View Full Version : Off Topic: Regulating a DC power supply

01-12-2004, 02:45 PM
I have embarked on a project to build a device using a number of audio kitsets. In order to allow the device to run for a long period of time, I intend to use a DC power adaptor.

I have two problems: I have an old DC power adaptor given to me by a friend, manufactured by Kerr Cowan Ltd in NZ, and appears to have been made in 1996 - a round label with 1996 written on it, and 1 to 12 around it. Although this is rated at 9VDC, the actual output measured by my multimeter is just under 15V DC. The maximum voltage the kitsets say they can take is 9VDC. (between 6 and 9). I will have to try with a better quality multimeter to make sure my reading is correct, however would this be a problem? The devices are kitset audio preamp (Pre-Champ) and amp (1W kitset amp).

Secondly, this adaptor does not have enough regulating to provide a "pure" enough DC signal, as a 50hz AC hum can be heard when the device is used by this power source. I have been told in the past that putting a large capacitor across the +ve and -ve terminals will help this. Can anyone offer me advice on how to regulate the supply to produce a "cleaner" DC signal?

01-12-2004, 02:51 PM
I forgot to add, the adaptor is rated at 300mA.

Graham L
01-12-2004, 03:25 PM
That power supply will be unregulated, so the 15V sounds OK when it's not loaded. It will probably give about 8 or 9 volts when loaded. It will have a rectifier and a capacitor in it. You might be able to measure an AC output from it, too. ;-) That would indicate that its capacitor is a bit small, or has become a bit tired.

I'm pretty certain that the back (information) part of the Dick Smith catalogue ($2) has a section about using voltage regulators.

A "7809" regulator will tame the supply quite nicely -- the "09" means 9V. You can get this series with various voltages It is a three terminal device ... "in, ground, out" .. and will give up to 1A out (but not when fed from your supply). South Island Components (http://www.sicom.co.nz), Disk Smith (http://dse.co.nz/) , Jaycar (http://www.jaycar.co.nz/) all have them.

A small capacitor (100nF) between the input terminal and ground is essential, and another on the output is a good idea.

If you try to pull 300 mA, a 9 V regulator will probably "drop out" because it needs a few volts over its output to work. You could use a 7806 or 7808 for a bit more margin.

01-12-2004, 03:27 PM
Thanks Graham - will those regulators need heatsinks?

Billy T
01-12-2004, 03:30 PM
Hi somebody

Your power adaptor is probably unregulated, and that would be why it gives 15volts out. It will also be putting out "dirty dc" that is to say, the output will have considerable hum modulation (which you can hear already).

Rather than attempt to redesign this adaptor you would be better to either procure something a little more suitable, or feed its output through a regulator circuit. Just adding capacitors to the output will simply increase the output voltage slightly and ensure that it delivers a destructive current if your connected devices can't handle that voltage.

I haven't looked lately, but the DSE and Jaycar catalogues contain info sections at the rear that might show you how to set up a three terminal regulator to reduce and regulate the output. They may also sell kitset power supply units that would solve your problem.

If you don't make progress or some other suitably experienced member doesn't offer more specific advice I'll come back to your post later with more info. I'm just a little busy right now.


Billy 8-{)

Graham L
01-12-2004, 03:41 PM
Just screw the regulator to the case. You don't need a "proper" heatsink for low currents.

If you add some more capacitors apart from the 100 nF ones, remember that electrolytics (the sort that are 100 or 200 microfarad) are marked for polarity. Connected in reverse, they explode. :D They have voltage reatings, too, so go for 50V to have some safety.

01-12-2004, 05:08 PM
Thanks GrahamL for all your help. I will ask around some of my friends who have piles of electronics stuff, and see if they have the relevant components - otherwise I will pay DSE a visit.

01-12-2004, 05:10 PM
Thanks Billy. I'll pay DSE a visit at some stage (but first see if any of my electronics friends have spare bits lying around).