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Old 07-24-2014, 02:42 PM #1
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Default [HOW TO] Split Supplies (+/- X Volts) Using single power source

Hey everyone,

I see a lot of posts (most are actually pretty old, come to think of it) on here with people building LPMs and interface kits with IC's or discrete parts that require Split (or Dual) supplies. In order to achieve this, most people tend to get two 9V batteries (or something of the sort) and hook them up like this:



Doing this has Pros and Cons.

Pros:
  • Very high current output possibility
  • Intuitive, easy to figure out and think about.
Cons:
  • Uneven load from either rail (inevitable) and the fact that all batteries drain at different rates will cause your +/- rails to fluctuate
  • Fluctuating rails or uneven spacing will cause your GND to drift, so any 0VDC biased inputs on your ICs (such as buffers or inputs from, say, a thermopile head) will no longer be at 0V. In some extreme cases your ICs will stop working correctly.
  • More batteries in your design means a bulkier, and much more expensive project. Especially if those batteries are 9Vs. Plus, you may (if the above is true) have the added inconvenience of having to replace them at different times.

That's all that comes to mind at the moment. So, you may ask, why'm I Bitchin'? Well, there is a better way.

Method 1: Rail Splitter

Pros:
  • Reliable
  • Minimizes battery/power supply requirement
  • Very well buffered (no uneven draw on your rails)
  • Smallest overall component count
Cons:
  • Only 40mA max output to your circuitry
  • Chip can be hard to come by



Using the TLE2426, you can easily set up a dual supply with a common ground point in between. If you get a version of the chip with a "Noise Reduction" pin, add a small capacitor from that pin to your negative rail. Either way, unless you're doing modulation of some sort the noise shouldn't matter all too much

The only drawback is the current limitation on this chip. Now, for most of your LPM/other circuitry, if all you need the dual supplies for are the ICs, this should not be a problem at all. If you need to drive a high current component, you could use separate supplies for your brains (this) and your muscle (high power component + brain controlled switch), but that would bring us back to square one, with more batteries/etc.

Method 2: Rail Splitting using an Open loop Buffer

Pros:
  • Relatively Reliable
  • Minimizes battery/power supply requirement
  • Buffered (Duh)
  • Output current is limited by Max Ratings on the buffer you use (can be very high)
Cons:
  • Higher output impedance, relatively (basically, this is undesirable)
  • Expensive (for the IC world) [~$10]

So you just gotta have higher current from your split rails, for that fancy solenoid you have connected directly to your comparator, or whatever you're trying to power. Go ahead and buy yourself a nice, high current linear buffer, like this one.

Hook it up like so:

With the V+ and V- pins connected to the + and - terminals of your battery

This setup should be fine. In some very rare cases, you might notice that the center point (your 0VDC ground) drifts heavily, and the rails are no longer balanced. If this happens, replace the resistor dividers (the 100K Resistors) with a TLE2426, like above. This should fix any issues, and will make your circuit look all fancy and complicated, with multiple IC chips

Method 3: "I'm too lazy to buy a TLE2426" (or, "I live in a third world country")

Pros:
  • Reliable
  • Minimizes battery/power supply requirement
  • Made from common, cheap components
  • Feedback (aka low output impedance )
  • You get to be like me (this is debatably a con)
Cons:
  • Output current limited by op-amp chip (usually low)



Don't want to buy a TLE chip? Don't need a buffer (or is it too expensive)? Then screw it, make a buffer. After-all, what is a buffer but a mere unity-gain op-amp? This one is by far my favorite. You can choose any Op-Amp chip, as long as it runs off of Dual Rails. Your limit is your imagination. And the Op-Amp datasheet.
Some of my personal favorites are:
ALD4702
TL074
AD8031/AD8032

Method 4: Scrambled Eggs

Pros:
  • Reliable
  • Minimizes battery/power supply requirement
  • High Current Output
  • Low GND output impedance
Cons:
  • Lots of parts
  • Expensive (debatably a Pro, if you're planning to hang the PCB around your neck on a chain and flaunt your electro-Bling )

So you're a connoisseur of the Everything Bagel. You order your In-N-Out Animal Style, Extra Everything. You also noticed that all of these circuits involve parts that can be combined... with other parts.

