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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Patrick wrote ...
Im wondering what to do with the UA741's pins 1 and 5, if im not wanting to use those pins? im also hoping the input of 2M ohms isnt a problem in my app.
it would take 3 of these analog "channels" to monitor 3 cells of a battery. this schematic is incomplete. the power enters through the the isolation transformer, then the signals under monitor are kept floating with the op amp, with the output signal coupled back to the whole machine ground through a opto-coupler.
Whoa, complexity overload.
How about (drawing a circuit with words) using easier to get components
Using the total battery 12ish volts, create 3 reference voltages for the cells a) Use a regulator/reference, supplied from 12v, to create a first reference 3.7v (or whatever) from the -ve rail, to comapre with cell 1 +ve. b) Now use a diff amp with 4 equal value resistors, to translate this first reference voltage up so that it 'stands on' the first cell connection, so creating a second reference voltage for the second cell +ve. c) Now use another diff amp with 4 equal value resistors to translate the first reference voltage up so that it 'hangs below' the 3rd cell +ve, to create a thrid reference for the 3rd cell -ve. Note that end of battery life, the second and third refs will be similar, but they are not equivalent, one is for cell 2, the other for cell 3.
Now arrange 3 comparators (from an o/c high voltage quad like 339) with the right logic sense for each compare, and wire-or their outputs at the +ve rail, and use the 4th comparator to get a full rail swing from the wire-or point if required.
Bill of materials - a single reference or regulator, a dual opamp and 8 good tolerance resistors to create the other two refs, and a quad comparator
<edit>
WAIT, WAIT, it's yet simpler, let's lose the dual op-amp
a) make a 3.7v reference off the -ve rail with a regulator or referencey thing, this is the compare voltage for cell 1 +ve b) Get 4 equal value close tolerance resistors, like you're starting to build a diff-amp, but notice that when the difference at one end of the pair is v (ie a cell), and the difference at the other end is -v (ie the reference voltage), their centre terminals are at the same voltage, AND nowhere near the rails, so you don't need a R2R input comparaot. Put a comparator here. Do this for cell 2 and cell 3. I'm sure that with this hint, you really do not need a diagram. c) wire-or the three o/c comparator outputs for a full swing output, put leds in their outputs for debug or display
</edit>
<editedit> OK, you want pictures
U1 generates the cell endpoint voltage. I've drawn it as a box, it can be a voltage regulator or reference, as accurate as you want, but it must be able to sink a little current (see later).
The bottom cell is directly compared with this voltage. If the cell voltage to the bottom compare drops below, the wire-or output gets pulled low.
To detect the second cell, its +ve terminal is potted to Vref- by R1 and R4, its -ve terminal potted to Vref+ by R2 and R3. Do the math to assure yourself that when the voltage on cell 2 drops below Vref, the second comparartor output goes low.
The third cell is handled identically, except using R5 through to R8.
The accuracy for cells two and three is diluted by the tolerance of R1-R8. 0.1% components are not much more expensive than 1% parts, if you need the last few percent accuracy. I've shown 1k, and that will do, but higher values will leak less current. The highest usable value is limited by the comparator input offset and leakage.
There is no interaction between the cells, each has its own unique comparator. The Vref generator output is low impedance, so it will source or sink current to maintain Vref at 3.65, or whatever you choose as the endpoint voltage. Note that R3 and R5 source current into the Vref terminal, so if it's a reference it must be beefy enough to sink the current. If it's a regulator, it must have sufficient loading on it to sink that current.
So, one regulator, and one quad comparator, and a handful of resistors.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Slack wrote ...
<editedit> OK, you want pictures
U1 generates the cell endpoint voltage. I've drawn it as a box, it can be a voltage regulator or reference, as accurate as you want, but it must be able to sink a little current (see later).
The bottom cell is directly compared with this voltage. If the cell voltage to the bottom compare drops below, the wire-or output gets pulled low.
To detect the second cell, its +ve terminal is potted to Vref- by R1 and R4, its -ve terminal potted to Vref+ by R2 and R3. Do the math to assure yourself that when the voltage on cell 2 drops below Vref, the second comparartor output goes low.
The third cell is handled identically, except using R5 through to R8.
The accuracy for cells two and three is diluted by the tolerance of R1-R8. 0.1% components are not much more expensive than 1% parts, if you need the last few percent accuracy. I've shown 1k, and that will do, but higher values will leak less current. The highest usable value is limited by the comparator input offset and leakage.
There is no interaction between the cells, each has its own unique comparator. The Vref generator output is low impedance, so it will source or sink current to maintain Vref at 3.65, or whatever you choose as the endpoint voltage. Note that R3 and R5 source current into the Vref terminal, so if it's a reference it must be beefy enough to sink the current. If it's a regulator, it must have sufficient loading on it to sink that current.
So, one regulator, and one quad comparator, and a handful of resistors.
