Science Buddies: "Ask an Expert"

Hopefully a simple question...

My son is in grade 11, and by way of starter project and a problem solving exercise we are attempting to fix an inflatable bed electric air pump. Upon disassembly I have noticed a few things I didn't expect and can't readily explain to him:

(1) The unit is recharged using a 12V AC/DC adapter, however the NiCad battery pack in 9xAA batteries in series ie. 10.8V.
1a. Does this mean the duty cycle for these batteries allows 12V charging? (the circuit is very basic no sign of any resistors only hook-up wires essentially.
1b. Would the battery pack show an open circuit voltage of around 12V anyway when fully charged? (1.2V rated Nicads usually show a 100% Voltage of 1.3 - 1.4 on a battery tester in my experience!)

(2) Manufacturer direction say to "not use the pump when connected to the AC/DC adapter OR when charging, as damage may occur. Why would this be the case? ie. why can't I run the unit while the charger is connected instead of having to wait for a full charge cycle (apart from the design not functionally allowing this). I read an awesome post in this forum on a DC motor project including reference to DC motor data sheets, duty cycles, 100% duty rated designs etc...

Is it likely that the unit is not a 100% duty cycle motor and that part of the 'fail-safe' is that the NiCad battery pack will be consumed long before the motor overheats and that if the unit was to run off the AC/DC adapter then the user could easily overheat the unit or is there more to it?

(3) I notice a small electrolytic capacitor (100uF, 35V) across the poles of the motor, what is the purpose of this capacitor and is this why the unit cycles (ie. the rotor vibrates back and forwards rapidly rather than rotating), when I press the on switch while the unit is charging? Or does the AC/DC adapter Jack physically change the circuit when the plug is inserted like an audio jack isolates the speakers when you plugin headphones?

Hope this is the appropriate place for this post, while it is not yet a formal project the thinking involved and the ability to interpret the circuit configuration and the manufacturer's instruction in terms of the underlying electronics is a great start for us

Tx

Xman01 - I'll try to address your questions as asked. By the way, good observations on many of the motor/charging issues.

(1) The unit is recharged using a 12V AC/DC adapter, however the NiCad battery pack in 9xAA batteries in series ie. 10.8V.
1a. Does this mean the duty cycle for these batteries allows 12V charging? (the circuit is very basic no sign of any resistors only hook-up wires essentially. Reply - The nominal terminal voltage of the battery pack may be 10.8 V (9 x 1.2), but as you observed, fully charged NiCad cells may show 1.3 to 1.4 volts per cell, without load. In order to charge, the charging voltage has to be greater than the NiCad cells, so a 12 volt charging unit is appropriate. I would guess that if you measure the output voltage of your 12 volt charger, it might even be 14-15 volts without a load.[/color]

1b. Would the battery pack show an open circuit voltage of around 12V anyway when fully charged? (1.2V rated Nicads usually show a 100% Voltage of 1.3 - 1.4 on a battery tester in my experience!) Reply - Your battery pack, assuming the cells are in good condition, will probably be over 12 volts as you commented in 1a.

(2) Manufacturer direction say to "not use the pump when connected to the AC/DC adapter OR when charging, as damage may occur. Why would this be the case? ie. why can't I run the unit while the charger is connected instead of having to wait for a full charge cycle (apart from the design not functionally allowing this). I read an awesome post in this forum on a DC motor project including reference to DC motor data sheets, duty cycles, 100% duty rated designs etc... Reply - The motor current is probably significantly higher than the charger output. The NiCad batteries likely have a healthy output current also higher than the charger, so when the motor is running, the charger may see a current demand higher than it's capability and may overheat. Therefore they want it disconnected. If the charger was connected to the motor, but not plugged into the wall, it would not be a problem.

Is it likely that the unit is not a 100% duty cycle motor and that part of the 'fail-safe' is that the NiCad battery pack will be consumed long before the motor overheats and that if the unit was to run off the AC/DC adapter then the user could easily overheat the unit or is there more to it? Reply - Whether or not the motor is rated for 100% duty cycle (probably in a consumer product you describe it is not a 100% duty cycle motor), the problem still is as I mentioned above. The charger is now subjected to a current higher than it's design specs and will start to overheat, especially as the battery terminal voltage declines as capacity is used.

(3) I notice a small electrolytic capacitor (100uF, 35V) across the poles of the motor, what is the purpose of this capacitor and is this why the unit cycles (ie. the rotor vibrates back and forwards rapidly rather than rotating), when I press the on switch while the unit is charging? Or does the AC/DC adapter Jack physically change the circuit when the plug is inserted like an audio jack isolates the speakers when you plugin headphones? Reply - Interesting observation. The capacitor is most likely there to suppress electrical noise from sparking of the brushes and commutator. It is permanently connected across the brushes. It is weird that the motor doesn't rotate while charging and you turn the switch on. That is fascinating. In any case, they don't want you to turn the motor on while charging anyway, only using the NiCad power source. Strange design bug.

Rick Marz

Hope this is the appropriate place for this post, while it is not yet a formal project the thinking involved and the ability to interpret the circuit configuration and the manufacturer's instruction in terms of the underlying electronics is a great start for us