OK here we go. Don't be freaked out by the amount of detail here. I intend to use this as a reference post that I can use to answer future similar questions.
Here's my suggested circuit.
Many of the component values aren't specified on the diagram because they depend on your requirements. I've explained them below, and provided lots of links to specific recommended components on the Digikey web site.
Your existing Magnetek 6415 is on the left. I've shown the relevant parts of the internal circuit. The positive DC output is on the blue wire.
Any appliances that draw a lot of current and that don't need a clean DC supply should be powered from this point. I've shown this with an arrow marked "UNSMOOTHED". This will minimise unnecessary voltage drop and power dissipation (heating) in the regulator.
The other dotted box represents the part you will need to build - a circuit board inside a plastic enclosure, with holes drilled for ventilation (or you could mount the heatsink on the outside).
In this description I will be mentioning I
L a lot. I
L is the maximum current, in amps, that you will be drawing from the output of the regulator. If you have no idea how much current your appliance(s) need, you could assume 5A for I
L.
F1 and F2 - PTC resettable fuses for input and output
Input current flows through F1 which protects the source against a fault in the regulator circuit.
Output current flows through F2 which protects the regulator against a shorted output. F2 should have a slightly lower rating than F1.
You could use a time delay or slow-blow fuse in these positions, but I've specified a device called a PTC resettable fuse, which has comparable performance but doesn't need to be replaced, for both positions. Just remove the fault and it will cool down and "reset". Very convenient.
DS - Schottky diode
The input current then passes through DS, which is a Schottky diode, with CS connected straight across it. Schottky diodes are like normal diodes but they have a much lower forward voltage, so they don't waste much power, and they don't heat up as much as normal diodes do. But they are susceptible to damage from short disturbances on the incoming voltage; CS protects DS from these disturbances - see later for component details for CS.
I have suggested a number of alternatives for DS. Choose one rated for at least 50% more than I
L. You can choose one with a much higher rating than you need; an overrated diode may be easier to mount onto a small heatsink (some kind of heatsink will be needed if I
L is more than 1A) and will reduce losses.
DS is needed so that positive sections of the "camels' humps" can charge the smoothing capacitors that follow it, but during the gaps, those capacitors will not discharge back into the loads that are connected onto the unsmoothed voltage rail.
CA and CB - smoothing capacitors
CA and CB are smoothing capacitors. I've shown two of them, but you may only need one, or you may want more than two, depending on how your requirements fit with the available components. These capacitors charge up to the peaks of the unsmoothed voltage rail, then hold the voltage during the gaps between the peaks. They smooth out the voltage at the input of the regulator so it stays high enough that the regulator can deliver a continuous output voltage. Capacitors in parallel add together, so the total smoothing capacitance is the sum of CA, CB, and any others.
The required amount of smoothing capacitance depends on I
L. I recommend at least 3300 µF per amp of output current. For I
L=5A I recommend at least four 4700 µF capacitors in parallel.
I've suggested a range of values from 3300 µF to 10,000 µF from Digikey with a 25V voltage rating. The best brands are the Japanese ones: Rubycon, Nichicon, United Chemi-Con (UCC) and Panasonic. These are all cylindrical radial (two wires emerge from one end) units that mount directly onto stripboard. Some of them have metal tags instead of wires; for these you will have to drill out the holes on the stripboard to make them fit.
If you want long rated lifetimes, use several of the long-life 3300 µF or 4700 µF units in parallel.
If you don't have enough smoothing capacitance, the first symptom will be dips on the output voltage at twice the mains frequency. This will cause a buzz in audio devices, and assorted symptoms in other types of appliances. Some may be greatly affected (but are not likely to be damaged) and some may be unaffected.
I this case, you can increase the smoothing capacitance, and/or reduce the regulator's output voltage.