</editedit>
im still simulating the above circuit, and using a calculator and paper to work it out in my head... im not seeing a definate trigger point for a comparator once the set point is violated... maybe im goofing it all up, im sure Dr. Slack has this idea all figured out.
Keep in mind if had a traumatic brain iNjury recentLy ,,, , bluada ...;osfsdchdsjcndjshdfh dfff
EDIT1: i do see how the first comparator works, its a direct comparison between the non inverting and inverting pins.
EDIT2: i had a bogus connection present, the signals now appear as follows: less than setpoint, out = -V setpoint = cell V, out = 0V greater than setpoint, out = +v
it does appear really sensitive, right down to mV, as needed....
EDIT3: i do see that the lower comps position sees twice the difference, as the upper two comps are referenced through voltage dividers. (explaining the 2:1 and 1:2 ratios)
EDIT4: this appears to be a very clever circuit.... Questions: First, im thinking of 10k resistors, that means 4.2 / 10k = 420uA, at 0.0018 W. (better than 1%) Second, the regulator should probly be a 4.096V regulator, divided down via pot to the end point voltage. about 3.65ish V. Third, i dont know if 420uA is to weak though, re- the input offset and such. Foruth, im wondering about the comparators outputs. i may need to buffer with logic or at least a power transistor? or perhaps a NAND logic fucntion instead of the current AND output....
pic of the schematic to convince myself it all works... weeeeeee!
EDIT5: a voltage below 2.0 can safely be considered "Alarm! Land Now !", above 2.0V is good to fly.... there is a special condition where a low cell still cuases 1.2 ish volts to appear, else low is like 230mV. This point should allow enough hysteresis to guarantee reliable triggering.
ATM: im looking through my Forest M. Mims books on Op-Amps for the remaining comparator to be used for power transistor triggering.
dam!!!! i found a special case where a low cell can still allow the output to be high....let me post that pic below.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Several things
a) Your simulated comparators. Are they voltage output or open collector? If voltage output, then 2 high might win over one low. The LM339 el-cheapo-est quad is open collector., which will do the logic correctly.
b) Always bear in mind that a circuit that is designed to work over a range of voltages might do strange things outside those voltages, and that a simulator may well not reproduce this fault behaviour at all. It is vital to test the real circuit when built, over the full full range of fault conditions that could occur. However, you are going to be using this to let you know that a cell is getting low *during* a flight when manifestly all three have been OK earlier, not to check a maybe dead pack before flight (for which you can and should measure the actual voltages with a meter).
c) By all means use a reference with a standard voltage output, but don't go potting things down. Let the algebra do the work. Trust me that by going to other than 1:1 in the voltage dividers, and by treating the bottom cell in exactly the same way as the other cells are treated, you can change the compare voltage at will. A further effect of this is that the detection sensitivity of each cell becomes the same. When I say trust me on this, the problem is that I'm very busy at the moment, and may not be able to post detail for a few days.
d) not sure why you think you need power transistors or logic. The LM339 is good for 20mA which is more than enough for a LED. Suggestion, put a led in series with each compararot output, each with a current limiting resistor going not to the top rail, but to the base of a PNP, emitter to top rail, collector is now the logic output from the alarm circuit. This will show you which cell is low. Again apologies for now detail circuit diagram, but I'm away from my normal tools and don't have the time to struggle to make a diagram.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr Slack, Im grateful for any help and time you can spare.
A) this may explain the bizarre result. B and C) yes, i plan to extensively test the real circuit, and ive kept bad batteries for just this reason. but i need a at least a credible circuit before i go to the trouble of etchig a possible final version. D) i need enough power to run a bank of power LEDs white and blue, and a high power piezo alarm. so im thinking 500-1000mA. i need to see and hear it from 100+ meters away....
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
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!
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It *is* adapted for n number of cells. In that picture, n=3. Simply cut'n'paste the 4 resistors and one comparator per cell to stack up to any number.
I drew the reference ungrounded because a) I couldn't be @r$ed to find a voltage regulator symbol and b) to emphasise that the voltage of the reference bottom terminal is not important as far as the basic operation goes, it can be grounded, refered to to top rail, or anywhere in between, just as the voltage at the bottom of each cell is not important. An example of a non-grounded (but not floating either) reference would be a two-terminal shunt reference device, with a resistor top and bottom. There are shunt references made that are not a simple zener, but a proper low impedance accurate reference, like the analog devices ADR5044 for instance.
The import thing when choosing where to pitch the reference voltage is that the common mode voltage at all the comparator inputs must be respected. An LM339 input includes ground but only goes to within 1.5v of the top rail. This means that if you use a reference voltage standing on ground, you won't have to do any sums, as it will be obvious that none are near the top rail.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I'm seeing what has been meant earlier, now, just the choice and sourcing and sinking of the reference is at thought. I'm looking at parts now...
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.
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LiPo Saving Op Amp In-Flight Blinker.
