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

FusionITR · Nov 12, 2009

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

$6 seems high? This is with the cheapest parts I could find with digikey. This is priced with only buying one item per part so this is not volume pricing. The most expensive part is the pwm (obviously) which has two outputs so I can get +/-3.3V rails. I plan on using the TL1454A which has a unit cost of $2.43. The rest of the $6 are passives.

Yes it does seem overkill to power 8 opamps and would rather use linear regulators. I'm not sure how to model the usage model of the opamps as far as load imbalance but I don't think it would be greater than 20mA. This is for a video processing and driving application.

I looked into just DC biasing the opamps so I can only use one rail but believe it or not the part count for that solution doubles and the price of doing that is actually more than just building a dual rail solution since you have to use large caps in between opamp stages for video applications. Plus quality suffers (line droop) if you don't use proper clamping. Basically a single rail solution for video processing is messy IMHO.

Also, building a this give me a template to expand on in the future.

indulis · Nov 12, 2009

Look into a biploar flyback with a 555 for the PWM and a TL431 for the feedback... if you can get the magnetics at a reasonable cost, you should be well under $6.

tcmtech · Nov 12, 2009

+-3.3 V / +-5 V SMPS power packs are not that uncommon. I had several I took off of some odd portable equipment a year or so ago. They looked like a standard wall power packs and had shielded multi wire cords coming out to a multi pin plug on the end.

So just find out what they cost to purchase from the dealers or the actual manufactures.

So what are you making anyway?

FusionITR · Nov 12, 2009

How would I get a customized flyback transformer for cheap? I have not seen any companies for sell standardized high frequency transformers that you can design around.

Speakerguy · Nov 12, 2009

This thread is full of fail.

tcmtech · Nov 12, 2009

Basic fail or semi epic fail?
We want to know where we stand.

Mr RB · Nov 12, 2009

Epic fail. Who the heck needs 8 opamps and +/- rails on a tiny cheap low-powered product? It's probably a 3 transistor jobbie.

FusionITR · Nov 12, 2009

Heh.

Excuse me for trying to enter the field of electronics. I guess I should sell my oscope, give up, and get a job flipping burgers. That's all I'm good for.

indulis · Nov 12, 2009

To start, what spec's do you need for the supply? Line reg, load reg, ripple, transient response, current limit, isolated or non isolated, thermal enviroment, regulatory approval... etc.

How would I get a customized flyback transformer for cheap?

Check with Pulse

http://www.pulseeng.com/

or

http://www.coilcraft.com/lpr4012.cfm

FusionITR · Nov 12, 2009

I looked into some integrated solutions, such as https://www.electro-tech-online.com/custompdfs/2009/11/LM2685.pdf. Oddly enough this is actually MORE expensive than just building one using a pwm and passives. I guess this is only a good solution if area is critical. Given that these IC's alone almost cost as much as it would in parts, $6 isn't that bad for a dual rail pwm.

Speakerguy · Nov 13, 2009

Basic fail or semi epic fail?
We want to know where we stand.

Just basic fail. So many good ideas and alternatives have been posted, yet the OP keeps looking for something else and seems intent on finding the worst solution.

FusionITR · Nov 13, 2009

speakerguy79 said:
Just basic fail. So many good ideas and alternatives have been posted, yet the OP keeps looking for something else and seems intent on finding the worst solution.

The only solution posted here was the divided down 6.6V or an integrated smps IC since I already said I won't be using an AC wall adapter.

BTW I swapped out some parts and got the unit cost down to $5.

Speakerguy · Nov 13, 2009

The only solution posted here was the divided down 6.6V or an integrated smps IC since I already said I won't be using an AC wall adapter.

