Battery backup write cache

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Battery backup write cache

Postby Jimster » Wed May 27, 2009 3:25 pm

We have had a few times where the power has been turned off to the DL1 and saved data has been lost. It would be handy if RT (or one of you guys here who are clued up on electronics) made a little circuit which either contained a battery or large capacitor, so when power was pulled from the DL1, the logger was stopped and it could keep the DL1 going just long enough for the DL1 to save the data.

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Postby grendel » Mon Jun 15, 2009 2:32 pm

That would be nice indeed.
I have noticed as this has happened to me several times that data isn't lost. Always, when there is a problem I can find the data in a folder 1.1.2004. Just need to find which is which. This I can do from the GPS time in the file.
Hope this helps.

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Location: Bridgend, South Wales

Postby Jimster » Mon Jun 15, 2009 2:45 pm

I can write all I know about electronics on a postage stamp, but surley this wouldn't be hard to make an external device to do.

I was thinking of maybe using a relay to start the device, so when power is given to the DL1 this also gets powered,
then if power gets lost, it powers it's self and the DL1 from a 9v battery. If the DL1 logging output is enabled then trigger logger, wait 5 seconds, then shutdown device and DL1,

So in logic, once the device has had power, it will be triggered by the loss of external power.

If logging output of DL1 = false then
turn off device and DL1
Trigger logging
wait 5 secs
turn off deveice and DL1

Have I oversimplified it a bit? I would have though this would be 10 mins work for one of you clever bods.

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Postby Turtle » Mon Jun 15, 2009 4:01 pm

It's not quite as simple as you think.

There is a thread here where I asked a few questions:

I did end up building a unit with a pair of quite large capacitors, plus fuse and diode (diode is needed so when the power cuts off the capacitors power the DL1 rather than trying to power the car...).

It works, but occasionally the DL1 doesn't power on correctly. This is likely due to voltage drop while the capacitors charge initially - but I haven't looked into it in detail.

I will be looking into this again at some point, but as I wanted a reliable solution I ended up just using an external switch/logging status LED. Then if the logging is messed up, blame the driver...


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Postby faraday » Tue Jun 16, 2009 2:19 am

Jimster, I think I understand your intention, but do not understand some of your logic.
Before discussing logic, what do you mean by "trigger logger", and then "Trigger logging"?

So, to the logic...

My first trouble is what is the starting point of the logic?

Your first line looks at the logging output - I assume you mean the logging status.

If you are driving and logging, what is the device doing? If you've been on for longer than five seconds, the device will have turned itself off. Will it somehow turn off the otherwise powered DL1?

Let's assume that the DL1 stays on. It is still logging data. This is good. The device is off. This might be bad.
Richard Cranium, the driver, has decided to come in. He does so and stops. He turns off the main power, thus turning off the DL1 and threatening the data.
What can your device do? It has already turned itself off.
Lets say it is still on, because it understands your thoughts. I hope it's concentrating and not fantasizing or laughing about sheep and bigguns.
The driver has killed the power, which I assume kills the status output as well.
Thus, the device turns itself off. Too much turning off :( I want to see a turn on 8)

:? I'm confused. Too much fantasizing :oops:

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Another interpretation

Postby pault » Tue Jun 16, 2009 7:31 pm

I think the thought was to have a device in the power line that will hold power for enough time to allow the unit to close the files on the CF card.

This can be accomplished externally (given that you can get a capacitor big enough). You connect a diode from the car power to the capacitor to the power of the DL-1. You also connect the car power to an input on the DL-1. You have setup the DL-1 to stop logging when the power to the input goes away.
The keys to the size of the capacitor are:
1. How much current does the DL-1 take? (I in amps)
2. How long does the DL-1 need to close out the files? (T in seconds)
3. How much droop on the power line can the DL-1 tolerate. (V in volts)

The capacitance (C) of the capacitor is then C=(I * T)/V.


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Postby Turby » Thu Jun 18, 2009 1:05 pm

Better still, how long does the DL1 need to close the current file safely if it is logging ? (ie stop writing and simply close the file)

... which then leads on to the DL1 monitoring the power supply istelf and close the current log when voltage dips sufficiently.

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