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Friday, December 20, 2013

Transformer Specification



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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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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