EEVblog #259 - PSU Rev C Schematic - Part 12

[EEVblog]

Are the Li-ion cells in series or parallel?  The choice of the charge controller (MCP73213) with its 8.4V VREG output seems to indicate they are in series, however the 3v3 LDO MCP1700 seems to have a max VIN of 6v.

Give this man some points!
That does look to be a critical error

Yep, points hereby awarded!
The MCP1700 was originally powered from the +5V reg, so I missed rechecking that.
Thanks.

Dave.

[Stephen Hill]

Are the Li-ion cells in series or parallel?  The choice of the charge controller (MCP73213) with its 8.4V VREG output seems to indicate they are in series, however the 3v3 LDO MCP1700 seems to have a max VIN of 6v.

Give this man some points!
That does look to be a critical error

Yep, points hereby awarded!
The MCP1700 was originally powered from the +5V reg, so I missed rechecking that.
Thanks.

Dave.

Easily fixed with two resistors

[robrenz]

Center tap the battery holder and power it  with one cell?

[tom66]

Dave, if the boost converter has a minimum output of 9V, how is the LT3080 going to cope when the load is a short circuit @ 1A? Or even a low voltage? That could be 9W dissipation. I hope it has thermal shutdown.

Do you need a SEPIC converter to provide the lower ranges? Or maybe an isolated flyback supply? (good fun to design, trust me!)

[SuperFungus]

Wow.  I must say your original design didn't really excite me too much, but after all these Rev. C design decisions I'm pretty damn excited.  I thought every single thing you changed was for the better and now I'd actually buy one.

Since you asked, I did have a few comments

1) Why not move the battery measurement divider to the 'system' side (pin 3) of the switch rather than the 'battery' side (pin 2) of the switch to save the quiescent current when the supply is turned off?  Not that it's really a big deal, if your battery is about 3AH I back of the envelope around 2 years to discharge the batteries into that 44k. Doesn't cost anything to change either though.

2) You've probably considered this, but at 20V out and ~7 Volts nominal on the battery string to get 1A out of the output you'll need close to 3A at the batteries.  Can your cells handle that?  You mentioned they were about 3AH capacity so 1.5AH each?  That would be a 2C discharge rate? Sounds a little quick to me. Maybe a good candidate for a firmware fix, just drop the maximum current limit for high voltages.

3) Looks like the 100pF compensation capacitor from Vo to the feedback pin on the switcher is missing from your schematic.  I also notice that the digital pot seems to have around 16pF of output capacitance.  I haven't gone through the math but could you be introducing a stability issue?  The series 820 ohms helps, but 16pF seems close enough to that 100pF to be worth a second look. Maybe another argument for some kind of analog control loop for the pre-regulator?

One last note: THANK YOU for using Ethernet comms and not USB/GPIB.  (Why isn't GPIB dead yet?)  Ethernet is perfect for test equipment: isolated, completely ubiquitous, run long lengths with no problem etc.  But it seems like all sorts of gear just still targets USB or GPIB and not Ethernet and I just don't get it.

[IanB]

Just a quick observation to the above poster that two cells in series at 3 Ah each is 3 Ah total.

[SuperFungus]

Whoops!  Of course you're right!  I always think about AH as the capacity of the battery which isn't strictly true. 1C discharge is much more reasonable, ignore that part of my post. Good catch, thanks!

[oPossum]

Are the Li-ion cells in series or parallel?  The choice of the charge controller (MCP73213) with its 8.4V VREG output seems to indicate they are in series, however the 3v3 LDO MCP1700 seems to have a max VIN of 6v.

Yep, points hereby awarded!
The MCP1700 was originally powered from the +5V reg, so I missed rechecking that.
Thanks.

Dave.

MCP1702 goes up to 13.2 V

[robrenz]

Dave,  I know that design euphoria feeling you were describing when a few inspired design changes cascade into many design refinements that satisfy your personal design sensibilities and you know "this is it".  Except mine occur in mechanical design. My life path was similar to yours except I was machining at 12 years old instead of electronics.  I think this "design euphoria" can only be experienced fully when you have sole control of a project and there are basically no constraints.  Probably why many may be unfulfilled at work where deadlines and costs and the boss may prevent that from happening.  Also may explain why personal projects can be so fullfilling.  Now because of your inspiration I may someday experience some "electronic design euphoria"

Thanks for all you do,

Robin

[inderpreet]

Any chance of getting a connector in there for an Xbee? Would be cooler than the ethernet since the battery powered supply would then be wireless. Also, There is some example code out there on how to get arduinos to upload wireless-ly over the Xbee. Just a connector...

