[time-nuts] Oncore question

[Arnold Tibus]

Hello Morris, 
I am not aware what is the voltage range for the backup battery in the GPS-RX, 
but using a super cap or gold cap instead of a (rechargeable) battery
should be ok for short backup times, Synergy does even propose the use 
of a SuperCap or UltraCapacitor as a backup power source for certain op. conditions.

What is the main difference to the use of batteries?
In capacitors, the voltage does directly follow the charging current amount 
but batteries do hold the voltage in a certain area quite stable. Therfore 
a battery with the same charge is more efficient but need long charging time 
and the special capacitors can be charged very quick.

After your question I did want to know myself for what time these caps would serve.
My result:
For a backup current of 25 µA and allowing a voltage drop during the backup 
of 0.5 V, the data will be safe for around 5 to 6 hours,

With only 5 µA backup current the time will be 5 times higher (abt. 27 h) 
not counting for leakage or other discharge factors. 

The very simple numerical assessment:

C = 1 F , U = 5 V (max. 5.5 V in gold caps?)
1 F charged to 5 V : 
Q = 5 As,  
U = I * T / C

Voltage drop per second for C = 1 F loaded to 5 V
U/s = 5 E - 6 * A * 5 V/ 1 A = 25 µV

Time for 500 mV drop down (from 5 V down to 4.5 V )
at a load current of 25 µA :
500 *10 ^-3 Vs / 25*10^-6 V = 20,000 s = 5 h 33 min

or:

Q = 5 V F = 5 V * 1 As / V = 5 As = 5 *10^6 µAs = 5.000.000 µAs
10 % of 5 V   => 500,000 µAs
A load of   5 µA gives 100,000 s = 27 3/4 h
A load of 25 µA gives   20,000 s =   5 1/2 h

Such big capacitors may be charged within parts of a second, if wanted, 
but don't forget the Voltage Limit!

Good luck and regards
Arnold, DK2WT


On Sat, 22 Nov 2008 18:52:22 +1100, Morris Odell wrote:

>Hi Fellow Nuts,

>I've been following the thread on parsing the Motorola receiver code with
> great interest. I have written AVR code to do this but have not gone to
> great trouble making it bullet proof as it was a non critical application 
>in
> a Nixie clock. I programmed the receiver to send only one message, and then
> looked for the @@Ba string before I started counting bytes. I used the
> number of SVs byte to decide whether the data was good. Worked fine for me!

> I'm about to start another project with an Oncore so maybe I'll experiment
> with a more complex program this time.

> My question relates to the older Oncore VP receivers. I have one which had 
>a
> dead memory battery. The receiver has an on-board battery charger for a
> rechargable Li cell but they are very hard to find down here. In the past
> I've used a conventional Li cell with a schottky diode to prevent the
> charger from forcing current down it's throat. I was wondering if there's
> any reason not to use a super cap. I only need it to support the memory for
> very short periods during power burps. Will the on-board charger be OK for 
>a 1F
> super cap?

>Any help or opinons much appreciated

>Thanks,

>Morris 


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