kankki

Hi! I have had great success with this circuit :D But I have a problem. I'm trying to use relays to change the transformer taps from 24v to 12v. ( Series to parallell :) ). It works great in CV mode, regulation is fine. But when I have it in CC mode the current limiting turns off a good five seconds or so, when the relays switch. There seems to be some capacitor holding a charge for a while.. I've tried to disconnect and connect all the caps, but still the same fault. Some parts that work together perhaps? Is there any way I can modify this circuit to take the tap switching better? :) Help is greatly appreciated! :)

Thanks a lot for the help! :)

Thanks for the reply! :) I fiddled around with my board a bit and lo and behold- the hysteresis thing just disappeared. Hope it ain't intermittent. The LED is still acting weird. Gonna do more research on that tomorrow. One thing though. Is there supposed to be a ~2v drop across the pass transistors when they are fully open? I measured the base voltage when they are supposed to be fully open and it was around 1.1v. Is the On-resistance that high on the TIP3055's that I'm using :o Anyways thanks for the help so far :)

Hello again. I've taken a closer look at the PCB and fixed up some suspicious looking solder joints and so. I haven't gotten it working yet, rather it became worse ::) Heh. I don't know what I did but now the CC LED is always on and it only becomes slightly brighter when the current is limited. Something wrong with Q3? And I still have the hysteresis problem.. I've noticed something though. It seems that in order for the current limiting to kick in the output voltage needs to rise to (the normal point where it would start to drop + forward voltage of a diode, about .6 volts.) Then it starts limiting the current and works until I turn I raise the current limit again. Could it have something to do with D9? I've attached my schematic with the original schematics component names so it's easier to see. I'm also attaching my board layout. Again, I'd really appreciate some help :) Oh, and the 20k pot isn't connected to GND it's connected to "before shunt-gnd". A drawing mistake :-[

Oh ,okay I'm gonna try and look for errors on the PCB :)

Hello everyone! 8) I am having some trouble with this circuit. I have milled out a PCB according to the schematic I attached. (it's from Eagle). Soldered everything up and it kind of works. The current limiting is acting weird though. Seems like there is some hysteresis on it. When I set the current limit to e.g. 0.5A you have to go to 1A for the current limiting to kick in. After that it works just fine. But if I then wind the current limit up and down again I have the same problem. The large heatsink on the board are for the flyback diodes and the smaller one for the BD139. The transistors and large heatsink is gonna be mounted outside the chassi. I have also tried with a .47ohm shunt resistor. Does anyone know what could be causing this problem? ??? Any help would be greatly appreciated. ;D (maybe this has been answered before, but I couldn't find an answer)

Oh, okay Thanks for the help :)

Okay :) What about the large voltage fluctuations under load? Are they because of the breadboard too?

Hello! I have just hooked up a working prototype on a breadboard. Everything seems to be in order. Current limiting works and all. I have some problems though. There seems to be some 50hz junk on top of the output. Is that supposed to be there? Also some high frequency stuff. It's about 10mV pk-pk. Does the breadboard bring in some stray capacitance perhaps? Also when I attach a computer speaker that draws about 250mA peak the output voltage varies like half a volt! :o That's unacceptable :p Anybody know what that might be about? :-\

^^Thank you!

Hello! What do you guys think about this schematic I made? I would like 7 amps output. That means I should use around a .22Ω shunt resistor, right? I also plan to have 9 TIP3055's in parallel on a large heatsink. Couldn't modify it for the TO-3 one. I left that part out because this is just what's going to be on the PCB! Thanks in advance!