Regulated Power Supply Question

[aggieee07]

Ok so I have found some schematics for building a regulated power supply and would like to do this . Here is the schematic

http://home.cogeco.ca/~rpaisley4/PSupply.html

I understand all the component stuff but my question is if I want to make a 9 V supply how do I calculate the values for the capcitors and know which regulator to use. I was hoping a could make my supply provide 2 amps. And for future reference if wanted to add a couple of 12V output jacks could I just run another transformer off the line ground and neutral.

Thanks and Gig' Em
Nick Wallace

[Peter Snowberg]

Welcome to the forum.

I assume you are using the top schematic. To answer your questions out of order:

- the regulator part number will indicate the output voltage. For a 9V supply, you want a 7809. There are tricks that you can use to make other parts work, but I think the best way to go here is to get a real 7809.

- For cap size, make them as big as you reasonably can on the input side of the regulator. The 2200uF shown is good but you can vary over quite a range depending on how much hum you can tolerate. The cap has to be large enough so that it will supply full load for a good piece of an AC cycle. If the input voltage dips under the regulator output voltage plus the internal voltage drop in the regulator, you will end up with output noise. Typically you want to make sure you always put in 3 volts more than you want out. The output cap is just there to stabilize the regulator and does not need to be large at all. Some 78xx regulators don't like large output caps at all. A value around 1uF will work in 99.99% of applications. Some 78xx parts work great with output caps down to .047uF.

- On the current rating you'll need to get the right 7809. I just searched Digikey and they sell 12 different 7809s with current ratings of 1A and 1.5A. If you want to get 2 amps, I would suggest just building two 1A supplies if that can work for your application. That way you will be able to power a bunch of devices and you won't have to search out more complex solutions.

Just to be complete, there is a way you can boost the output current by using an external high current PNP "pass transistor". Take a look at the schematic in this thread: http://www.electro-tech-online.com/viewtopic.php?start=0&t=1747

Any power PNP device will work quite well as long as it can supply what you need for current. Just sub a 7809 for the 7805 in the schematic. You'll also notice the 470uF cap on the output side. Also notice the 1N4001 pointing from the output back to the input. When the input power is cut, the diode makes sure the output cap discharges just as fast as the input cap. Without the diode, if you have power on the output and nothing on the input, you may let the smoke out of the regulator.

[gez]

This covers all the basics:

http://www.amazon.com/gp/product/0900162961/qid=1134566932/sr=8-1/ref=sr_8_xs_ap_i1_xgl14/104-2479082-6682365?n=507846&s=books&v=glance

Haven't got round to ordering this one, but if it's like the above it'll be well worth the money:

http://www.amazon.com/gp/product/0859341666/qid=1134566932/sr=8-2/ref=sr_8_xs_ap_i2_xgl14/104-2479082-6682365?n=507846&s=books&v=glance