Re: Power Supplies

["Keith Doxey" <ukha@xxxxxxx>]

--- In ukha_d@xxxxxxx, "Steve Morgan" <steve@s...> wrote:
> In order to minimise heat dissipation, you choose a supply voltage
as
> close to that required to provide the required output voltage,
taking
> into account the voltage drop of the regulator. A 12V unregulated
PSU
> can be used with a 12V linear regulator to provide a 12V regulated
> supply. A 12V regulated PSU used with a 12V linear regulator will
> provide an output of less that 12V because of the inherent voltage
drop.
>
> Regardless of how conservative you consider manufacturers 'should'
be
> with their designs, if they say an unregulated supply is required
you
> should not assume that you can blindy replace it with a regulated
one.
>
> Steve

Exactly.

Taking HomeVision as an example. It runs internally of 5V DC which is
provided by a 7805 regulator. This need the 3V overhead to operate
correctly so a 9V power supply is specified. In this case a regulated
or unregulated unit would do the job equally well as the product
incorporates a regulator to give the correct internal voltage.

As has been stated, the unregulated power supplies can give a voltage
much higher that the stated voltage but usually only when not loaded
correctly.

For example, an unregulated 9V 300mA power supply will often give
about 13V with no load but will usually drop to about 9V when
delivering the stated current. the voltage dropping gradually as the
current increases.

The voltage regulator built into HomeVision can happily accept any
voltage upto 35V but it would not be a good idea to try this as the
heatsink built in is designed for the anticipated input voltage of
between 9-13V from an unregulated PSU.

The one thing you should never do it fit an unregulated power supply
to something that came with a regulated PSU unless you know that the
product has internal regulation.

Fitting a regulated supply to a product that was expecting an
unregulated one will not do any damage but it may also not work
correctly if it was expecting a slightly higher voltage than
expected. Having said that, if that is the case then the marking is
incorrect as there is insufficient headroom for the regulator to work
correctly.

KAT5 is designed with onboard Polarity Protection and Voltage
Regulation which means that a cheaper unregulated supply can be used
safely.

Keith