[curiousbob]

Center tap the battery holder and power it  with one cell?

This would work... until the battery voltage drops below (3.3V+dropout voltage)
For a few more cents, the MCP1702 will do the job as oPossum suggests.

[chavotronic]

Can someone explain to me the purpose of C24? Its between the Arduino reset and the CN3 FTDI Connector. I personally would have tied the other side of the cap to ground to build a lowpass together with R44, and then paralleled the lines. But i am probably wrong on the understanding of this way of connecting it.

[A Hellene]

C24 is needed for the good operation of the AVR microcontroller (see its data sheet), in order to decouple any possible noise that could be randomly resetting the AVR. Of course, it should be connected between the nRESET pin and the ground.


-George

[chavotronic]

So george, so you also think this could be a mistake in the schematic?

[A Hellene]

I think that it is rather about what it actually is than what I think it is!

Kidding aside, yes, of course it is one of the countless oversights or mistakes that could be introduced in any new design. This is one of the side effects of one's overly self-confidence, I usually also excel in...


-George

[oPossum]

The schematic is correct. That cap will reset the AVR on the falling edge of the handshake line. This is an ugly hack that has been in the Arduino from the start.

[A Hellene]

If the case is this, then I fail see any components that discharge the coupling capacitor C24, or any other components that decouple the AVR nRESET line noise, or that use the same capacitor for this task.


-George

[oPossum]

The internal pull-up resistor on the reset line will discharge the cap. It also provides some limited noise immunity - the pin isn't floating.

[A Hellene]

Yes, of course the AVR nRESET line has an equivalent of a >50 kohm silicon pull-up resistance built in on the die. But, when the component manufacturer strongly suggests the use of an external low-pass filter for enhanced noise immunity in ther engineering application notes, I will ostentatiously disregard any quick-and-dirty hacks of non-engineering communities like the aforementioned one; of non-engineering communities that introduced that imbecile baby-talk by renaming the add-on PCBs to "shields" and the programs to "sketches"...


-George

[curiousbob]

Not sure if someone has already mentioned it but  the LM7805 5V regulator requires VBATT>7V.
Now that Dave will use only 2 18650 batteries, this will be a problem once each battery goes below 3.5V.

[oPossum]

Yes, of course the AVR nRESET line has an equivalent of a >50 kohm silicon pull-up resistance built in on the die. But, when the component manufacturer strongly suggests the use of an external low-pass filter for enhanced noise immunity in ther engineering application notes, I will ostentatiously disregard any quick-and-dirty hacks of non-engineering communities like the aforementioned one; of non-engineering communities that introduced that imbecile baby-talk by renaming the add-on PCBs to "shields" and the programs to "sketches"...
-George

It's not hard to find poor design practices in the Arduino - both hardware and software.

I prefer the Microchip Microstick or TI Launchpad for simple DIY projects. They both have a full featured IDE with source level debugging and cost less than a genuine Arduino.

[oPossum]

Not sure if someone has already mentioned it but  the LM7805 5V regulator requires VBATT>7V.
Now that Dave will use only 2 18650 batteries, this will be a problem once each battery goes below 3.5V.

LD1117V50 would solve that problem

[A Hellene]

Yes, oPossum, that is right! However, Dave's PSU being compatible to the Arduino platform does not necessarily mean that it will be of that non-quality, also. Dave, as well as many other members of this community, is a seasoned designer. I would not expect him to adopt poor practices. After all, I cannot find a better quality for someone to have, other than their reputation as effective designers.

By the way, Dave, on the battery matter I would rather prefer to restore the third battery cell back in order to increase the PSU's autonomy by more than 50%, with the insignificant additional burden of swapping the two-cells battery charger with a three-cells one and restoring the SEPIC pre-regulator that will reduce the thermal losses dramatically. I would also take BaW's advice to ditch the ee-pot in favour of a faster regulation loop, a better power efficiency, a lower CPU load and, most importantly, of a better overall autonomy and, thus, efficiency of the device.


-George

[Shale]

Question about microcontroller choice?

Dave. why have you chosen to use the standard arduino 168 micro vs, the 328 or the one on the arduino Mega? After you changed the entire design to smd i would have thought you would change the micro as well, but i seem to be wrong. with all the people wanting to mod your design seems like the extra pins could just connect to headers and be left for people who might want them. as I am not yet an engineer I would like to know what drove those decisions.

Thank You

[IanB]

I think Dave wants the micro to be socketed so it's easy to swap out if necessary.