verifying voltage of wall chargers--all types (and Olivetti Logos 9)

[Duane Hess]

Wasn't sure of posting here or in not remotely HP calculators. My question specifically involves a non-HP product. But, in general would apply to all vendors. Please reassign category location if needed.

What should one expect when measuring the voltage levels stated on the label of a A/C adapter versus actual results? The label has an output voltage and type (DC) and a milli-amp rating. I unplug the charger from the calculator then measure with a volt-meter (i.e. unloaded charger).

My understanding is loaded (attached) chargers may read a different voltage than on the label as the charger is unloaded. Granted that is dependent on the charge circuit construction. However, it always seemed measured voltages was "reasonably" close to the label when unloaded.

Is there anyone with a background in charger circuits who could comment. i.e. does unloaded values vary from the label by ... 10%?, 20%?.

How about 50%? This is the situation I am in and seems a bit much.

I have several Olivetti Logos 9 hand-held calculators and the USA version of the power pack:
DC-PACK Charger
Model QBK-930A1
Input: AC 120V 60 Hz 4W
Output: DC 9.6V 30 mA
positive tip
(I am in the USA)

The charger was used before (over a year ago?) and charged the Logos 9 attempted. Have no recollection whether voltage values were measured. In the last few weeks, tried using it again, neither unit charged. Measuring the output voltage produced about 4.6V-DC (unloaded).

Granted both units have the original NiCAD batteries. Could bad batteries damage the charger? It did warm a bit, but didn't seem warmer than any other charger I ever used. The calculators did not heat.

I obtained another charger, used of course. Right out of the box it measured the same (more or less) 4.5V-DC unloaded.

At this point I assume both chargers do not work correctly. Less than 1/2 the stated value seems a bit much. Is it worth fabricating a test cable so under load voltage can be measured (attached to calc)? Personally I assume not.

The units are sealed, so opening one is not overly feasable. Both adapters are the same model, vary from label values by same amount, same age. Seems reasonable should charger model X fail due to age, likely another model X may fail in the same way.

I have ordered a generic voltage selectable charger. Hopefully the tips fit. Hasn't arrived, but thought would ask for comments before using it.

Anyone have any comments/suggestions concerning verifying voltage levels of loaded/unloaded chargers? Or any other suggestions?

[dxaren]

Charger output is commonly stated at nominal load condition. The voltage is usually within +/-5% at nominal load.

If the charger is a simple transformer (often combined with a rectifier) the no-load voltage can be up to 10-30 % higher. This might be a calc-killer (!) if no batteries are in place. The same goes for open-circuit batteries! Beware of old batteries!

If the no-load voltage is lower (10% or more) than the spec label, the charger is damaged!

Batteries might get internal shorts (more or less severe). This may drain such a load, that the charger output might drop to zero!. If the charger is designed properly it will survive due to its internal cut-off circuits. It might become warm, but not critically.
However, badly designed charger might be damaged and if so, usually sends out lower voltage, that drops drastically under full load.

[John Keith]

Actually 50% over the nameplate voltage is not uncommon, especially for low-current chargers. Not usually a problem but as stated above, beware of power supply circuits that rely on batteries to limit voltage.

[Hiwi]

Hi,
i have a full working adapter and with no load i measure about 15V, with load 11V.
My nicd battery was dead and i replaced with NIMh cells with 250mA (original 110mA).
The logos 9 has a special charging circuit which limits the current to around 10mA and cuts off at somewhat 9,4V - 9.8V.

I think the charging characteristics Nicd vs. NimH differ a bit, so i could not fully charge the new battery. The cut off is to early to get the battery full. I tried with a diy circuit (LM317) regulated to 11,6 V and later 12,2 V, this works better, but current is limited to 10mA.
So it takes 3 days to recharge the battery!

I have to make further investigations on the charge circuit and modify it.

On my other Logos 9 i have resolderd the battery connections so that the charger socket is directly connected to the battery. I had to add a shottky diode, because the plug did a short when plugging in. Now it works correct.

The new battery lasts longer, i think twice the time.

My Logos 9 has an standby current of 900uA so it is empty after 8-10 days.

I had some other Logos 9 for repair, they always had none working clocks. In most cases the crystal had to be replaced. And often the soldering of the smd parts were weak.
I replace the two electrolyt capacitors in wise foresight :-)

Ralf

[Jeff_Birt]

I did a video about this a while ago and did some experiments to show the difference between a regulated and unregulated supply. I also tried to provide tips on selecting the correct wal-wart for a piece of vintage gear, etc.

https://youtu.be/f2ShjK9uq9Y

[Hiwi]

Hi Jeff,
great video, one thing when updating nicd to nimh cells in vintage gear is to bear in mind the internal resistance, nimh have more than double of internal resistance in comparison to nicd.
Eneloop cells three or four times.

However some computers have buildin charger circuits, so it make no difference whether regulated or unregulated powersupply is used.
Ralf