Simple 3v power supply for Prop

william chan

Hi,

1. Can I use 3 IN4002 diode drops to get 3v from a 5v regulated supply to power the propeller?

2. Does the Propeller have built in clamping diodes like in the SX?
How many milli-amps can the clamping diodes handle?
If it does, then I should be able to read RS232 data from a PC just by having a 5K resistor in series to lower the current flow right?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.fd.com.my
www.mercedes.com.my

viskr

You really want 3.3V, but I still wouldn't recommend using 2 diodes, as the voltage will vary with load. There are a lot of 3 terminal regulators available in TO-92 which can easily do the supply. Radio Shack may even carry them. LT1211, LP2950 and others

Its not a good idea to rely on the internal clamps, as if you overdrive them you've just destroyed the prop. Use a bigger resistor and those diodes you have lying around to clamp 3.3V and GND (remember the signal out of the PC is inverted)

VIRAND

I'm running a Propeller on a pair of alkaline batteries with good results so far.
Two AA or D size cells work well for me for graphics and sound demos.
When the batteries get too low then the prop starts repeatedly rebooting.

deSilva

I made extensive tests with these things, and there are some threads from others as well. My results are:

(1) 3V is the barest minimum for the Propeller; when runnning from 2 alkalines (which start at 1.65 V per cell) you will hardly tap their capacity - look for further use of them in any case!

(2) 3 NiCd or NiMH rechargeables (or one Lithium cell) are absolutely fine IF YOU charge them only upto 3.7 (at most 4) volts. This can be regulated with many chargers. When you are not so sure about the max voltage use a 1A Schottky diode in series.

(3) You need a truely regulated supply voltage in rare cases only..

(4) ESD-diodes are generally rated for 100µA, but will not die immediately when pulsed with 1mA for a short time. That means for say 15 volts a resistor around 100 k. 5k is definitely NOT good..

(5) 2x4001 will work of course, but they will show a somewhat high minimal load voltage.. It might be a good idea to add a 100 Ohms load - an LED indicator, drawing 10 mA will do the same. 3x4001 will reduce the voltage too much at higher current.

Note that the dissipation with linear regulators is not better than with such diodes...

Post Edited (deSilva) : 2/18/2008 9:35:05 PM GMT

william chan

DeSilva,

4. The curious thing is that whenever I use a 10k resistor with a SX, I had difficulties reading data from some notebooks.
PCs are no problem. When I switch to 2K, all PCs and notebooks work fine.
Based on your estimation of 100uA to 1mA, the SX clamping diodes would have died long ago in many of my projects, but my experience shows that they are still very much alive.

What gives?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.fd.com.my
www.mercedes.com.my

deSilva

I did not say they MUST die. When the notebook outputs do not produce more then 9 Volts and are current limited to 10mA, which means an output resistance of 1k and you use a 2.2K series resistor there will be 9 - 5.7 = 3.3V/ 3.2k = 1 mA. This is not good, but acceptable. As -9 Volts comes in pulses only, this is worse but not necessarily deadly.
100k is needed for the worst case scenario of +/- 15 Volts of higher current agains the Propeller's 3.3V

10k contributes to the low pass filter of the around 10p input capacity to RC = 100 ns. There should be no bad effect below 1 Mbps; however a 100k will influence the signals above 100 kbps.

I don't know why it had not worked with your 10k resistors.