U1 - low-dropout (LDO) linear regulator
The next section is the actual regulator. U1 is the regulator IC, and as you can see, I have specified several options, depending on I
L. These regulators are members of the MIC29xxx family of low-dropout integrated regulators from Micrel. They're all documented in a single data sheet:
http://www.micrel.com/_PDF/mic29150.pdf.
They're ideal for this application. The same circuit can be used for all members of the family, and they all come in 5-pin packages that can be mounted onto a heatsink. The 1.5A, 3A and 5A devices are in a "TO-220" package with 5 pins:
The 7.5A device is in a similar but larger 5-pin package.
The 'x' in these part numbers is either B (non-RoHS) or W (RoHS-compliant); both are equally suitable.
Heatsink for U1
U1, and probably DS, will need some heatsinking.
The main parameter for a heatsink is its thermal resistance, which is measured in °C/W (degrees Celsius per watt). This number tells you how many degrees Celsius the device's temperature will increase above ambient (room temperature) for every watt of power it dissipates.
The regulator will dissipate around 3~5 watts for every amp drawn from it (let me know if you want an explanation of that figure). So if I
L is 5A, the regulator will dissipate up to 20 watts. A heatsink with a thermal resistance of 3 °C/W will increase in temperature by 60 °C so if ambient temperature is 20 °C, it will run at 80 °C, which is the hottest I would recommend.
U1 can be screwed firmly (but not unnecessarily tightly) to the heatsink without any insulator. Its tab is internally connected to its middle pin, which is 0V, so the heatsink will be at 0V potential. Nothing should be allowed to short onto it.
The heatsink should be ventilated. It will draw cold air from below.
These heatsinks are specifically designed for TO-220 packages. You can also get generic heatsinks and drill your own mounting hole(s). See
http://www.digikey.com/product-sear...thermal-heat-sinks/1179752?stock=1&quantity=1 for more heatsink options.
You can mount DS on the same heatsink as U1, but you need to use a TO-220 insulating kit (
http://www.digikey.com/product-search/en/fans-thermal-management/thermal-accessories/1180797?k=to-220 kit) with DS otherwise it will short the input voltage to the grounded heatsink.
CT and CS - 0.1 µF film capacitors
CT is a 0.1 µF capacitor that must be connected between pins 2 and 3 and located close to U1 for stability. CS protects DS from noise and disturbances on the input voltage that could possibly damage it.
0.1 µF, 63VAC film capacitor:
http://www.digikey.com/product-detail/en/R82EC3100AA70J/399-5861-ND/2571296 USD 0.22 x2
R1 and RA - 10k resistors
R1 pulls pin 1 high to enable the regulator. RA forms part of the voltage divider (along with RB) that sets the output voltage.
http://www.digikey.com/product-detail/en/SFR2500001002FR500/PPC10.0KYCT-ND/596847 USD 0.26 x2
RB - resistor to set output voltage
The value of RB (in conjunction with RA) sets the regulator's output voltage. The formula from the data sheet is (RA / RB) = ((V
OUT / 1.24) - 1). Substituting 10k for RA and rearranging for RB gives RB = 10k / ((V
OUT / 1.24) - 1). So here are some values for RB for different output voltages.
Or you could use a trimpot for an adjustable output voltage:
http://www.digikey.com/product-detail/en/PV36W202C01B00/490-2880-ND/666507
CD
CD is the output decoupling capacitor. It must be 22 µF or more, and it must be a standard aluminium electrolytic. This is the best choice available from Digikey:
http://www.digikey.com/product-detail/en/EST227M035AG4AA/399-6615-ND/3083030 USD 0.42.
CONSTRUCTION
You can build the circuit up on stripboard - Google stripboard construction techniques. The layout is not critical apart from CT and CD which should be connected directly between the pins of U1 as close as possible to the device.
The schematic shows "pin 6" of U1 connected to 0V. This is just the mounting tab; it's internally connected through to pin 3 inside the regulator. You don't need to connect the tab or the heatsink to anything.
