Advice on hacking a power supply

Paul Baker

I have an LCD screen that needs -22V, the flyback DC convertor I assembled doesn't work and I dont have the equipment and knowledge to·fix it (I will do a perfboard assembly of the circuit in case parasitics are affecting its operation). But I am considering trying to use an off the shelf supply and modify it to my needs so I can continue working on it. Allelectronics has a switched supply that has 5V, -48V and 48V. I am thinking of using a -24V regulator but most have a minimum voltage greater than -48V. So im thinking of using some rectifiers to drop the initial voltage, then a couple more to drop the final voltage to -22V. What are peoples thoughts on this?


-48V -|<|-|<|...[noparse][[/noparse]-24V Reg]-|<|-<|- -22V

Post Edited (Paul Baker) : 4/17/2005 10:10:39 PM GMT

SteveW

Could you just use a little 5V -> +-12V converter?
http://www.farnell.com/datasheets/191.pdf is a nicely isolated little unit, that I've used for exactly this.
(edited to point at the dual output version - oops)

Steve

Post Edited (SteveW) : 4/17/2005 10:33:17 PM GMT

Paul Baker

need -22 Volts, dont see how that would work.

SteveW

The output is isolated from the input (to 3kV), so just tie the +12V output to ground, ignore the ground output (it's now at -12V) , and take your -24V from the -12V output. You can regulate it if you like, but I've not found that (STN / FTN) LCDs are that fussy. (A bit of extra capacitance to remove any remaining ripple will stop the LCD shimmering)

Steve

Chris Savage

Paul,

·· How much current do you need?· If it's less than 1A, you could use a standard transformer, bridge rectifier, filter caps, and a -24 volts regulator (7924).· This is of course, if the thing will handle the extra 2 volts, if not add your diodes...Think that will work?

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Paul Baker

I need less than 1/2 Amp, I could go that route but I am concerned about weight. The application I had in mind will be wall mounted. It will be a SX connected to the phone line listening for incoming calls, It will display the caller ID on the LCD screen so that it would be visible from the living room couch and Im thinking of getting an EMIC text to speech module so that it will announce whos calling. The phone is set up in the kitchen and on the other side of the apartment from my room, my roomate gets alot of phone calls and Im tired of sprinting from my room to check the caller ID to see if it is for me. Im also contemplating using a wireless module to relay the caller ID info to another module connected to my computer so that a little program would pop up the caller ID but thats a future feature.

Also what is the rule of thumb for correlation between the AC RMS voltage and the rectified and filtered DC voltage?

Chris Savage

Paul,

·· One final thought on your project...What about a DC-DC converter designed for that voltage?· That is a small package and can handle a reasonable voltage.· As for your other question, if this is what you're looking for, the voltage after rectification and filtering is around 1.4 times the·RMS voltage.· If memory serves (It IS Monday!).

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Paul Baker

That was my original plan, but the circuit doesn't work.

SteveW

Can you post your circuit, and I'll have a look?

(That Farnell part I suggested will do exactly what you want, for less than $10, straight out of the box. How hard do you want to make your life?)

Steve

Paul Baker

Ill have to wait until I get home to get which flyback convertor I have, I know I constructed the circuit exactly as described in the spec sheet for the part. BTW I also need 5V, whould I just stack that on top as well?


·- 12V + - 12 V + - 5V +
························^
························· Ground

Post Edited (Paul Baker) : 4/18/2005 5:25:33 PM GMT

SteveW

Ah - I thought you already had a source of 5V. If not, the 5V -> -24V converter may not be ideal [noparse]:)[/noparse]
What are you running the CPU from?
(In general, for stuff like this, I'll try to use a single rail supply, and generate everything I want locally - it saves conectors, switches, and hassle, and lets me sequence power supplies the way I want, if I need to. (If you look at your LCD data sheet, it almost certainly requires a careful power supply sequencing, with the -22V only applied when all the other supplies are stable and the data is streaming properly, or you'll risk damage)
The 5V -> -24V DC-DC makes this easy, because it's isolated. It's easy to wire it's +5V input pin to +5V, and ground it's 0V input with a small FET switch to turn it on when appropriate. (It's easier to switch a ground rail than a +vV Rail...)

Steve

Paul Baker

Ok, the chip I have is the MAX749 ( http://pdfserv.maxim-ic.com/en/ds/MAX749.pdf·), I have to check the circuit, but I think I am using the circuit on page 6.