Transformer Specification
How do you calculate how much capacity you need for your
transformer? There are many rules of thumb and users don’t really know what to
estimate, being afraid that they underestimate the capacity needed.
Overestimate is OK. You just spend more money and build more muscle! No big
deal.
If you do not know or do not want to calculate, let us do
the mathematics for you when ordering. Just send us the schematic and we will
customize the power transformers and chokes for you. No big deal (again).
Why do we need to do all these calculation? It is because
there are losses on the transformer. In real world, what you put in is not
equal what you get out due to losses or >100% efficiency. Transformer lost
has 2 components – the core loss and the copper (wire) loss.
Core loss is in terms of heat as there are Eddy current and
core flux resistance. It can be calculated per core weight and material. For
example, H18 has core loss of ~1.8W per Kg and H50 has core loss of ~5W per Kg.
The Z11 that we always use has core loss of 1.1W per Kg.
Copper loss is simpler – it is the loss caused by resistance
on the wire. Current passes through resistance creates heat. These 2 losses are
combined as transformer total loss. These 2 generate heat and is dissipated via
the transformer surface. These 2 also raise the transformer temperature until
equilibrium. If there is too much loss, the transformer temperature will be
very high.
For short term use of an hour or so, one can tolerate higher
losses (with thinner wires and smaller cores) but for long term use, we need to
make sure both are taken into consideration, seriously. Normally, we would rate
out transformers to be 1.33x – 1.4x the actual VA needed, which is almost good
for long hours operation (>12hrs to 24 hours). In other words, the VA is
only 30% efficient – a 10VA transformer is only good for 3.33VA.
For push-pull output transformer, you can rate the size to
3x-4x of the required output power. For
single ended output transformer, you will need to have 6x – 7x of the required
output power. This will ensure enough headroom to spare on the core side to
prevent saturation. If you calculate it to be smaller, don’t even bother
buying.
How to calculate the VA based on dimensions – you can Google
for it. There are many methods on doing so.
Also, below is a nice PDF on what you need for different
types of rectification.
J&K Audio Design
20/12/2013
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