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The mains transformer's secondaries should be rated:
18-0, 18-0 (twin secondaries), or 18-0-18 (centre tapped), at 80VA
This will give you a power supply that should be theoretically capable of just over 1A each rail
assuming your heatsink and smoothing capacitors are up to the job.
Mounting your PSU2 on a 4U or 5U 19" panel as detailed earlier will be more than adequate
for a 1A supply. However, it is up to you to verify that your chosen panel is up to the job of
keeping the two power devices cool. Remember that air flow to the panel is essential so make
sure any tests you make are representative of the situation your heatsink will eventually be
fitted to.
The transformer secondary voltage is suggested to be 18V and it is not wise to use any more
than this as this will require more heat to be dissipated, and stress the smoothing capacitors
and regulators. You may be able to use a 15V transformer and I have found that in the UK, at
least, 15V transformers work very well as we typically use a 240V input compared to the
rated transformer input of 230V which gives a little more voltage than the specified 15V.
Remember that the maximum output current rating of the transformer is true only for a simple
resistive load on the secondary. That is, the manufacturer has specified their device when
taking a sine wave current from the transformer. This is not the case for most power supplies
as the AC output of the transformer has to be rectified, smoothed and then regulated.
Therefore the current drawn from a transformer secondary when connected to a standard
linear power supply is about as far from a pure sine wave as you can get. It is a complex task
to calculate the exact secondary current so a quick rule of thumb is often used to calculate the
approximate secondary current with any given load on the output of the PSU. I have found
that this rule of thumb is surprisingly robust given the amount of variables that actually affect
the current.
Iac = 1.8 x Idc
Iac is the RMS current flowing through the secondary winding and Idc is the load current
attached to the power supply. So drawing 1A from each your 15V rails will require
1.8A(RMS) of secondary current. And if you want to give yourself some leeway, so as to
keep the transformer cooler, then you can use a factor of 2 instead of 1.8.
As an example of the equation's usage I will use an 18 – 0, 18 – 0, 80VA transformer.
Total AC root mean square (RMS) voltage across both secondaries is 18 + 18 = 36V
The power rating is 80VA and so the maximum RMS current drawn can be 80/36 = 2.22A
The maximum DC load is therefore approximately 2.22/1.8 = 1.22A
Because we have considered both secondaries together this means that we can take 1.22A
from the +15V and 1.22A from the -15V. This, however, will heat the transformer up by the
amount specified by the manufacturer. And the temperature rise under maximum load is
surprisingly large, normally 55C, which is probably not what we want in our modular case. So
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