Mindboards

There were references in an earlier thread about using high voltage batteries with the NXT about a 10.5V limit with the H-Bridges which drive the motors (can't find the thread).

I wondered if that limit is based on the maximum voltage applied to the NXT circuitry or only the voltage supplied by the motor commands (which seems likely).

If I used higher voltages but restricted the power % so that less than 10.5V was sent, would that work or would they get damaged anyway?

The reason I ask is that I was thinking of using more than 8 Ni-MH batteries to bump up the power level available.

I have 8-cell battery holders which give about 11.25V with 8 fresh Fujicells.

Oh, I'd be interested in knowing this as well. I bought some solar panels, which might give of some more power in sumer than they do now

At Dexter industries they say you shouldn't use more than 3 of theirs in parallel, so I wonder if there is an upper bound in amps as well?

BTW, the page you linked to in your other thread also mentions 10.5V: http://www.philohome.com/nxtsupply/nxtsupply.htm

10.5v is the absolute limit according to the datasheets of the motor drivers.

When you specify a certain % for the motor speed, it is not controlling the voltage. It is controlling the duty cycle of the PWM.

PWM (Pulse Width Modulation) is a method of speed or intensity control, used for things that need a certain voltage when powered (like LEDs and motors). Think of a time-slice (for example 0.1 seconds). Divide that into 100 segments (1ms intervals). Now, at 100 % power, the PWM output will be on for all 100ms of every time slice. At 75% power, the PWM will be on for 75ms, and off for 25ms. At 10% power, the PWM controller will be on for 10ms, and off for 90ms. In theory, it allows you to control the motor speed without effecting the torque (although in reality this isn't totally the case). Another advantage to this is that the H-Bridge transistors are fully on or fully off (not half way on, dissipating 50% of the energy as heat).

You don't normally need to be concerned about a "limit" of parallel supply. The only possible danger is that with an unregulated supply, the actual voltage could be significantly higher than rated. Using multiple of such supplies in parallel allows them to share the load, and raise peak voltage.

There is no reason to worry about the amps available being too high. It's like with a water tower vs. a well with a pressure tank. Both of them supply maybe 15-30 PSI of pressure, but the water tower would be able to give a lot more water at once, in a high demand situation. It's like that with amps, if you have 10 packs of 6 series D cells in parallel, you would still have only about 9v, but you also have enough current at your disposal that you could probably run more than 40 NXT motors at full strength.

OK, thanks.

If I use the alternative supply I will stick to 7 cells which gives just under 10V (fresh batteries).

I would be very careful with the 7 batteries thing. I have seen an NXT go "poof" with 7 freshly charged NiMH batteries. They give off considerable more than the 1.2V when they're fully charged. LEGO replaced the brick, thankfully.

- Xander

Thanks Mightor, I measured a set of 8 new Fujicell Ni-MH batteries and the average was 1.42 v straight off the charger.

This gives 9.94 v total (no load) for 7 batteries and 6 fresh Duracells at 1.62 v gives 9.72 v.

There is a loss of about 0.2 v from resistance inside the carrier which puts them almost equal.

I believe that the newer types of Ni-MH batteries might give higher voltages but I will stick with my Fuji's so should be OK.