I think you have to first decide how long of a back up time do you need, and what are the total power requirements during the backup. Are you trying to back up only the Prop? The entire board?
Backing up a board can be as simple as a few diodes, but there is some method of charging the battery required.
Post Edited (Originator) : 11/7/2008 8:16:53 PM GMT
Let's say ~1 hour, but it may happen up to 10 times per year and I wouldn't want to change a battery that often. so if a battery can get me a few days of run time, that's ok, or if it can be recharged on the fly like a super cap, an hour run time is fine. Is that confusing enough?
I have use lead acid batteries with floating chargers left on them for years, but you do have to replace them every few years though.
A supercap may work too, I experimented with them once, they were very expensive, maybe $50, they are very big too, but they didn't meet the need at the time for powering a complete system for very long. When you say 1 hour, you also have to know what the power consumption is , i.e, say your board pulls 1amp, that is a different factor of cap life versus 200ma. You could meter your boards current with a standard multimeter to get some ideas of what your backup plan needs to be to obtain 1 hour.
Oh, sorry. I forgot to mention that I only want to run the prop, not the remainder of the board. Does the power consumption of the prop vary based upon what it is doing or how many cogs are running? Could I use something like a coin cell that didn't recharge but only got used when the power was out? Maybe some type of black out sensing circuit?
The power consumption of the Prop varies on a few factors, frequency, number of cogs running etc. A cog can be 'running' but not consuming power if it is in waitpeq, waitpne for example. So there are various tricks to conserve power, you could refer to the manual for some more info on this and power consuming modes. The trick with powering only the Prop is, how are you going to separate the power to the other board components that use the same 3.3 volts if you use a button battery? To power on the Prop would require some additional switching to power only the Prop, which is doable.
You may want to explain what exactly you are trying to accomplish with the back up plan, there may be tricks to accomplish the same thing without backup power.
Post Edited (Originator) : 11/7/2008 9:05:13 PM GMT
If it's just the Prop, and only for an hour, then I think the simplest circuit would be the one used in the STM32Circle...· Basically,·the NiMH cells are always there, getting charged through a resistor.· But, if the line power is gone, then the battery provides power.
I am using it to keep track of time (of day) and ultimately display the time with led strips that are set up in a seven segment layout. It would be similar to the Spark Fun 6 foot wall clock if you have seen it. I will be using clk to keep track of time and not an external RTC, since my experience with them has shown them to be less than accurate. Using the prop I can run the clock for a week or so, see how far off it is and then "tune" it accordingly. At any rate, I would like to keep the clock ticking and keep track of time, but don't need to drive the led's or drivers when A/C is off. The other option is to just forget it and reset the clock each time the power comes on, but I would like it to be a little more hands off if possible.
Rayman is right, unless you already have this running and just want a quick fix, get a wall wart lead acid charger from somewhere, connect it to a battery with the same voltage your Prop board requires, let the Prop board run off the charger/battery. The battery can run you 15$ for a 5AH, the charger maybe $15-20$, and you are good for days of backup.
You can get the batteries to last longer if you shut down everything in the Propeller except the time keeper. This would require a power sense signal so the Propeller knows when it should shut down non critical cogs and bring them back up when power is restored.
Benj
Posts: 66
Comments
Backing up a board can be as simple as a few diodes, but there is some method of charging the battery required.
Post Edited (Originator) : 11/7/2008 8:16:53 PM GMT
A supercap may work too, I experimented with them once, they were very expensive, maybe $50, they are very big too, but they didn't meet the need at the time for powering a complete system for very long. When you say 1 hour, you also have to know what the power consumption is , i.e, say your board pulls 1amp, that is a different factor of cap life versus 200ma. You could meter your boards current with a standard multimeter to get some ideas of what your backup plan needs to be to obtain 1 hour.
You may want to explain what exactly you are trying to accomplish with the back up plan, there may be tricks to accomplish the same thing without backup power.
Post Edited (Originator) : 11/7/2008 9:05:13 PM GMT
Here's a screenshot from the manual:
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Paul Baker
Propeller Applications Engineer
Parallax, Inc.