Propeller activity board voltage

Zap-o

I just received my propeller activity board and know the pcb states 6-9 Volts, however; I have limited resources and looking over the schematic I see no reason why I could not supply up to 20 Volts. Could someone point out an issue if any before I supply 13.2 Volts to the Propeller activity board.

Always thanks in advance.

Mike Green

Main issue is heat dissipation. Any voltage over 5V has to be dissipated as heat (times the current drawn). Also the 3.3V regulator dissipates as heat the difference between 5V and 3.3V (times the current drawn). With 13.2V supplied, that's about 10V. With just the Activity Board and one or two cogs in use, that's not a lot of heat. The Propeller with several cogs running full speed plus the EEPROM and ADC and a few LEDs might draw 60-80mA. If you add a WiFly module, that might add another 40mA. That's somewhere on the order of a Watt of heat. The regulators have no external heat sink except for the PC board itself. They're going to get hot and might shut down. If you add other current loads like a servo motor or a few LEDs, the heat level will rise further and faster.

So yes, you can run the Activity Board off 13.2V, but you will be limited in what you can do with it due to heat. I have some old power cubes with voltages from 6 to 9V and current ratings from 600mA to 1A that I keep for this sort of use. People throw them out all the time. Check garage sales too.

Zap-o

Mike

Awesome - thanks for that tip. I forgot about the "heat" detail. Ill look into gluing a heatsink on to it and a fan if needed. Otherwise sounds great. Again thanks.

Zap-o

Following up on this,

I once again went over the schematic and found no reason 13.2V would not work so with Mike Green in mind I went ahead.

The Activity board woks perfectly and the voltage regulator never appeared hot to the touch. I have a heat sink ready to glue on said IC, but have not needed it yet.

I am operating 2 servos and as many cogs as one imagines in a day programming the Activity board, sounds etc. nothing abnormal accrued.

Mike Green

Yep. The Propeller and its boards are all pretty robust and you can get away with a lot of abuse. Servos draw a lot of current, particularly under load. I assume you're running the servos using the regulated 5V for power. Setting the servo header jumpers for Vin will, in your case, provide 13.2V to the servos which will cause arcing in the servo motor brush assembly and shorten the life of the servos markedly. Setting the jumpers to Vdd will use 5V for the motors avoiding the arcing, but increasing the current drawn through the 5V regulator.

Zap-o

Mike

Thats a good point I am using the servos at 13.2V for now as this is just a proof of concept design I am having fun with. I have the real servos still in a box and they will run from a PCB I design after this all works out. I did run the servos on 5V but I was afraid the load would be excessive as the voltage regulator reset the code a few times.

Mike Green

Figure on a peak current in excess of 1A per servo with peaks occurring when the servo is first beginning to move, particularly under some kind of mechanical load. Near stall situations can cause peak current draws for a prolonged period. Many regulators have a maximum current draw of 1A, some have a limit of 500-600mA. Above this they start to reduce the output voltage in order to limit the current draw. The regulator's datasheet will have details. If you have a large electrolytic capacitor across the servo power connections (1000's of uF), that will help the first situation (brief peaks), but not the second one (prolonged overload).

A 13.2V supply will definitely cause arcing ... thus pitting of the brushes. Use some kind of external regulator to reduce the voltage to the servos.