Hi Everyone, I'm new to this group and new to electronics but have been reading the messages for a while and I was hoping someone may point me in the right right direction to solve my problem. I have used cpemma's (Thanks ) PWM II as the basis of my project but I now need to swap the fan for 3 rows 6 (2V) LED's in series. I have a 30V 6VA transformer that I'd like to use to power circuit. Would the following arrangement allow the 30V source to be modulated? The 12V side of this diagram is taken from http://www.cpemma.co.uk/pwm_erg.html. I hope the above makes some sense . Many Thanks Mike.
Do your diagrams in Notepad with Courier New set as the font, then Cut&Paste into {code} {/code} tags (using square brackets). Code: ||||||||||| || || | 0 ~ | ( ^ ) | ===== | !!!!!!!! You can use my existing circuit as-is, just break the positive supply line before the top right end so everything except the load and transistor switch runs at 12V, with your 30V supply to the load, through the transistor to a common ground. "Level shifting" is easy with NPN transistors and n-channel MOSFETs. edit:the circuit you're showing will give a short through the first transistor, it needs the other resistor R moving to give both a current-limit load on the first and base current to the second one; then when first transistor switches on it will turn the second one off. Only needed for heavy loads (over about 1A) and a single MOSFET would be better.
Hi cpemma, Sorry for my slow responce. I'm having a little difficulty in visualising the circuit your proposing could you do a quick little diagram? Thanks for the help Mike
its hard to tell for sure what cpemma is thinking but it sounds a little something like this: EDIT: oops, it looks like i put a P type instead of an N type just flip the arrow around and youll be alright
Note the 30V ground and 12V ground are joined. Actually, if your 30V supply is accurate you can run the whole thing on 30V, the max voltage rating for an LM324 is 32V. (Bit close though, and the capacitor C2 would need the higher rating). For the leds, go play with linear's calculator wizard, it says a string of 13 with a parallel string of 5 leds would be most efficient.
Yeh I should have mentioned that the transformer is unregulated(?) therefore I believe it's output is higher (about 42v). OK I think I've got it, he components will be here tomorrow so I'll build it and see how I get on. If I get stuck I hope you won't mind me dropping a line again Thanks again for the help Mike Edit: Unfortunatly I'm stuck with 3x6 LED's, I built a cabinet for the living room about 8 months ago with very little understanding of electronic's (not much more now ) and wired it all up 3x6. If I'd studied this 8 months ago I would have gone a parallel and series route and a 12v source. I made a rod for myself
