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Old 16th August 2010, 04:04 PM   #1
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Default Help : any tips to get +/-24v from SLA 48v Battery

Help : any tips to get +/-24v from SLA 48v Battery?

temporary using a centre tap 48v(4x12v) SLA battery, slight problem ... as they get unbalance voltage after some usage

need 100mA
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Old 16th August 2010, 05:18 PM   #2
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Something here may work for you:

Virtual Ground Circuits

Cheers,
Dennis
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Old 16th August 2010, 05:43 PM   #3
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tried, the opamp operates +/1 15v max , I cant find any higher, or if anyone can introduce me 1

Tle2426 insufficient current...
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Old 16th August 2010, 06:05 PM   #4
star882 is offline star882  United States
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Charge pump made from discrete components.
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Old 16th August 2010, 10:21 PM   #5
HRMMI is offline HRMMI  Croatia
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Any OP amp can be used as voltage splitter. Including OPamp output amplifiers like LT1010. Any OPAMP can be powered over transistors with higher voltage then rated. But if you want to keep it simple try OPA 551 or OPA552 or OPA547 or OPA548 to make simple voltage splitter.
On picture you can see basic idea of powering OPamp with higher voltage then rated.
As it was mentioned, Charge Pump is also god option now days.
Click the image to open in full size.
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Old 17th August 2010, 03:23 AM   #6
gootee is offline gootee  United States
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Look at the rail-splitter circuit in the LM675 datasheet at national.com and remember that you can use higher-voltage chipamps in place of the LM675, if necessary.
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Old 17th August 2010, 03:54 AM   #7
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Measure the idle current draw from both rails (-+ 24v) and place an
equalization ballast resistor across the rail with lower draw so that both rails draw the same current from the batteries.
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Old 17th August 2010, 04:16 AM   #8
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cool ideas, I will try the LM675 for simplicity

for Charge Pump, does it mean a lower voltage input, and charge pump the splitted voltage output
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Old 22nd August 2010, 06:17 AM   #9
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The easiest way to get virtual ground is to use the discrete rail splitter shown on the Vitrual Ground Circuits page (see link above). It uses only 8 discrete components. Depending on the output power requirements you may use more powerful transistors on the output. You can try e.g. TIP100/TIP105 (high power Darlingtons): http://www.fairchildsemi.com/ds/TI/TIP102.pdf . The only drawback is that some power will be dissipated by the transistors.

Mark
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Old 26th August 2010, 06:59 PM   #10
TechGuy is offline TechGuy  United States
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The potential issue I see with a Virtual ground circuit is power consumption. Since your running off battery I assume you want something pretty efficient. Plus a virtual ground will be about +24V which might present some challenges if you need to connect the devices chassis ground to Vbatt ground, or Earth Ground.

Use a Buck regulator but with two windings on the inductor. One winding is connected using the standard configuration for a buck regulator. The other would be used to generate the -24V.

Found this App note of the idea:
http://cds.linear.com/docs/Design%20Note/dn100.pdf

I bet this design would provide at least a 80% efficiency.Just use the LM2576HV or another simple switcher in place of the LT1376 since the LT1376 is limited to 26V input. The LM2576 "HV" design tops out at 60V. Since your load is very low go with a reasonably high inductance of around 100 uH to improve low current output efficiency. Note: note the dot on the inductor windings, The secondard is winding if flipped in relation to its connection to the switching diode (D1).

FWIW: you might be better off just buying a COTS DC to DC converter. a 48V to +/- 24V should be commonly available, and would have an effficient rating in the 92% to 96% range.
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