HP 82241A charger replacement polarity

[jebem]

(03-09-2018 04:13 PM)Dave Frederickson Wrote:  It's now apparent that both printers will accept either AC or DC.
Dave

And that makes sense, since I believe that most of these printers (if not all of them) have a full bridge rectifier where its input is connected to the printer panel power plug.

This bridge of 4 diodes converts external AC power to DC with negative ground, but also converts external DC on either polarity to the correct negative ground polarity.

Basically external DC is treated as external AC.
The only small difference is that AC is converted to DC but the AC voltage nominal value results in AC times sqrt(2) DC value under no load.
Because the bridge output is loaded and the smoothing capacitor have limited capacity, the real DC value will be lower with a certain amount of ripple.
Of course this ripple must not reach the printer electronics, and this is why the bridge output is connected to a 7806 6Volt regulator.

On both AC and DC external input power supplies, the bridge will, on average, take 1.4Volt (0.7Volt per diode, as only two diodes are in series at a given time).

So, if the external power adapter is a 10Volt DC type, the regulator will get around 10-1.4 = 8.6Volt, just enough to be able to operate under its normal conditions delivering 6Volt regulated output (the common 7806 type is not a low drop voltage regulator, so it requires a absolute minimum of 2Volt higher at its input pin).

The above is true only if the power adapter have insignificant internal resistance to be able to maintain those 10Volt DC under the required printer current load.
Otherwise, we will see a voltage drop at the input of the 7806 regulator, and if this value goes under 8Volt, the printer will not be able to work using that adapter alone, and if batteries were left inside, they will supply the required energy to let the printer to continue working.