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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686
Posted: 10:32am 28 Feb 2010
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Ok.... no genius here, but here is my take on it.
You have 2 op amps. you can use either or both, the output is on pin3, where you can get the feedback sorted... the same as any other op amp. This solves your question [quote]i did a quick checkup on the TL494, and i could not spot any OPout pins, they seems to be connected inside[/quote].
However, you can use the chip with the pulse width modulation. Just give it a fair bit of positive feedback (pin3) so the on/off is fairly sharp, and you get the hysteresis you want..... then this will drive the PWM from 0 to 90% or more.... this is on or off for a relay. You can switch a fet or whatever you wish and that can drive a relay if you want isolation.
The Vref amp is very stable, and designed to give offset for the op amps.... better than your 7805 I suspect.
Remember, if you want to use the pwm (and I would) then the result of the op amps is.... high= narrow pwm (off) and pin3 low will turn up the PWM to full power.
I know this is counter intuitive, but it beats the triangular sawtooth from the oscillator with the output from pin3 in an op-amp......think about it. If pin3 is greater than the peak of the triangle (non-inverting input), then the output is low, as there is no time that the triangle V is greater than the pin3V (driving the inverted input).....
Now as pin 3 drops below the V triangle, the length of time it takes to get from the ascending side, over the apex to the same point on the decending side will be the width of the pulse.... so pin3 going lower, makes the pulse width wider.... simple... (so don't be surprised if you get it back to front first time)
With the opamps, either one can drive the pwm on or off. Which ever one pulls the voltage up on pin 3 turns the pwm off
So, with one chip, you have dual comparators, ref voltage and an output selectable emitter follower or common emitter .... output selectable from pushpull to single ended drive (pin13) output stage to drive a fet etc. Keep the frequency lowish, and driving the fet will be childs play, and efficiency will be high as well.... less current to keep the contacts closed than full on I would think.
So, pin 13 (output control ) allows for using it's outputs as push pull or single output as well...... lots to fiddle with
............oztulesEdited by oztules 2010-03-01Village idiot...or... just another hack out of his depth
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 11:24am 28 Feb 2010
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I have normally used PWM IC's at the 20-100kHz rate, but there is probably no reason they couldn't work at the 1-100Hz. This would be quite soft on the heaters.
There would be no need for the hysterisis, only a precise setpoint, as the duty cycle for the diversion load would vary from about a max of 90% ON, above the setpoint to fully OFF below it.
I am not putting my hand up to make a cct, but there is potential.
Gordon.
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333
Posted: 11:32am 28 Feb 2010
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[quote]Pete- i was not sure what voltages to use for cut in/ cut out. please advise me on that.
i suppose i could post the question on some electronics-forum, know about any good ones? [/quote]
What are we cutting in and out for me to advise you on??
As for a good forum on electronics i am sure there are a few but i have never got any answers worth while to the questions asked.
There is one site that has a fellow by the name of OZt????? or something like that on it that gives very constructive information but dont remember where i found it now ... Thebackyard or something like that.
Pete.
Ps. there is another bloke there too, Gordon someone who is a smart cookie too.Edited by Downwind 2010-03-02Sometimes it just works
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 01:06pm 28 Feb 2010
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I would not cycle a 12V battery below 12V. The actual voltage would depend on the current drawn from the battery at the time. 0 amps, and 12V. 20A, and maybe 11.8V. Depends on the battery capacity.
Gordon.
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686
Posted: 01:19pm 28 Feb 2010
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Hmmmm...
Yes Gordon, nice for a dump load..... but I think in this case he was going for a low voltage cut off device.... to protect from deep discharge. That makes a heater a bit unlikely
Cut off voltage will be a bit tricky to nail down, as the battery versus the drain will dictate different voltages for different drain.
eg, in my electric car, the batts are only 220ah. They see current draw of over 200A (peaks of 280 I suspect), so the 40v or more at full charge , dives to 35v on full acceleration. As soon as it settles down to 50 - 75A, the voltage recovers to 38v or so, until the pack gets actually well discharged. I have yet to see them below 36v at rest.... for a 36v battery bank.
I have however, seen them dip below 34v under hard acceleration when rest voltage was 37v or so.
So it becomes a "how long is this piece of string" question....... Answer.... it all depends on the load at the time versus the bank impedance.
You can see from above, that the unloaded voltag will jump up very considerably when you drop the load ie. when you release the batts from under full load.....so each setup will require a separate guess...... depending on the average current draw expected.
I have the same problem determining the shut down voltage on my little mills.
Being strong axial flux units, they can deliver over 100A@ nominal 36v (more like 45v or more at that rate).
So I tend to design dump/shut down devices that look at performance over time. Currently if the batt sees over 44v for more than 4 consecutive minutes... then shut the mills down.
In quieter days,(only 10A or so) this works well, and the batts are probably well charged and a bit equalized, but on windy days they would shut down only 3/4 charged, as 100A pushes the voltage up silly high too early..... on windy days I use only 1 mill heavily furled (furls early... 1kw max), and this seems to get close.
Suck it and see. You will need to use it and check voltage under normal load, then shut it off and measure the sitting voltage to get an idea of where to shut down sensibly.
I know that is not what you wanted to hear.... but???
Dump loads are much easier to guess at, and for my setup, anywhere from 41.3 (float) to 45V (cycling a bank) would be ok for dumping a 36v bank.
Gordons pwm idea would do well at this.....but I think 5khz rather than the low freq of 100hz would be more normal (two transistors (2n4401/03 etc. in totempole) and a decent fet or two driving the load would do it up to 20-40A I think.
The supply you get the tl494 out of will have a totempole pair you can hijack as well The switching transistors from the supply will probably be 80w devices good for 7A or more each (more in this application ).
You can get pretty fancy pretty fast if you dont take care
Edit: Sorry Gordon, you posted while I was scribbling You said basically the same thing in a whole lot fewer words..... I must ramble too much me thinks ...........oztulesEdited by oztules 2010-03-01Village idiot...or... just another hack out of his depth
adonis Regular Member Joined: 13/02/2010 Location: SwedenPosts: 42
Posted: 08:18pm 28 Feb 2010
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Oztules and Gordon, i dont know quite enough to understand your inputs i think, will have to read them very slow and cross check some things i guess... and then i will reply more specific. i was hoping there was some way to control the transistors (pin 8-11) directly by the OP-amps, and maby thats just what your replies answered, just i didnt understand?
if so, they should be powerful enough to control the relay directly i think (200mA each).
Pete, i referred to your earlier statement that 11v seemed to low for a safety cut out voltage, thats what i ment for you to advise me on. but i think Gordon and Oztules anwered that.
Very well, thanks for the help and inputs so far, all of you.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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The switching transistors from the supply will probably be 80w devices good for 7A or more each (more in this application ).
