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Hi Slo, i hope you will solve it and make it working. I still did not have time to do the full testing. Anyhow good luck and maybe post you progress here. Cheers Tim

You ask many good questions. I'll try my best to answer them. What kind of caps and how many, before and after the regulator: As you can see from the cool graphics that are at the start of this thread, the job of the "smoothing" capacitors is to average the voltage across the cap. To "smooth out" the peaks and valleys. The two most important specs of a capacitor are the capacitance and the voltage rating. The capacitance tells how much smoothing you'll get (more is generally better) and the voltage rating is a maximum for that cap that you must stay below. Bigger capacitors are usually Electrolytic, these are usually polarized, and so they should have markings to show which leg is + or -. Another high capacity type is "tantalum". They have high capacity in a small package, and usually don't degrade over time like Electrolytics, but on the downside they are more expensive, and they are not tolerant of voltage spikes. How much capacitance do you need? That is math beyond my ability. It depends on the current demand of your device, the properties of your transfomer, and how smooth you need the voltage to be. Especially with electrolytics it's a good idea to go a bit over the minimum requirement, as they may vary by 20% right at the start, and will degrade with time. As long as there is load on the supply, there will always be some ripple after the first capacitors. Each regulator requires a certain voltage to do it's job. A typical 7805 may require 7 volts to assure a good 5 volt output. If the ripple (at it's worst under full load) ever falls below that minimum, the 5 volt supply will suffer dropouts. See the datasheet for your regulators to learn about the minimum input voltage requirements. The Texas Instruments sheet I just read states that the input must be 2 volts over the output for a 7805, 2 and a half for most other voltages. So if you're feeding a 7812, you'll need no less than 14.5 volts at the input under worst case conditions. The total capacitance needed may require a bigger cap than you have room for. In the case of the example given above by Ray Wilson, there are 4 4700 uF caps. These 4 together are less tall than one much bigger cap. I expect he's planning for a low profile case, like a 1U rack case or something. So if it's about "total capacitance", why include a dinky little 1uf or 0.1 uf ceramic? These smaller caps are more effective at smoothing higher frequencies. The main ripple in the supply will be at 100Hz or 120Hz, depending on your country, but much higher frequencies can be stopped much better with little ceramic caps. OK, all that for the "smoothing Caps", now on to the regulator. The popular 78xx and 79xx regulators are "linear" devices. They require a bit more voltage on their inputs in order to provide a smooth, clean regulated output voltage. As an example, let's say we have about 15Volts DC, and we want regulated +5 and +12 volts. We supply the mostly smoothed +15 volts to the input of the 7805. The output should show something very close to +5 volts. We do the same with the 7812 regulator, and we get 12 volts out. It all tests fine, so we connect our toys, and after a few minutes, it shuts down with a burning smell. What happened? These "linear" regulators have to "burn off" the difference in voltage between the input and the output. The greater that difference, and/or the greater the current, the more heat is generated. In our example, we might be drawing 50ma at 5 volts. The difference between +15 volts in and +5 volts out is 10 volts. that 10 volt difference at 50ma = 500 mw of heat (1/2 watt) That's a bit warm. Now try 500 ma at +5 volts, and suddenly you're dissipating 5 watts through your little 7805 regulator. This is why heat sinks are needed. They are radiators that help to dissipate that heat into the air. Here's a tip. the positive 78xx regulators happen to have their mounting tab at ground. So you can mount them directly to your metal project case, which should be grounded anyway. This lets you use your case as a big, free heatsink. Don't try that with the negative regulators though, their tab is not grounded. Get a bit of "thermal paste" or "thermal grease" and put a dab between the tab and heatsink. It'll help carry the heat across to the heatsink. After the regulator, there are a few more caps, though not usually as big as the previous ones. These help to smooth the load being placed on the regulator. It is also a common practice for smaller ceramic caps to also be placed at each chip. These help to reduce spikes generated by (especially digital) signals being created. You asked about the "extra" diodes and resistors in Seb Francis' designs. These may be needed for some systems. Here's what they are about. Some regulators require a minimum load to regulate well. Many designs have a very light load, especially on the negative supplies. Adding a load resistor may help the regulator to do a better job. In some systems, when the device is turned off, the input voltage may fall faster than the output voltage. This is a bad thing for the regulator. If your input voltage is gone, and you still have a bit of charge in the caps after the regulator, the regulator may be damaged. Adding a diode from the regulator output to the regulator input prevents this problem. Another diode that is sometimes added is from the output voltage to ground, so that if ground ever rises above VOut, the diode will shunt that voltage. You can see examples of all these things if you download the data sheet for a TI 78XX regulator. Finally, extra rectifiers to drop the voltage? You'll drop between 0.7 and 1.4 volts per diode, but it all adds up to the same amount of heat. You're just moving some of it from the regulator to the diode. That works. You can also get high wattage resistors to drop the voltage. Just calculate for the maximum current you'll be drawing, and make sure that the result never falls below the input voltage required for your regulator. I know these answers are for an old question, but I think power supply basics are useful for a lot of people here. I hope this response helps someone. LyleHaze