0

I'm planning to make a variable split power supply from a 9V DC adapter.

So far I found two easy approaches. One is with a 555 timer here:

Making dual power supply from single for opamp?

And the other is just with two resistors and capacitors here:

What are the ways to make a dual power supply from a single voltage source?

I have two questions:

1-) If I want to make a variable split supply between 5V to 12V which is controlled with a poti, can I use the same circuits without changing any component values?

2-) How reliable and regulated 9V DC adapters in general? I mean should I use a voltage regulator between the 9V adapter and the splitting circuitry?

Community
  • 1
user16307
  • 11,802
  • 51
  • 173
  • 312

2 Answers2

1

I'm planning to make a variable split power supply from a 9V DC adapter.

You are going to be disappointed if you expect more than a few mA from the negative supply and also you will need a buck-boost regulator if you want to create 5V to 12V rail from a single 9V adapter.

Same is true for creating -5V to -12V - you need a much more rigorous approach than either a 555 negative voltage generator or a couple of resistors and capacitors.

How reliable and regulated 9V DC adapters in general?

You can get very reliable ones and can can ones that are really crappy. Best rule here is do some research and pick a reputable supplier.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • omg cant i just buy one? the power supplies sold in market are single rail. i need split one for powering small projects – user16307 Dec 14 '15 at 09:19
  • Buy two and wire the outputs together to give you 0V, 9V and 18V. They have isolated outputs so they can be used this way. The 9V centre becomes the new 0V giving you -9V, 0V and +9V. – Andy aka Dec 14 '15 at 09:26
  • One's plus terminal will be connected to other's minus terminal right? I think for opamp rails I can always use fixed split 9V right? So no need to make it variable – user16307 Dec 14 '15 at 12:46
  • Correct and making it variable is a bonus but not needed for 95% of op-amps. You can always use a 7805 and a 7905 regulator to give +/- 5V and again that's suitable for the vast majority of op-amps. – Andy aka Dec 14 '15 at 12:48
  • how much should i concern about power supply ripples feeding the opamp rails? Would 8V regulators after 9V reduce the ripples? – user16307 Dec 14 '15 at 13:15
  • Ripples are at 100Hz for normal power supplies and these are quite easily dealt with by op-amps but, on switching supplies, although the ripple is smaller it is a much higher frequency. It it generally better to use linear voltage regulators to be on the safe-side but you need to find low-drop-out regulators because 9V -> 8V is too little V(difference) for a 7808 standard regulator. 7xxx voltage regulators need 2V headroom guaranteed. – Andy aka Dec 14 '15 at 13:19
  • how bout using caps with 9V supplied u mentioned? Would they help the ripple issue? – user16307 Dec 14 '15 at 13:24
  • 1
    I think plan on it working without regulators and then add caps if it's a bit noisy then add regulators as optional lower voltage outputs. – Andy aka Dec 14 '15 at 13:28
0

If you want to create a power supply for a general load (like for lab purpose and not only for a specific circuit) only the first solution is possible.

1) The 555 circuit does a simple inversion of the voltage. You can not influence it. If you want to change the voltage, you need to add a regulator in line (I would do that anyway) or find a different circuit.

2) I never had problems, but they may have a lot of ripple. If you have an oscilloscope, measure it.

Botnic
  • 2,245
  • 2
  • 19
  • 33