Slow power supply voltage rise
william chan
Posts: 1,326
Let's say I use a small solar panel to charge up a 1 Farad Super Cap which also powers a SX running at 4Mhz.
1. During night time, when the SuperCap voltage falls below 2.8 volts, and the SX is held in reset by brownout, how much current does the SX consume in this state?
2. When the SuperCap voltage slowly rises as it is being charged by the Solar Panel, will the SX start-up correctly all the time?
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1. During night time, when the SuperCap voltage falls below 2.8 volts, and the SX is held in reset by brownout, how much current does the SX consume in this state?
2. When the SuperCap voltage slowly rises as it is being charged by the Solar Panel, will the SX start-up correctly all the time?
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www.fd.com.my
www.mercedes.com.my
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In the process of testing, I am also using a second SX to record light, voltuage and current to/from the solar panel, the super cap (or NiMh) and tech tool.
I have had trouble finding super caps above 5.5 volts. Where are you getting yours? Here is one work-around: http://www.hackaday.com/2005/12/06/supercap-9v-battery/· There is a product called the Capettery at 11 volts from Evans, but cost about $400.
In theory 5.5 volts might work, but Tech tool wants 6 volt input (I think becuase 1 volt is lost in the regulator). With a 5.5 cap the discharge drops to 5.0 volts quickly and then you brown out. So you can desolder volt reg on a sx prototype board and replace with 3.3 volts. BUt in my system that leaves me with a problem running my 5 volt EEPROM chip for data storage.
I think with more time effort & all this can be solved. I'm curious about your experience.
In the end it would be great to develop for the community a complete solar-powered SX system that can be built on a protoboard.
The current an SX consumes in the Reset state depends on a number of things, the biggest one is voltage. Lowering the applied voltage dramatiacally drops the current consumed.
That said, please realize that in Reset the clock continues to run and consume power. To the best of my recollection, this can be in the order of 350 uA, again very voltage dependent.
If the device is put in sleep mode, then the current consumption can be reduced drastically; my experience is that it drops to less than one micro amp, but it is temperature dependent. On wake-from-sleep, you do need to contend with the rather unpredictable DRT timer issue. In the case of an SX48/52 (only) you can combat that somewhat if you opt to keep the oscillator running during sleep, but at a current penalty.
As far as certainty to start when the voltage level rises more slowly than then specified requirements, I can't answer for certain..... need to do some experimenting.
Cheers,
Peter (pjv)
Thanks a million for your help.
John,
For such solar / supercap systems, I normally would not use any regulators.
If you use regulators, you may need a bigger solar panel.
Just a 5.5v zener to prevent the supercap from exploding.
All parts you select must be able to run 3v to 5.5v
The SX can run 2.8v to 5.5v
Choose a 3V eeprom as well.
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Forgive me for jumping in with questions, but I haven't been around recently.
How you you handle in-rush current to the super cap? Or isn't it a problem with solar cells as the power source?
Also, do you have to have a blocking diode to prevent the solar panel from discharging the super cap if the panel is in a dark state?
This would be great for gather daytime data of ambient light in a greenhouse. The system naturally shuts down when light is no longer available. In that way, it also provides you data about useful period of light compared to actual astrological sunrise and sunset.
In some locations with morning fog or having a mountain effect an early dusk due to shadow, that data could be very useful to a grower.
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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
I used a voltage regulator to connect the solar cell to the bus (bus serves the solar cells, supercap (or NiMH) and SX board). William uses a zener which I will try next.
My solar array (pre-made 6volt unit) includes a diode for night time.
My goal is to get a solar-supercap system working, then figure how high of a duty I can support overnight & on cloudy days for SX wake/sleep That will then determine how often I can wake up, grab data and go to sleep. A six second wake every ten minutes gets the duty cycle to 1% and every ten minutes is still a good sampling rate for environmental data.
