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Getting the most out of a MFOS Wallwart Supply...
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Getting the most out of a MFOS Wallwart Supply...
Revok
...or building something entirely different. I'm open to all criticism and advice. I have a MFOS Wallwart Power Supply board and I'm trying to get the most out of it. I don't really care if that entire idea is scrapped as I'm mostly interested in learning right now.

I'm relying on the Power Supply Design Chapter from the Musical Engineer's Handbook and also referencing Understanding the Terms and Definitions of LDO Voltage Regulators from TI. Are there some other sources I should be looking at as well?

Something I'm feeling like a dumb-dumb about is the current rating of a center tapped transformer. If I am looking to provide the maximum amount of current per regulator do I need a transformer rated for the sum of the current supplied by the two devices or the maximum current of the higher rated device? Also. once the voltage is rectified, does the current = transformer current/1.414 as I've seen in some posts online?

The other confusing part is obviously the heat sink requirements. The power dissipation I calculated in the image below is at the peak voltage but the Musical Engineer's Handbook states:

"For low currents, the input ripple is small, and the power dissipated by the regulator is essentially (Vin-Vout)*I. However, we discussed earlier the fact that the actual input voltage in a proper design has a waveform something like the one shown at the right[image from text shows ripple with top labelled Vpeak and bottom labelled Vdrop]. This means that the average power dissipated is more like one-half of the larger value."

I'm not sure what "one-half of the larger value" means. Is there a proper way to calculate this outside of guessing the power dissipated and using the touch test? I've seen some equations for calculating the the heatsink requirements but they really hinge on the power dissipation being correct.

Please feel free to point out and flame mistakes I made in the image below. I am a humble noob.
Outside of kludging two more diodes onto the board and using a center tapped transformer, here are the changes:

U1 = L78S15CV
U2 = LM7915CT
C1-3 = tbd...
C4 = 0.33uF (specified in datasheet)
C5 = 0.1uF (specified in datasheet)
C6-8 = tbd...
C9 = 2.2uF (tantalum specified in datasheet)

Here is Ray's schematic:



Here is the changed schematic with notes:

roglok
how are you going to install adequate heatsinks for the regulators? the pcb doesn't really allow for that (unless you use flying wires or move the regulators to the bottom of the pcb...
Revok
roglok wrote:
how are you going to install adequate heatsinks for the regulators? the pcb doesn't really allow for that (unless you use flying wires or move the regulators to the bottom of the pcb...


The flying 18 AWG wires to small boards with the regulators, C4, and C9 is what I was envisioning.
roglok
ok, then. i like to refer to oskar salas' educational posts over at e-m for power supply derating and calculations:

http://electro-music.com/forum/viewtopic.php?t=51694&start=0&postdays= 0&postorder=asc&highlight=

i think you'll find the answers to your questions in there...
Revok
Thanks, roglok. This looks really helpful!
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