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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
EDIT: Dr. Slack, have you specifically chosen the resistor values to better suit the voltages and LM339? And ill use two resistors in series to get accuracy and precision, for the first physical prototype.
No, they just have the right ratio. I've not looked at an LM339 data sheet to see how large the values could be before input offset current reduces the accuracy (input bias current does not reduce the accuracy if the two sides have the same values) as I don't know what your accuracy requirement is, but 10k region certainly and 100k maybe. With resistors as low as I have shown, the current drawn per cell is around 1mA, how much do the motors draw? (rhetorical question, don't answer that)
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Im going to simulate a new similar circuit, and then make preparations for etching a prototype.
The similarities will differ, in that i should have the reference pushing above ground, so as Dr. Slack has stated, ill arrange some changes and simulate till satisfied, then start etching for final verification.
Final verification will involve ground testing, with known good and bad batteries. then eventually flight testing with good batteries till (safe) depletion, while in hover.
EDIT: (added later)
Dr. Slack wrote ...
....how much do the motors draw? (rhetorical question, don't answer that)
Ha! 17 Amps times 2, more or less..... :) i suppose i could live with 1mA for now, with 1/4 watt IMT resistors. But, if for the next gen i what to switch to SMT resistors, i may need to reduce the current.... we'll look at that later then...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
When I ask myself a question, it nags at me until I have a proper answer. So, how big can those resistors be?
For LM339 ... Input offset current is 5nA typical, 150nA absolute worst case over temperature. Input offset voltage is 1mV typical, 9mV absolute worst case over temperature.
That means that using 160k and 180k resistors, the offset error due to input current is of the same order of magnitude as that due to voltage. Using 16k and 18k resistors, the offset voltage completely dominates the offset current.
So my 'certainly 10k, probably 100k' of a few posts ago was a remarkably good estimate, or maybe I've just been doing this stuff for long enough.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
If I understand correctly, this only monitor cell voltage, which is important. But you should be able to, for the most part, compensate for esr by adding a current shunt and an inverting opamp. Which should result in a more stable voltage and can be used as a crude fuel gauge.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Slack wrote ...
OK, not quite as busy as I thought (well, my wife's on the phone, which is enough time to learn to play the violin).
This is eesntially the same circuit as before, but it's been drawn differently to emphasise the symmetry between all the channels, and the 45 degree lines show the crossover in voltage between the cells under test, and the reference (which is just a floating voltage source here to simplify the drawing, but it can be referenced to either rail, or somewhere in between). Don't forget that it needs a low outupt impedance to source or sink current into those resistive dividers properly, so a 'simple' pot down of the output would not give the current into the dividers that you would want. Choosing the divider resistors properly however does give the right result. Reference designators will be different.
I've chosen the values here from the E24 series that give the ratio nearest to 3.65/4.096. Alternatively make one arm of each divider two resistors in series, then it's easy to make more precise values from two coarsely chosen resistors.
Do make sure I've got the comparators the right way up, I think I have.
I've drawn the 'per cell' LEDS. I'm letting the emitter-base diode of Q1 supply all the current for the LEDs. Transisitor data sheets will give an Ibmax, though any cooking grade transistor ought to be able to handle the max 10mA that all three LEDs on at once would draw.
Consider a smoke pyro for the 'OMG I gotta land' signal. Visible from huge distances, and needing very little energy to set it off. I've made firework igniters that need only a few watts for a few seconds, with a 1/8th watt resistor, glue, and a crunched up match head.
Oops, she's off the phone, gotta post and do the sharing a bottle of wine thang!
im drafting this schematic, and putting the board layout together tonight. ill try to get it etched tommarow.
heres the full power management plan, the shematic above is just the red circle and arrow part.
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LiPo Saving Op Amp In-Flight Blinker.
