What is the easiest and cheapest method for acquiring +12V and -12V for supplying LM358 Op-Amps? In my school's laboratory we are using Agilent voltage supply but that kind of supplies are really expensive for me. What is the easiest and safest method to obtain these values?
PS: I've tried using 2x9V and 4×1.5V batteries in series with a voltage divider. And making the ground of my circuit their mid point. It was useful until I was dealing with AC signals. All my signals are distorted because of it. I've searched for voltage regulators found +12V voltage regulator circuit from 24V however I can not figure out how to obtain -12V at the same time.
Thank you for your kind helps.
Electronic – +12V and -12V for supplying Op-Amps in the circuit
12voperational-amplifierpower supplyvoltage
Related Solutions
The issue with using linear regulators (LM78xx) is that each regulator dissipates as heat the power represented by current to the load x Voltage dropped by regulator.
For at least the servo motors mentioned, this would be substantial: 6 x Servo Current x (12 - 5). For one thing this will need a good heat sink on the LM7805, for another, the battery will discharge quickly as a lot of power is just being wasted as heat.
A scheme that could work is to use DC-DC buck regulators powered from the 12 Volt battery for the 9 Volt camera and the 5 Volt servo motors, and (optionally, if needed) a linear regulator instead of a buck regulator, from the 9 Volt camera rail, for a clean 5 Volt supply to the microcontroller.
Also, one would use separate 5 Volt regulators for the microcontroller supply rail and the servo supply rail, to avoid EMI from the servo motors messing with the microcontroller.
Buck or switching regulators do not waste as much power or generate as much heat as linear regulators, they have efficiency in the 80% to 92% range. Thus batteries would last longer too.
There are integrated buck regulators available as drop-in replacements for the 78xx series. See the Murata OKI-78SR series as one option, there are others as well:
I agree with others that switchers are a better choice in terms of efficiency, but they can be somewhat complicated to deal with if you're inexperienced, and there can be lots of weird effects that aren't immediately obvious (precharge sinking, beat frequencies, etc.) that can make life difficult. Assuming you've figured out your power dissipation and know how much current each rail can deliver, if the linears will work for you, stick with them (at least for the first pass).
If you're trying to achieve a variable-amplitude square wave output on your adjustable rail, the chopping may introduce noise into the main 24V rail, which could show up on the other rails. You may want to have an LC filter between the main 24V rail and the regulator input to provide high-frequency isolation, and will probably need extra capacitance on the adjustable regulator output (bulk electrolytic as well as low-impedance ceramic) if you expect the square wave edges to be sharp.
1, 5) There are some dangers with your scheme.
Power dissipation in the linear regulators will be
\$(V_{out} - V_{in}) \cdot I_{out} \$
which is significant, especially for the lower output rails. 78xx-type regulators have built-in thermal protection around 125°C, and (without heatsinking) a junction-to-air thermal resistance of 65°C/W. Your thermal management will be challenging.
Another potential problem - if the series-pass element in any of your low-voltage regulators fails or gets bypassed (shorted), you'll present the full 24V input to the output. This could be catastrophic to low-voltage logic. You should protect your low-voltage rails with SCR crowbars that can sink enough current to put the DC/DC brick into current limit and collapse the 24V rail (they'll need big heatsinks too). Fuses are unlikely to be good protection since the 24V brick likely isn't stiff enough to generate the \$I^2 \cdot t\$ needed to blow a fuse.
2) Whatever floats your boat.
4) Meters aren't huge loads. Just use one of your rails.
3) Correct - all regulators have headroom requirements. If you want the maximum 24V out, you'll need a direct connection, and will have to rely on whatever intrinsic protections the brick will provide you.
Best Answer
You don't need a voltage divider. Just connect 4 sets of 4x1.5V cells in series and make their midpoint the ground, like this:-
simulate this circuit – Schematic created using CircuitLab
When connecting batteries in series always use the same type, and don't mix old and new cells. When any of the cells go flat the others will soon follow, so replace all of them at once. If you use standard AA cells and buy in bulk they may be cheaper than any other solution.