've just agreed you can measure) and the voltage across it (the terminal voltage of the AC adapter) then you can
01-13-2002, 10:53 PM
I was going to say "because the resistance measurements are less accurate than the voltage measurements" but I thought I should check the specs and make sure. Now I wonder how the resistance measurement can be more accurate than the voltage considering the voltmeter function is involved in making the resistance measurement? The specs say the ohmmeter (on the lower ranges) is a little better than the voltmeter, except between 90 days and one year after calibration. The voltmeter is better on the 3V range than the 0.3V range, but even then the ohmmeter is equal to the voltmeter in accuracy. This seems to suggest that the current source used for the resistance measurement is perfect! Could this be like Microsoft giving the Office people better info on the Windows API than outside developers? Did HP give the ohmmeter function a better voltmeter that it gave to the user via the front panel? |
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