Running on 3 rechargeables

tonymac

Hey Guys,
I know the datasheet says 3.6 is within the range the propeller is good for, I'm just wanting to know if any kind of hickup will cause failure at that voltage (like getting a transient on TTL being run at 5.5 V and having catastrophic failure). Otherwise I'm going to throw a diode in there and run on 2.9 V.

hippy

Be aware that three fully charged batteries may give a voltage greater than 3.6V.

Peter Jakacki

Catastrophic failure only occurs when there is sufficient current to fuse the mask or hotspot junctions etc. The absolute max on the chip itself is 4V but bear in mind that 3.9V is the "absolute max" of a 3.9V zener. Now, if you get the point about the diode you will take precautions on the I/O. The power for the chip itself should not exceed 4V or else the semiconductor substrate structures can breakdown and even if it's momentary they can go into latchup condition and self-destruct milliseconds later (haven't you read about magic smoke?).

I would avoid the series diode and simply use some form of shunt regulation which would clamp the supply. When the batteries are freshly charged their unloaded terminal voltage can be around 20% or so higher than normal so the shunt regulator quickly brings the batteries back to "normal" [noparse]:)[/noparse]

*Peter*

Gavin

Try a very low drop out linear regulator.

NiMH/NiCads wll run the prop down to 1.1volts/cell maybe even 1.0V each if you underclock the prop at 3.0volts.
Better solution is a single Lithium Ion/polymer, charged they are 4.2V and nearly flat at 3.3V.
Worried about batery life, add switchmode reg.

Gavin

deSilva

I tried both approaches mentioned above and encountered no problems till now:

(1) 3 NiCd cells (600mAh) in a small and very low cost "pack", in series with a 4001 diode. As I recharge often, the voltage at the batteries is generally around 4 V (starts with 4.2)

(2) 4 NiMH AA cells (2600mAh) which have become astonishingly low cost for some time! I reduce to 3.3 V with a LOD regulator. The main reason for this is that I need an exact voltage for the delta sigma circuit, and I use some devices that will run better "around 5 V" which are fed directly by the - somewhat random, but unrippled - raw batterie voltage.

Ale

TI has many buck-boost of pump.charge regulators, one of those can give you stable 3.3 V at good efficiencies (>75 %), require few external parts, a coil and two capacitors, sometimes a pair of resistors. Is worth looking

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=661&uiTemplateId=NODE_STRY_PGE_T

They are relatively cheap at 1 to 3 €

This is not advertisement. National has also many, and Freescale also.