240ac to 10dc

I think I might be able to help you out on this, but I need to know why you want to make this, mostly to figure out what kind of load requirements you have? Such as; are you building this more for a low voltage dc reference, or to actually supply power to a load? Because there’s quite a few things that I personally think are completely wrong with this design, but part of that could also depend on what your intentions are for this circuit.... also, I’m pretty sure you are planning on hooking this up to mains power for that +300 Vac input... is that correct?? (Because if so, this circuit would definitely not work)

Im trying to fix a bench power supply i fried. I have been traceing out the circuit paths for a few days to see if i can isolate the problem the power supply has a lit of sentimental value. Output is 0-5v 5w max. There is also a more current limiter but i didnt have room to draw it. As well as complex circuits after the secondary of the transformer. I recorded the values of the transformer after desodering it and hooking it up to my lc meter

Forgot to mention. The 1k resistors around the transformer are not part of the real circuit. Im trying to get the app to quit crashing

Ok cool... So first, is the transformer damaged at all? If not, than you will definitely need to reuse that or at least salvage one from somewhere else if you have to. If it’s still good, than next let me know if it has any center taps, and measure the resistance of both sides of the transformer; if there is a center tap you should see two low resistances, and then a larger resistance somewhere (which should be the addition of the two resistances forming the center tap; for ex: 1.2ohm + 1.2ohm = 2.4ohms-(the whole coil)

The transformer has 4 taps. The primary and secondary. Both read the same resistance of .48 and almost the same inductance. 1.3ml and 1.31ml It looks to be just a single layer coils on top of each other on a rod. 22awg. It actually looks more like an inductor than a transformer. It shows a little heat discoloring. And is about 1.25in long. Maybe 1/4 thick. When i opened the unit the fuse to the main was blown and the zener shattered. The 555 is damaged to. Im guessing when the zener failed, line voltage hit the circuit. The resistor values are what ive read off the components on the board and rounded to the closest common value.

Just found a missing component. Looks like there is a series cap in line with the mains. I cant tell what size. But shouldnt be hard to figure out

That actually kinda makes sense, because I was thinking, the transformer would have to be supper big to actually be able to connect the primary directly to mains because of how low the current has to be... but, with this the right size capacitor, it would have enough reactance at such a low frequency that the primary inductance wouldn’t have to be so large (large as in like a crazy 12 Henry’s, based on the low current requirements on the secondary). In this case, we need a reactance of about 4800 Ohms total, and say with a theoretical primary inductance of about 0.5H, then the primary’s reactance would be about 188 Ohms at 60Hz, which would mean we still need 4612 Ohms of reactance from the capacitor, which would equate to (1/2(pi)fc)= 575nF. But then that would also mean that the voltage across the inductor wouldn’t be a full 240v any more right? I’ll have to do a little research to make sure first, but after we figure out the transformer part, the rest I know much more about and should be easier

60,000 hz 5v across the transformer. This board is so badly damaged i didnt see this cap hiding next to the fuse.

I have come to the conclusion that my transformer is damaged and giving me the wrong L readings. It looks as if the cap on the mains provents the second cap in the circuit from reaching full charge, their by limiting the voltage to the 5v regulator. But the transformer isnt consuming enough power to handle the circuit at full load. Im going to unwind this thing and check for damage

Ok sounds good, let me know what you find. It may be a lot easier to pull a transformer out of another device, but if you do find something, carefully check what the actual voltage is across the primary and secondary with it plugged in before you de-solder it. I believe you could also find a doorbell transformer sold separately at Lowe’s home improvement (I have seen them carry transformers individually for doorbells, in the light bulb isle I think)

I bet you could also find a good transformer inside a drill battery charger.... but also, there are a lot of phone chargers that are designed to handle more power than your bench power supply even, but I think the transformer circuit in those might be more complicated, not 100% sure though to be honest, like I said previously I don’t know as much about transformers, but I do know that you can’t just hook up any transformer to mains unless it’s designed for that cause it’ll draw tons of current.

