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Article #3: Power Supplies

copyright ©1999 Paul Blitz



When building any form of electonic equipment, you will invariably need a PSU. If you are used to building PSU's then this won't be a problem for you! However, I know that whilst many Hospital Radio people are happy to follow a proven design, they are rather more wary at "trying to do their own thing".

The simplest way (and usually the most expensive) is to buy a ready-made and cased regulated PSU from RS or Maplins. However, you often find that the ones on sale aren't the right voltage, or a way over-powered for what you need. If you take this route, there should be no CE approvals implications (eg that cover insulation of mains voltages) with the PSU, as this has been taken care of by the manufacturer of the PSU. Take care to avoid switch-mode PSUs with audio circuits (see below)

The next possibility is to buy a (rather cheaper) UNregulated cased PSU, and build a small voltage regulation circuit (see article 3a) into your piece of equipment. This solution is actually a very nice one, as you don't have to worry about the safety of the mains side: that is all done for you in the PSU... all you are dealing with is a safe low voltage. Again, there should be no CE approvals problems.

The third possibility is to build the whole thing yourself. This should really only be done if you are happy that you can build it 100% safely, with the mains side of things properly isolated, and everything fully earthed. Theoretically, you will be 100% responsible for making sure the design and build standard meets the CE requirements.

Finally, it is possible to buy encapsulated PSU modules, and pre-made PSUs on Eurocards. They are typically aimed at "embedded systems" so are usually regulated, but are not cased. These have the advantage of being "ready built", but the disadvantage that you will have to sort out the safety of the mains wiring etc.

Another type of PSU, which doesn't quite fit the above descriptions, is the "DC to DC converter": this does what it says it does... it takes one DC voltage, and converts it to another. This can be convenient if you are building something that uses mainly +5 volts, but you need a very small amount of +/- 12 Volts. The DISadvantage of using DC to DC converters is that they use switched-mode technology (see below), and are thus not so suitable for audio circuits.

What is "regulated" and "unregulated"??

An UNregulated PSU has 2 potential "problems". The first is that their output is usually not very "smooth"... by that I mean that there is a noticible amount of "mains hum" on the DC (the technical term for this is "ripple" - a volt of ripple can be quite normal for an unregulated PSU) - this would cause problems if the PSU were used with an audio circuit. The second problem is that the voltage can change, depending on how much current / power you are taking from it. The problem here is that some ICs have limited voltage ranges.. a "15 volt unregulated PSU" may well put out 20 volts under no load / low load... and 20 volts could easily be too much for some audio ICs.

The advantage of an UNregulated PSU is that it is very cheap: typically it contains no more than a transformer, a rectifier, and a capacitor.

By comparison, a regulated PSU has close to no ripple (typically a few millivolts), and the voltage output hardly varies between no load and full load.

For audio and logic circuits you need a power supply that gives a constant (withing tight limits) voltage, and with as little "mains hum" as possible: in other words, you need to use a regulated PSU.

However, we said above that a suitable pre-built / cased regulated PSUs may not be avaialable: don't worry, you can buy an UNregulated PSU, and then regulate the voltage yourself: this is often the most cost-effective way to make a power supply.

The Nitty Gritty

Using a regulated PSU

Let's start with the first option: buying a ready made regulated PSU. To be honest, there's not a lot to do if it is already case: attach a mains plug, and find a way to connect it to your circuit.

The obvious thing to do here is to put some sort of low-voltage plug (eg a DIN plug) onto the PSU, and the matching socket onto the box of your circuit. Yes, you CAN hard-wire it, but experience shos that this usually ends up being a BAD IDEA. It is inconvenient at the best, and mis-handling can easily damage the cabling.

Alternatively, you may decide to mount the PSU into your box (tie-wraps maybe?), and have the mains lead dangling out - if you do this, then not only must you make sure that the mains lead is not likely to rub against any sharp metal edges, and create a safety hazard, you must also consider the safety-earthing implications (see below).

If you are considering using a pre-built regulated PSU, then you need to be aware that there are two types: Linear and Switchmode. Linear PSUs are less efficient, and tend to be available in smaller sizes. Switchmode units tend to be more powerful - eg used in PCs - but have the disadvantage that they can easily introduce audio- or radio-frequency interference into your circuits: that can be very difficult to get rid of. By comparison, Linear PSUs do NOT introduce such noise, and are to be preferred for audio circuits.

Using an UNregulated PSU

Ok, what about the idea of using a pre-built & cased UNregulated PSU. Again, you need to put on a mains plug, and connect the output wire to your box with a suitable connector.

Inside your equipment, you need to take the input, and regulate it. This is actually simpler than you might think: Maplin sells "regulator cards" (see the "Projects" section of the catalogue), both pre-built and DIY that you can use. They can be made to supply almost any voltage needed.

Alternatively, you can build the same regulator circut onto a bit of "Veroboard".... if you are also building something else, then this may be convenient. Article 3a gives more detail on what to do.

In either case, do not try to leave out any of the capacitors: the smaller ones (ie < 1 microfarad) are essential to stop the regulator IC from bursting into oscillation. If you build it yourself, then fit these as close as you can (and in any case within an inch) of the regulator IC.

For audio circuits you often need twin voltages: +/-12 or +/-15 volts. In this case you will need either a dual unregulated PSU, or two single PSU's (connect the "+" of one to the "-" of the other... this is your 0 volt point), along with two regulator cards, one +ve, the other -ve.

For the regulation circuit to work properly, it needs to be supplied with 2 or 3 more volts on it input than it gives on its output. Thus for a 12 volt regulated supply, you need a 15 volt unregulated input.

What about earthing?

Unless you are using a double-insulated power supply, the rules are very simple: any "touchable" metalwork must be earthed to safety earth (ie the big pin of the mains plug!) with a suitably low resistance.

If you are using a plastic box, then theoretically this means that all externally-touchable screws must be earthed. This makes the use of a METAL case a good idea! To ensure that all screw in a metal case are earthed, fit a star washer.

If the PSU *is* double insulated (most are: but check that it is MARKED as such!), then you can do what you like.

Another question that comes up is: should I earth the zero-volts of the psu? Generally, the answer is no, as you can very easily end up with earth-loops. In this case, you have to be careful you don't do it accidentally... for example, you can't use phono-sockets that simply bolt to the metalwork, you must get insulated ones (and maintain that isolation). Also watch out how you wire din and XLR connectors.

And Finally....

Finally, if you need any help with the circuit in any way, then feel free to email me with your questions.  


(plb, rev 1, Sept 1999)  


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