So lets put it all together:


With this circuit, you get all of the benefits. Attach your low current consuming parts (Op-amps, digital logic, IC's, yadda yadda yadda) to the weak GND, and your high powered relays, solenoids, LCDs, etc etc to the BUFF GND.

So Now, when building your DIY LPM's, instead of making yourself (or your customers) suffer through dual 9V batteries, just employ a rail splitter to do the job. If you need to drive even larger currents, you could even go for a Class AB push-pull circuit, which is also simple and made up of discrete parts. I can add that later if anyone is interested.

Feel free to post questions down below, I'll try to answer as best as I can.

Best,

amk


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Old 07-24-2014, 03:00 PM #2
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Default Re: [HOW TO] Split Supplies (+/- X Volts) Using single power source

Cool. I had never seen the TLE2426. Neat chip. Doesn't seem at all hard to come by though. Digikey has them. Only $1.90 too.

eBay also has them.
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Old 07-24-2014, 04:30 PM #3
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Default Re: [HOW TO] Split Supplies (+/- X Volts) Using single power source

There's also another easy way, a charge pump IC and a couple of caps Good for low current things, like opamp driving.
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Old 07-24-2014, 04:49 PM #4
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Default Re: [HOW TO] Split Supplies (+/- X Volts) Using single power source

Don't forget another very easy (but not cheap) way to get dual and isolated supplies.

I use this in my LPM. Miniature DC-DC converters.
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Old 07-25-2014, 12:21 AM #5
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Default Re: [HOW TO] Split Supplies (+/- X Volts) Using single power source

Quote:
Originally Posted by rhd View Post
Cool. I had never seen the TLE2426. Neat chip. Doesn't seem at all hard to come by though. Digikey has them. Only $1.90 too.

eBay also has them.
True, I just have a million assorted op-amp chips lying around everywhere, so to me having to mail order a new part makes it "harder to come by", haha. Also I know texas instruments blocks access to its chips/datasheets in some countries, but I guess that would be a very rare problem.


Quote:
Originally Posted by ARG View Post
There's also another easy way, a charge pump IC and a couple of caps Good for low current things, like opamp driving.
Very true. Unless you're planning to modulate anything, for... like, a scanner. The capacitors will interfere with your frequency response.

Quote:
Originally Posted by djQUAN View Post
Don't forget another very easy (but not cheap) way to get dual and isolated supplies.

I use this in my LPM. Miniature DC-DC converters.
DC-DC converters tend to have good buffering capabilities as well.
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Old 07-25-2014, 08:48 AM #6
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Default Re: [HOW TO] Split Supplies (+/- X Volts) Using single power source

Quote:
Originally Posted by djQUAN View Post
Don't forget another very easy (but not cheap) way to get dual and isolated supplies.

I use this in my LPM. Miniature DC-DC converters.
I wouldn't call them exactly "cheap." That one in particular is like $7.60 on Mouser. I accidentally fried one because I put a tantalum capacitor in backwards on a line connected to it. $12 down the drain (at least the one I got). That said, they're definitely convenient, but you can get bigger DC-DC converters for like $5 on Amazon too with more backing power.

Anyway, good post. It reminds me a lot of Tangent's virtual ground info page.
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Old 07-25-2014, 09:36 AM #7
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Default Re: [HOW TO] Split Supplies (+/- X Volts) Using single power source

Quote:
Originally Posted by Bionic-Badger View Post
I wouldn't call them exactly "cheap." That one in particular is like $7.60 on Mouser. I accidentally fried one because I put a tantalum capacitor in backwards on a line connected to it. $12 down the drain (at least the one I got). That said, they're definitely convenient, but you can get bigger DC-DC converters for like $5 on Amazon too with more backing power.

Anyway, good post. It reminds me a lot of Tangent's virtual ground info page.
To be fair, he did say not cheap

The CFB design on that page is pretty awesome too. I don't like how they did the transistor design though. Using MOSFETs would eliminate the need for the resistors on the emitters, as there would no longer be any thermal runaway.
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