Power supplying question

[Dr.EM]

Hi. I need a dual polarity -9 - 0 -+9 type power supply. I don't trust myself to build my own, so I would like to derive it from a plug in unit. The output is very low current, it is intended to be powering op-amps and 7555 cmos chips, aswell as some discrete transistors (tis97 mostly) in a synthesiser. I have found a switch mode power supply which is rated at 24v:

**broken link removed**

And I thought of the below circuit to derive the twin 9v. I am worried about noise though, with the SMP which apparently can be noisy, and the zener diode. Any thoughts? Can anyone find a "normal" type dc supply of either 18 or 24v?

 

[Nigel Goodwin]

The 100K resistors are FAR too high to generate a split supply, unless you are drawing exactly the same current from the positive and negative rails.

If you want to generate a split supply in this way I would suggest you use an opamp to generate the spit supply - it's a common technique, there are examples everywhere, and it's often used in commercial equipment as well.

If an opamp won't provide enough current to balance the two rails, you can add extra output transistors to increase it's current handling - again, a common technique!.

But probably easier, how about buying two cheap switch-mode regulated wallwart type supplies?, set them both to 9V, and use one for each rail?. Ones I have here are rated at 1A current.

 

[Dr.EM]

Think that last suggestion might be best actually, wasn't sure if you could do that. Are these ok for the job:

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Note that they are unregulated, if regulation was needed, I suppose I could install some regulator ICs into the unit?

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Would I need to use a +8v and a -8v, or two +8v ones?

 

[Nigel Goodwin]

Think that last suggestion might be best actually, wasn't sure if you could do that. Are these ok for the job:

**broken link removed**

How about using regulated ones instead? (as I suggested!), try from the same site as you listed.

 

[Dr.EM]

Thats very expensive! :lol:

If I could use the reg chips, it would be a lot cheaper.

 

[Sebi]

Poor man's supply...