For example, I picture a system like this going between 2.799 to 2.801 volts (with 2.8 brown-out) as voltage rises when charging in the morning. SAme will occur downward even when powered by the cap (as it slowly discharges). I need something like a Schmidt trigger where the threshold going up & going down are different.
I was thinking of systems external to SX:
- maybe a 555 that holds off change of state until ten minutes after last change?
- maybe use comparator to determine if voltage is above some level and if not go back to skeep for another ten minutes?
But ideally I'd like to think of a way internal to the SX. Still thinking...
BTW, since this project records data for a month, I'd like to include a DS1302 to time stamp data if the power budget allows. That real time will drift with temp, so when data is retrieved I get an accurate time for hte last data collection. Back in Excel I can then create a correction for each time stamp (decreasing back to zero at the time of deployment). NOt perfect, but good enough for this application.
In this way, it would go down below the off level before cycling into a recovery and then charge to a higher voltage than minimally needed before turning on.
If you want to complete the trick, I have an odd little 4-wire 3.3 volt/ 1 amp voltage regulator, that would be ideal. The fourth wire turns the output on and off. In fact, you might be able to use it without bothering with an external op amp. You might be able to use a zener diode and resistor.
Take a look at the Farichild KA78R33 Low dropout voltage regulator. I suspect it might also eliminate the need for blocking diodes, but that depends on where you put the supercap [noparse][[/noparse]before or after the regulator]. The 1 amp rating maybe a bit of overkill. One senario with a blocking diode is to put the supercap before the regulator, have the regulator trigger appropriate full on and off, and then put a large-ish storage cap after the regulator - say 1000mf. This particular regulator wants greater than 47mf on the supply side for smooth operation. Have a protective diode running in the reverse of normal current flow from the output side to the input side.
I would be interested in trying to include an Ultra low power RS-422/485 transceiver to moves data over distances. But in reality, I don't think one could get more than 10-20 seconds out of a 1 farad supercap when using a BS2 alone.
Most of the time you would be relying on the KA78r33 to manage staying power and it may be very important to protect it from back current. But in theroy if the supercap on the supply side is bigger than the regulating cap on the output side, no damage would occur. A 7805 can be damaged by anything larger than 10mf if suddenly unpluged with a protective diode and it is its documentation which brings me to be concerned.
The KA78r33 shuts off when the monitored input voltage is below 1.4volts. It can be adapted to turn on at 1.4volts or anything higher. Since this is an op amp input, it takes very little current away from charging the supercap before power is turned on.
You could have another voltage monitor provide early warning of voltage drop to provide orderly shut down. Beam Robotics builder use a little 3 wire device. If you tie the trigger to the RTCC, it can call a shut down routine. If that is no good, any of RB port can provide an interrupt.
You've fired my imaginations. I forgot about these regulators. Originally I got stuck with them because they were the only 3.3volt regulators in town and I had considered the 4th wire a hassle. Now I begin to see it as real neat.
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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
There is no rush of current from the solar panel (small one) that the supercap cannot absorb easily, since the panel is always connected to the supercap. There is no switch.
Yes, if your solar panel has no blocking diode, you need to add one.
If you use a 7805, it will leak 3mA even when there is zero load. This is unacceptable.
Even a very good regulator may leak 500uA when there is no load.
John,
If you run the SX at 4Mhz, it will only consume about 5mA of current, 1Mhz will be even less current.
Use any one of the SX's IO pin to directly power the eeprom. No need for transistors.
This way, before the SX goes to sleep, it can literally power down the eeprom first.
Assuming the board has only the SX and the eeprom and no regulator, the total sleep current would be less than 5uA.
At 5uA, lets hope the 1 Farad capacitor can last the whole night.
To save power at night, use the watchdog to wake up the SX every 2.5 seconds.
Then quickly sample your data in a few milliseconds and then go back to sleep.
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www.fd.com.my
www.mercedes.com.my
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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········