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Old 11th April 2012, 06:05 PM   #751
heb1001 is offline heb1001  China
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Quote:
Originally Posted by janneman View Post
One thing to remember is that for real high performance servo's, the used opamps have to be very high (audio) performance also.
Many people know that the servo 'works' at very low frequencies but don't forget that they are required to work as a perfect integrator up to the highest audio frequency.

So basically all that you would want for an opamp for the signal path also is valid for the servo opamps!

jan
I think the ones connected to the audio signal need to be good. I'm not sure if I can get away with cheap ones for the active low pass filter after the integrator. The integrator should compensate for any DC offset and the output of the low pass filter should be -200dB so hopefully it won't matter if there is a little distortion from those op amps.

Something to measure to find out perhaps.
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Old 11th April 2012, 06:17 PM   #752
bcarso is offline bcarso  United States
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Typically, two-pole lowpass filtering will entail a lot of ringing, if not outright oscillation.
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Old 11th April 2012, 06:21 PM   #753
heb1001 is offline heb1001  China
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Quote:
Originally Posted by bcarso View Post
Typically, two-pole lowpass filtering will entail a lot of ringing, if not outright oscillation.
Is there a way I can use spice to tell me if it's going to oscillate? The frequency response doesn't seem to have any nasty peaks in it.
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Old 11th April 2012, 06:30 PM   #754
bcarso is offline bcarso  United States
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Sometimes Bode plots lie, so transient response simulations are really the safest.

I should qualify my statement a bit: if there is sufficient loop gain and two close poles, you will see ringing. But an integrator followed by a little passive lowpass with a different tau may be a stable solution. This also filters out some of the broadband opamp noise, which appears at the output of a Sallen-Key lowpass.

If you get a complete schematic with real opamp parts I could take a look at it in sim, although I'm a little busy right now.

Brad
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Old 11th April 2012, 06:40 PM   #755
qusp is offline qusp  Australia
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Patrick: ANR26650M1B lifepo4 batteries like The Shaman is using are 2500mah vs probably something like 150-200mah from your 8.6-9v nimh, so given the minimum 600 cycles before it doesnt reach their high performance specification (not lifetime, they will last a lot longer) its not an issue.

Take however long your batteries last in a cycle (20hrs is a number I remember you mentioning I think?), multiply that by 12-16 (240-320hrs/10-13.33 days 24hrs a day) and multiply x 600 (144000-192000hrs/6000-8000 days/16.4383-21.9178 years non-stop) still seem like an issue? thats before they start to show their age. basically I expect battery technology to have been revolutionised again several times by then. dont let them ho below 2.8-3v though, they wont explode like some lithium cells can, but they can age quicker, though nowhere near as badly as nimh do if you deep discharge them.
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Old 11th April 2012, 08:21 PM   #756
EUVL is offline EUVL  Europe
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600 cycles is 600 times you play music. If you play 3 times a week, that is in theory 4 years, not 16~24.

But do feel fee to use what you think is appropriate.

I personally do not see any issue with NiMH.
They will do 2000 cycles without too much trouble, have no memory effects, and will take trickle charge of C/40 continuously.

I have not used LiFePo4 myself, but a friend used them a lot for his bike lamps.
And he said one should expect to change them every 2 years.
I have not changed my NiMHs for 5.5 years now.

Unless one can prove that there is a performance advantage, like lower internal resistance, or noise, or whatever else.


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Old 11th April 2012, 08:39 PM   #757
bcarso is offline bcarso  United States
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Quote:
Originally Posted by EUVL View Post
Unless one can prove that there is a performance advantage, like lower internal resistance, or noise, or whatever else.

Patrick
Had I seen someone claiming lower noise for LiFePo's, or did I just dream that?
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Old 11th April 2012, 10:52 PM   #758
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Quote:
Originally Posted by EUVL View Post
600 cycles is 600 times you play music. If you play 3 times a week, that is in theory 4 years, not 16~24.
Patrick, they are referring to full discharge cycles - which are in fact not recommended.
Normal charge cycles like the one you describe are in the order of 1000s... Check here for example.

Click the image to open in full size.

Their lifetime is considered higher than that of NiMh.
It mustn't be a coincidence that the automobile industry is moving to said batteries for new electric vehicles (I've read about Renault, GM and even Porsche, among others).

Quote:
Originally Posted by EUVL View Post
Unless one can prove that there is a performance advantage, like lower internal resistance, or noise, or whatever else.
Not sure about noise but I think their internal resistance is a lot lower than other types of batteries up to very high frequencies. I recall reading a paper with such measurements @electrochem.org but I can't seem to find it now.
Anyhow, there are plenty of references to the advantages of LiFePO4s in the literature (e.g. Ref.1, Ref.2)
A less scientific test can be found here.

Click the image to open in full size.

Also here's a quick reference from "Battery University":

Click the image to open in full size.

Patrick, I can send you three of these batteries and a charger to test them (objectively and subjectively ) and you send them back when you're done.
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Last edited by TheShaman; 11th April 2012 at 11:02 PM.
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Old 12th April 2012, 03:52 AM   #759
EUVL is offline EUVL  Europe
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I have access to them. I even have access to proven charging circuits.
But no time to test right now.

So you guys go ahead first and report back results.


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Last edited by EUVL; 12th April 2012 at 04:01 AM.
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Old 12th April 2012, 02:54 PM   #760
bcarso is offline bcarso  United States
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Of possible interest regarding charging of LiFePO4 batteries: LiFePO4 Batteries Help Consumer Devices COME TO LIFE
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