AArgh! Power supplys!

[audioguru]

I checked the voltage after the rectifier and it says 12v (ish).

My DMM very accurately shows the exact voltage, current, resistance, capacitance and frequency. Yours must be crap. Do you use a clock that tells you only if it is daytime or night-time?

 

[philba]

My DMM very accurately shows the exact voltage, current, resistance, capacitance and frequency. Yours must be crap. Do you use a clock that tells you only if it is daytime or night-time?

I didn't see a smiley here so since you are nit picking, how do you know your DMM is accurate? Just because it shows lots of digits to the right of the decimal point doesn't make it accurate. It simply shows a precise number within the accuracy of your DMM. beware false precision.

what do you call it when it says 11.9874? I'd say 12, does that make my dmm crap?

 

[Paul Obrien]

With each LED requiring 20mA that means you need 1Amp to power them all at the same time, so that means 5watts of power is required. To reduce 25V @ 1amp to 5V @ 1amp means 25watts has to be inputted, 20 watts of heat to dissipate, 5watts for LEDs.
https://www.electro-tech-online.com/custompdfs/2006/08/LM2576PDF.pdf
**broken link removed** while switching regs may seem scary, they are quite nice people once you get to know them, and while they require a few more parts the saving in heatsinks and power offsets these.

 

[philba]

If the power dissipation is too much for the regulator, just segment your LEDs into, say, 4 groups and power each group seperately with a different 7805. thus, you've cut the power dissipation for each 7805 by 1/4. that may be cheaper and simpler than building a switcher (though not as intellectually satisfying...).

 

[HackinBlack]

I notice you mention capacitor discharge unit;do you mean a motorcycle type CDI ignition unit? if so be VERY careful when playing with these things,they can kick out several thousand volts in an instant;and if your holding it at the time OOOHHHCCCCRRRRIIIIISSSS...lets just say it hurts a little.i would say the current draw would be more like 4amps when the'yre charging,all depends on what coil they are feeding.wossit for anyway?you've got me interested.

 

[audioguru]

how do you know your DMM is accurate?

It is made by Fluke so it is very accurate. My cheap DMM has an accuracy nearly the same.

I mentioned accuracy because a 12VAC transformer makes a rectified and filtered 15.5VDC (or more without a load) which is not "12v(ish)".

 

[angie1199]

AudioGuru - yep, ish, 12V and a few decimals which are unimportant when I'm trying to get to 5VDC and as this 12v transformer is apparently more than a 12V transformer judging by all the diodes/resistors/at least 1 capactior and two NPN transistors in it and keeps switching on and off under load from the 5VDC regulation circuit!

Philba - what do you call it when it says 11.9874? I'd say 12, does that make my dmm crap?

Exactly. I'm no lectronics genius, just need something simple as a first project, but that seems too difficult for some....(and I don't mean you)

Philba - If the power dissipation is too much for the regulator, just segment your LEDs into, say, 4 groups and power each group seperately with a different 7805

Thought of that but my own logic dictates there must be a way to do it other than four seperate circuits.

Paul Obrien - Thanks for that. I'm definately learning fast here. I didn't even know what a voltage regulator did till last week! and now I'm looking for a LM2576. I found a 2575 but that's only 1A. The search goes on. The circuit is no problem for me to build, it's just getting the part.

HackinBlack - No, it's smaller but I don't touch the outward wires anyway. I learned long ago not to trust wires even when you think the fuse is out