the coil which doesnt reduce voltage is probably an inductor, and yea 3A should br more than enough for most hobby circuits.
it is a switching psu the theory is that at higher frequency you can convert more power with a smaller transformer this is important i have a small power supply the transformer is rated at 25v at 2 amps and it must we 3x3x3 inches and rather heavy- there is no room for those in a psu so they uase a high frequency like 400-1000hz so they can use a smaller transformer so first they rectify it then thy switchit on and off at the highfrequency and pipe it through a second transformer to get the right voltages they can also vary the out put voltage by changing the switched voltage so they monitor the out puts and feed it through a special chip containing a precision voltage regulator and comparator.
Thank you for the explanation…I probed the other trans just to the left of the big heat sink and got the stepped down voltages but they are too low for what I need. Another question if I may; why does the bridge show 116V on the AC side and only 1V on the DC side. My multi is a Wavetek DM10XL, not the greatest but not junk either. Thanks for everyone’s help on this project. I think it best to build a whole new circuit, step down trans and all and tap into the mains.
Can I use -12V line and the +12V line to give me the starting 24V for the variable circuit? Thank you…
Oh well...back to the drawing board (kinda) Thanks acrimonious...you told me this wasn't going to be simple. I do have another question or two (for now). The 3A adj. circuit calls for a 4K7 Trimmer...am I right in assuming a trimmer is a pot and 4K7 is the top resistance of the pot?
The resistor before the pot is .5W. Would a 1/2 W pot be ok or should I get a 1W? Should I get a copper winding type or something else?
The circuit is 1.2V-25V/3A…does the meant I need a 75W pot... The biggest on in Digi-Key is only 5W. I gota be doing something wrong…
You've just discovered why its impractical to use a pot directly for voltage regulation, you could use a voltage divider circuit with a highpower transistor (such as a TIP1xx) to provide an output, however that has a very steep Vbase / Vemitter curve (as transistors are primarily used for switching). The best way to do it is with a variable voltage regulator.
Sorry i thought you were tryign to use the pot straight for regulation, missed your circuit. The pot for that doesn't need to handle much current at all since its just providing a refrence and not dealing with the full voltage.
It looks like you're using a momentary switch between the green wire and ground. If so, it won't work. You need a latching switch.
I was starting to wonder what was happening. So…how do I figure out what size pot I will use? Which material is best?
Found two types of pots that max out at 5K one is wirewound at 5W and the other is Cermet at 2W. Would it be ok to use the 2W cermet?
Well this has sat around long enough…the resistor in front of the trimmer is .5w so I’ll get the 2W pot and hope for the best.
Looky what UPS brought me...the new case for my mobil PSU. I decided to split them up. Time for a move out of electronics...I guess.
Do the maths. With the pot wound right down you've 2.5v over 440R, 5.7mA, plus a couple from the regulator, call it 8mA tops. RI^2 is 0.015W at most for each 220R resistor. With the pot turned up, input permitting, you'd get 30v output, 1.25v over R12 and 28.75v over the bottom 220R resistor R11 and 4700R pot, so current is about the same, a 1/4W pot and 1/4W resistors are more than ample. If you've only got 12v max input from a computer PSU a 4k7 pot is too big, as the output can only go to about 10.5v. Read the LM350 datasheet and do some more maths.