It's my dads old ps, he gave it to me years ago. Im trying to breath life back into it. There is just so much damage its hard to identify what the components are and their values. When i use a usb wall adapter and feed the isolated side she comes back to life and works like a champ. The more i experiment with this cir uit the more surprised i am that it took this long to blow up

This entire part of the ps takes up about 1x2 inches of the board. This transformer is tiny. Little bit bigger than a 10w resistor

Man, I’m sorry that happened, I’ll try my best to help you get it bring it back to life... we’ll get it working one way or another!

Hey did that power supply have a little square wave function generator? Because I really cannot make any sense as to why else the 555 would be there..?

This thing is as old as me. Early 80s. My dad used it to fix vhs players. There are a lot of wave form adjustment. Mostly for adjusting color and v-shift. And some driver outputs for running dc motors but each perriffial a different supply rail. This one is magnetically coupled to 6 other circuits. Im thinking the 5w is really .5w. The old silk screen is hard to read. But after following the circuit paths it ties directly to the coaxial output that hooks to an old school tv.

Redrew circuit in a more standard format, as far as I know I have not changed any thing other than to get rid of the meters to make more room. It looks to be just a standard power supply. 1 power rectification. 2. 5 V zener power 3. 555 driving push me pull me output stage... a few queries, why only 5V for switching power supply? Where is the output feed back. & With no feed, where are the power storage capacitors? Also the output transformer should ideally be a step up device. http://everycircuit.com/circuit/4589834096869376

BillyT, do you know why the 555 is in there, and why the transformer is being driven from it instead of directly to mains? Why make a pulsing several kHz square waveform to amplify and feed to a transformer? That part doesn’t make much any to me

This is personally the way I would go about making a 240v ac power supply, although it definitely could be improved, this works pretty well. I’m case it’s confusing at first, The 240v input comes from two 120v ac sources 180 degrees out of phase with each other.... one source simulates the “Hot” while the other simulates the “Neutral”. http://everycircuit.com/circuit/6063146375315456

Normally the feedback would be fed back thru pin 5 of the 555, it is a common way of working these types of circuits, the voltage is picked off a voltage divider and sent to control the 555 to back off when the output is too high and speed up when the output is a little on the low side.

Your circuit basics are not too bad, just remember to run the program and check the different voltages when you design it. A few points, the voltages should be peek, and those EC centre tapped txfmrs are not useful for v-0-v.

Check this http://everycircuit.com/circuit/4559166910496768

@PrathikP, a lot work has been put into that circuit..

After a lot of thought, this circuit really needs a fair bit of work, starting with the input voltage dropping capacitor.

Thanks for the redraw, makes it a lot easier to see whats happening. The cap next to the mains real value is unknown. And the wattage is labled 5w on the board but i think thats is not accurate. The transformer soaked overnight and i after unwinding it ive got 32 turn primary and 32 turn secondary around a solid 1/8 in core material. But the math works out for the 1.31mL. With an iron core

Once again thanks for all the help. Here is some more information that might be useful IC=7555IBAZ. All the transistors are titip29c or titip29o.(very small print)

I still can’t figure out why this circuit is driving the transformer from an smps signal... and looking at the load (the 2.5 ohm resistor I’m assuming); please tell me if I’m wrong, but it sure appears to do a horrible job of actually creating any steady output voltage, it’s just a noisy 68kHz signal and its output voltage drops pretty low even with not that big of a load... Am I just seeing this totally wrong, or is it really just a bad design? It makes no since to me, and especially considering this circuit doesn’t even use the transformer to step down any voltage, but instead puts a lot of stress on the transistor voltage regulator??

I really appreciate all the help but its time to finish this repair. I know if i apply 5v to the cap at the secondary of the transformer the tv tunes in amd i can watch the output if the vcr being tested. Im just going to slap a modern 5v power module in there and abandoned this effort.(i got more of an education over the last 4 days than i bargained for) besides, if the original design blow out, then it wasn't that good of a design anyway

Good luck man! Please let me know how it turns out after you finish all the repairs, hopefully it works well! Also, don’t hesitate to ask if you still have any questions

Sounds like the best thing to do.

Works like a champ. Had to add a 300k resistor in series with 5v+

Nice! Glad to here that! What kind of 5v module did you use?

I stripped a usb wall charger. That worked great to test the unit but caused major contrast problems on the video out. Adding the resistor to lower the current fixed it

Ok sounds good, interesting