Split power suppy (+/- 3.3V or +/- 5) from +9-12V DC solution for under $6?

[FusionITR]

So I am looking to take a 9-12V DC wall adapter and make split supplies, preferably +/- 3.3V. I can do a switching power supply solution for about $6 worth of parts. Does anybody have a solution with less costs? I know you can get 3.3V linear regulators with pennies on the dollar but I haven't seen a regulator that does split supplies with the inverting circuitry integrated on the chip.

Only need about 100mA for both supplies (so 200mA total). Looking to keep part count low, as my part count with my switching power supply solution is out of control.

 

[Speakerguy]

Since the wall supply is isolated, you could generate a mid-rail voltage and use it as a virtual ground. Then just use cheap linear regulators to get +/-3.3V around the mid-rail ground. Connect the GND of any external equipment you are connecting to to the virtual ground.

 

[FusionITR]

Since the wall supply is isolated, you could generate a mid-rail voltage and use it as a virtual ground. Then just use cheap linear regulators to get +/-3.3V around the mid-rail ground. Connect the GND of any external equipment you are connecting to to the virtual ground.

I'm not sure what you mean, do you mind drawing a diagram?

 

[crutschow]

If you can get a wall wort transformer with a center-tap, then you can just use that with a bridge rectifier and two linear regulators to generate your two voltages. That way you don't have to generate a virtual ground.

 

[Speakerguy]

Diagram too hard. This part does exactly what you want, except it is only 20ma.

https://www.electro-tech-online.com/custompdfs/2009/11/tle2426.pdf

You feed it from the +9V and GND of your power supply. It makes a 4.5V rail.

Now since the wall wart is floating relative to earth ground, just treat the wall wart ground as -4.5V, the positive output as +4.5V, and the output of the tle2426 as the circuit GND. Then use pos linear reg (ldo) to get +4.5 down to 3.3, and -4.5 to -3.3. It's a totally analog/non-switching solution.

ETA: good link here
https://tangentsoft.net/elec/vgrounds.html

 

[mneary]

How accurate do the voltages need to be? How similar are the load currents?

 

[Mr RB]

Use a single LM317 to make 6.6v. Then use 2 resistors to divide it in half, giving you +/- 3.3v.

 

[Speakerguy]

Actually the best bet would be to get an AC to AC wall adapter, and then use a power supply like this:

**broken link removed**

You can simplify this somewhat (eliminate 100nf input caps if the regulator is close to the electrolytics, eliminate the electrolytic caps on the output (keep the 100nf on the output though) if you're not going to have huge load transients, eliminate protection diodes since they will not be needed at 3.3V for most regulators if you eliminate the electrolytic output caps. Very simple, but must use an AC wall wart, not a DC one.

 

[FusionITR]

I can't use the wall solution because I'm not using AC off the mains, i'm going to be using a +9V unregulated DC adapter (that plugs in to the wall) so all I'm going to have available to me is +9V and ground.

Use a single LM317 to make 6.6v. Then use 2 resistors to divide it in half, giving you +/- 3.3v.

Unless those resistors were very low, wouldn't the load current effect the resistor divider?

 

[kchriste]

Instead of using a "+9V unregulated DC adapter" use a "6Vac unregulated AC adapter" and rectify and filter it yourself as mentioned by speakerguy79.

I'm not using AC off the mains, i'm going to be using a +9V unregulated DC adapter (that plugs in to the wall)

 

[Speakerguy]

WAU12-500 Triad Magnetics Plug-In AC Adapters

Something like that, except maybe lower than 12VAC so you don't burn up so much power in the regulators. All kinds of equipment use them, I have a box full of old ones from electronics that have long since been thrown away. It just has to be AC output.

But the final circuit would be two diodes for rectification, two electrolytics for filtering/smoothing, two regulators, and two ceramic output caps, for really nice regulated +/-3.3V.

 

[FusionITR]

Hmm, call me stupid but I did not know they made wall plugs that output AC. I thought all wall plugs were pre-rectified DC. Are these common and what is the cost difference between an AC wall plug and DC wall plug? When I go to the store all the wall adapters I see are DC

 

[kchriste]

They are not very common as far as finding them retail is concerned. They are cheaper to manufacture and output more power for their size than a DC output type because the diodes and capacitor are missing. You could scam one from a dead cordless screwdriver charger, etc.

 

[FusionITR]

Yeah I didn't believe it was that common, I never see them.

This is going to be for a production product so I don't want to use a hard to find item like that. 9V dc wall adapters you can get anywhere. Any other solution when using mains is not possible? The part count, area, and ~$6 cost for the smps is not attractive, especially when I only need about 100mA for the 3.3V rails.

 

[Speakerguy]

This is for a commercial product, and you're asking on an internet forum how to design an extremely simple power supply? That's kinda scary.

 

[FusionITR]

This is for a commercial product, and you're asking on an internet forum how to design an extremely simple power supply? That's kinda scary.

I can design a power supply, linear or smps. What I was asking for is a cheap/low part count way to get split 3.3V from 9V dc wall adapter. This would be my first attempt at a commerical product.

 

[Mr RB]

I've already suggested a good cheap simple low part count way. It will work perfectly fine unless you expect to have large unbalanced DC loads. Since you haven't said what the PSU is powering or even basic specs on what quality of regulation you need etc it makes it very hard to suggest options.

People ask mystery questions expecting forum users to do MORE work guessing at every possible solution and then you want to go and make profit from our work. Thats pretty uncool.

 

[FusionITR]

I've already suggested a good cheap simple low part count way. It will work perfectly fine unless you expect to have large unbalanced DC loads. Since you haven't said what the PSU is powering or even basic specs on what quality of regulation you need etc it makes it very hard to suggest options.

People ask mystery questions expecting forum users to do MORE work guessing at every possible solution and then you want to go and make profit from our work. Thats pretty uncool.

I asked about. Problem is as you said unbalanced DC loads will change the values rails unless you are using very low resistor values and then it will always be burning power.

The application is powering 8 opamps, with 3 of them driving ~10mA (AC).

 

[mneary]

Must your 9V (-) line be considered "ground"?

Post #5 has a good solution, if your imbalance will be less than 20mA.

 

[indulis]

Your BOM cost for a sub 1W DC-DC seem way too high. What knid of volume are you talking about?