power supply options

[4krow]

Now, I realize that posting this topic here is a bit of a stretch, seeing that it has nothing to do with BH products. It might tell you all however, that I trust your opinions more than others. So the situation that I have is about an old piece of Carver equipment, that in this case, I am more curious about than anything else. Looked to me that the transformer used in this piece was pretty weak by anyone's guess. Just happened to have a good sized tranny that would work just fine. Turns out that in the new circuit, it produces 25V rather than the required 12-15v. Of course the easy answer here is about a 10M resistor in series. But then I thought, would there be better ways to do this(price not being counted)? I mean, there could be a small step down tranny, or voltage regulator. Fact is, I have made an enclosure that the new tranny sits in, so there is new space in the old circuit board for something.

    Just couldn't leave well enough alone....

[Jim R.]

Greg,

No center tap on the secondary -- or perhaps the ability to wire the primary for 120 or 240 v?

Any idea how much current the piece draws -- the droppig resistor is not my fave way to deal with this and some sort of simple regulator circuit that can handle the current (and assuming the piece of gear has the room for the heatsink if one is needed) would be the approach I'd take.

Of course if you can wire the primary for 240 v then that's the easiest way.

HTH,

Jim

[4krow]

Wow!  OF COURSE! I promise, I was better in school than this. Thank you very much, and yes, it DOES have a 220 winding. Now, back to undoing some of my madness....

[4krow]

Ok, that was still worth a shot, but the fact is, the 220v wiring only dropped the voltage about 2-3 volts. As to what the circuit draws, that is about 10 watts. Being so little, I might try a resistor on each leg of the secondary. I meant to say each hot lead, if you will. Looks like 10 mega-ohms will be in the ballpark, and maybe only a 1 watt size? Probably carbon type....your thoughts.

[Jim R.]

Greg,

Something doesn't sound right there.  Maybe the primary windings need to be in series for 220?

-- Jim

[Wanderer]

What piece of Carver gear do you have?

The Magnetic Field Power Amps used a very different switching type power supply and I am not so sure tossing a lot of iron at it in the form of a big transformer is going to do much.

I don't know - I am just guessing - I have been wrong before


Here is the patent

http://www.google.com/patents?vid=4218660

  

[4krow]

Actually, I own an old Carver C-9 unit that is intended as a signal processor. It draws very little wattage as proven by the 1/4 amp fuse. It lists the circuit draw as 10 watts.
  I will have to take closer look at the transformer that I intend to replace the original. Worst case scenario, the old tranny can be switched back as intended. It's just that you can see corners being cut in this first version of the C-9, and power supplies are a good place to start. I'll keep in touch. Also, I will give you the two readings from the windings on the primary side.

[4krow]

I just called my lawyer, and he says 'No you don't have to confess every stupid thing you have ever done'.....Whew! which means I am not even tempted to tell anyone the reason for the slight voltage drop with the '220v' wiring. Isn't it more important that progress was made? That I as a basement electrical tech was not harmed in any way? And what a triumph that the output is now at 11.63 volts, just as it should be. I can rest tonight with out 'what if' on my mind until 2 A.M.   

      Thank you, and good night.

[Grainger49]

As Paul Joppa mentions elsewhere, transformers put out name plate specified voltage at name plate current.  There is an internal resistance that comes into play here.  As the load decreases the voltage floats up (less I²R drop in the windings).

Luckily you can shed voltage that isn't needed, but you can't put it in if you don't have enough.

[Jim R.]

Greg,

Thanks for at least letting me know -- that one had me puzzled.

Glad you got it all worked out.

-- Jim

[VoltSecond]

10W at +/-12V is 0.41 amps

1. Snub the transformer secondary.  Pick a cap with a voltage rating twice the DC output and place it from each transformer secondary to the secondary ground.  This should be two >25V film or >25V X7R capacitors in this case. (one cap for +12 and one cap for -12V)  I'd use a 1uF cap. Each cap sees 12.6V peak. The 25V rating is for margin and to control self heating losses if you buy a ceramic cap.

2. Make the diodes >40V schottkies. If there are 4 diodes, a 1 amp diode would work or something like a MBR360.  Note: I've actually soldered schottkies across the existing diodes in a CD player MOD I did.

3. Bias the output of all the op-amps up to ~1mA to the negative rail with a CCS or a resistor to a filtered -12V.   Note: You can also use the positive rail for the CCS. 

Below is a 2.2 mA CCS I used in an ArtDIO.   I've used this same CCS in other circuits too. Make the 280 ohm resistors 620 ohm for 1 mA. With 75K, the circuit works down to around 10V.  0.47uF is the smallest I'd use for C1.  100uF the biggest.  C1 should be rated for 10V or higher.

http://www.siteswithstyle.com/VoltSecond/ART_DIO_MODs/ART_DIO_MODs.html
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.siteswithstyle.com%2FVoltSecond%2FART_DIO_MODs%2FART_DIO_IS_SCH.gif&hash=12e4cc3a153f9549bd956cebe8e8c11bc1c7c328)

[4krow]

Absolutely right sir. Thank you very much for taking the time to post the schematic. And the ideas are just what I have been looking for. Before I go too far I first have to find the poor component that didn't ever see 25vdc coming...snif snif     Looks like it may be an output transistor, as the integrated circuits all made it through the storm. This is an easy circuit to work with, so it is helping me learn some more basic stuff.

[4krow]

Just keeping in touch about the project. Surprisingly, all the transistors survived, and instead I found a solder bridge(no surprise) that I made while desoldering. The product that I have been using(chip-quik) is a really great product, but DO watch out for solder bridges. Amazingly, the C-9 sounds much better than I thought with the power supply upgraded, so I may not sell it after all.