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 Post subject: Scope clock mod
PostPosted: Sep Sat 21, 2019 6:16 pm 
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I have a dutchtronix scope clock board which has a RTC built in and used a 32.768KHz XTAL for the RTC. I swapped the XTAL with a DS32KHZ as it is more accurate.

I'm wanting to move away from a battery being used to keep the RTC and DS32KHZ operational when the clock is off.

Is it possible to use diodes to drop the voltage from 5Vdc to whatever the battery voltage was then use one of those super capacitors in place of the battery? I would of course add a resistor to reduce the initial charge current.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Sat 21, 2019 11:01 pm 
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It should be possible. I've used a couple of 1N4007's to drop a 5 volt logic supply voltage down to about 3.6 volts to power a bluetooth audio module that is normally powered by a lithium battery.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Sat 21, 2019 11:43 pm 
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What value of capacitor in farads should I use.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Sun 22, 2019 4:39 am 
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It depends on:
- how long a power interruption you need to survive;
- how much current the capacitor has to supply (both to the RTC chip plus other drains on the capacitor);
- how low the RTC supply voltage can go before the RTC drops out.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Sun 22, 2019 3:09 pm 
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The clock would only need maybe a day or two.

Current i believe is microamps.

Not sure how low the chips can be before they stop working.

Think the rtc can go lower than the DS32KHZ.

That said I think perhaps it was a fluke that the time reset but the date didn't and maybe the battery is fine.

I'll test the battery to be sure.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Mon 23, 2019 5:34 am 
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If you need to ride out a 2 day power outage, then I suggest you stick with the battery.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Mon 23, 2019 12:04 pm 
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Figured that would be the case.

I do need to measure the battery voltage though to see what it is at before trying anything else.

If the battery was drained enough I could always use a higher capacity battery.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Tue 24, 2019 4:25 am 
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It would be interesting to put a ammeter on the battery to see how much current is being drawn by the RTC. Sometimes these things can be out of spec and have high leakage current. I've seen this a couple of times. One where the backup battery in a clock radio would go dead in a couple of hours if the power failure lasted that long. Another in a smoke detector that would suck batteries dead in a couple of days.


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 Post subject: Re: Scope clock mod
PostPosted: Sep Tue 24, 2019 4:42 am 
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Now had the battery been dead I suppose the clock and date would be at their default values.


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