Need some clues

[xtrmi]

Success it works! Thanks for the help. I will post schematic later, but for now here is the deal. The power supply can go to 20v unloaded. At a load of about .8amp it can create a difference of 17v max. There is a little v droop at the max volts , but below 16v there is very little droop. I set the supply at 12v then added a 10ohm load and there was no droop. So I think its good. I really appreciate all the help. I guess I'll really have to dig deep and make sure the schematics I'm looking at are right even on the datasheets. I probably ask here.

Just curious: I put this in a old psu case so I have an 80mm fan and I want to give it about 7v. what is the best way to do that? I was thinking another seperate linear power circuit.

 

[marcbarker]

I have an 80mm fan and I want to give it about 7v.

Wire a simple resistor in series with the fan to drop the unreg voltage. You'll need a 1.3 W zener across it to clamp it for a few 100 us before it starts. Be better if you had 2 fans in series. Needs to be a fat resistor. DOn't make the resistor too small because when the fan is at speed too much current will pass through the zener.

 

[xtrmi]

Still haven't got the schematic made, but I tapped the transformer on the edges and I was able to draw 1.1amps @ 25v with a small droop. So I'm happy with this project now, but I still want to take this to school and check the quality of the output with an oscilloscope.

 

[xtrmi]

Ok, the bottom part is the linear power supply I have working good now and the top half is a circuit I'm thinking about using to output 7v to a 80mm fan. The reason I thinking of the separate circuit is so the regulator doesn't have to drop as much voltage. Plus I dont have room for another huge sink like the one I got for the variable supply part. What are your thoughts on this?
**broken link removed**
I'm also going to throw a fuse in there somewhere once I finalize the whole circuit.

 

[audioguru]

I think your power supply will blow up because you are shorting both of its grounds together but the grounds have different voltages on them.
You can join the grounds only when your transformer has two completely separate windings.

You can ground one outer wire of the center-tapped transformer winding and use the center-tap for half the voltage and use the entire winding for full voltage. But then the rectifiers will be half-wave which needs the main filter capacitors to be doubled in value.

 

[marcbarker]

Yes it 'works', it doesn't blow up, but there's some duplicated functionality going on with the rectifiers.

Nah don't put a fuse in, fuses blow.

 

[RCinFLA]

Eliminate D2 bridge and tie center tap of secondary to C8.

Both supplies will be full wave rectified.

 

[marcbarker]

Eliminate D2 bridge and tie center tap of secondary to C8.

Both supplies will be full wave rectified.

Via a single diode in series?

 

[xtrmi]

Ok, I tried this with a diode as I thought that it would work, but I got an error. Please excuse my newbness here, but I'm still trying to figure it out. The simulation with a connection directly to C8 worked. Here is the schematic. Is this what you suggested?
**broken link removed**
I'm guessing that there is no ac coming from the center tap when it is connected to the ground of the bridge? I there is no ac then why would the diode give an error? I could be all wrong here, these are my thoughts. Any suggestion are welcomed on this. I have no design experience just theory that hasn't reach semiconductors or adavnced circuits yet. I also will need a 5v supply in the future for my digital voltaga read out circuit.

 

[audioguru]

You cannot do it like that. Try it and see why.

 

[xtrmi]

I dont think I'd gain much from that. What is wrong with RCinFLA's suggestion?

 

[xtrmi]

It seems to work. I'm getting about 20v to the regulator and I'll set the output voltage to 12v so it wont have to drop so much and it can stay cool.

 

[xtrmi]

I have everything in there just like the schematic and it works good. I put it in its case and ran it for a half an hour at 8v drawing .4amps. Is there something I'm missing. I would like to a hint to what I should be looking for or hints of a failure because its seems to be ok. Is there a better way? I ask this because when I need 5v for the voltmeter circuit I plan to use another regulator connected to c8. Remember I'm a complete newb here so I just trying to feel this out and learn on the way.
**broken link removed**
**broken link removed**
**broken link removed**

 

[xtrmi]

Any comments out there on the above schematic???? I've ran it for several hours under load now and it seems ok, but my limited experience may lead me to miss something important.

 

[marcbarker]

Any comments out there on the above schematic???? .

In the box running now, it's what's drawn in the schematic right?

Some of us thought it wouldn't work and 'you'd find out why' when you tried it.

It's working OK is it?

 

[xtrmi]

Yeah its the schematic just above here and it seems to work good. The fan is going nicely and the power supply supplies power without any voltage droop when the load is applied. I've stress tested it and nothing bad happend. I was wondering if there was something more specific to look out for that I may be missing. As of now no fire, smoke, or funny smells.

 

[marcbarker]

Yeah its the schematic just above here and it seems to work good. The fan is going nicely and the power supply supplies power without any voltage droop when the load is applied. I've stress tested it and nothing bad happend. I was wondering if there was something more specific to look out for that I may be missing. As of now no fire, smoke, or funny smells.

What's the DC voltage readings on C1 and C8?

What puzzles me is how T1 powers U2, seemingly without a rectifier of it's own. I reckon you can earn some brownie points by explaining how!