The virtual ground was a perfect solution using a DC wall wart adapter, and frankly your reasons for not using an AC wall wart adapter are flat out stupid. Every major electronics supplier offers them, and about 25% of the ones I've collected from old electronics are AC output and not DC. They are not hard to find, and they do not cost more. If you are making a product that needs + and - rails using an AC to AC adapter is just a no-brainer. Buy them in bulk and include with your product. Instead you are looking at SMPS stuff - do you even know about EMI compliance or what kind of a hassle you'll be getting into if you go that route?

Like I said, basic fail.

FusionITR · Nov 13, 2009

speakerguy79 said:
The virtual ground was a perfect solution using a DC wall wart adapter, and frankly your reasons for not using an AC wall wart adapter are flat out stupid. Every major electronics supplier offers them, and about 25% of the ones I've collected from old electronics are AC output and not DC. They are not hard to find, and they do not cost more. If you are making a product that needs + and - rails using an AC to AC adapter is just a no-brainer. Buy them in bulk and include with your product. Instead you are looking at SMPS stuff - do you even know about EMI compliance or what kind of a hassle you'll be getting into if you go that route?

Like I said, basic fail.

I'll re-visit the AC wall adapter I suppose, and I agree it's a better solution for my application if AC wall adapters are both cost effective and easily replaceable. It's just for the most part nearly every single electronic device I have and know of uses DC wall supplys, even devices which I know for sure have opamps with +/- rails. I also know they use DC-DC smps. They may have bigger power requirements than me though.

I don't know how tough EMI compliance is but I don't think it will be a problem since my design is small and my smps is not offline.

Speakerguy · Nov 13, 2009

Do the video signals have any DC content? Or are they pure AC? I am an audio guy, so I don't know much about video.

FusionITR · Nov 13, 2009

speakerguy79 said:
Do the video signals have any DC content? Or are they pure AC? I am an audio guy, so I don't know much about video.

"AC", but you often have to have very low cutoff frequencies on your HPF because the nature of video can carry DC level signals + sync tips in which you have to be able to pass signals around 20hz. Also, because video isn't sinusoidal and is more like a staircase function for different color/luminance levels you have to deal with signal droop if you AC couple. It's complicated. You can get a full explanation here:

**broken link removed**

Speakerguy · Nov 13, 2009

OK, that's cool. Just wanted to make sure AC coupling wasn't an option. I will look at the Maxim link tonight.

Thanks for considering my post even though I wrote it when I was miffed. You took my criticism well. For that I am going to try and help out a little extra. I will post back more later tonight when I'm not at work.

indulis · Nov 14, 2009

The only solution posted here was the divided down 6.6V or an integrated smps IC since I already said I won't be using an AC wall adapter.

No... a 555 timer as a PWM was suggested. That is not a "integrated smps IC" and 555's cheap, very cheep!!

I don't know how tough EMI compliance is but I don't think it will be a problem since my design is small and my smps is not offline.

It doesn't matter if it's off-line or not... even DC-DC have regulations that must be met.

tcmtech · Nov 14, 2009

This all seems like a long and drawn out and overly complicated way of getting a simple power source designed to me.
I would likely just use an of the shelf +- 5 VDC SMPS three wire power pack and then use a pair of LDO regulators in the actual device to get the remaining +- 3.3 VDC power off of the +-5 VDC lines in the actual device.

Or use a 9 - 10 VAC center tapped three wire power pack and then do a simple regulator circuit in the device itself.

All you need is four regulator IC's and 6 small electrolytic capacitors to get a regulated +-3.3 VDC and +- 5 VDC output.
Being its only running at 10's of ma loads they can be standard issue three pin regulator IC's that sell for about 50 cents in reasonable quantity purchases.
Same with the capacitors as well. You would likely need four 6.3 VDC 100 - 470 uf and two 10 VDC 470 - 1000 uf capacitors. Each sells for around 25 - 50 cents in volume purchases also.
Maybe toss in a few snubber caps on each side of the regulators if needed.

kchriste · Nov 14, 2009

Considering that the output is from an OpAmp, it is unlikely that the current between the "ground" and the power rails would exceed the capability of one OpAmp. Hence this solution:

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

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