To them, it is always Watt that they heard mostly and learnt in secondary school, or even university. How do they use Watt versus VA? What is the best method to power transformers? What are the differences between VA and Watt? Some often see that we usually up-rate the transformers per the Vdc and Idc they use on their tube amplifier and they wonder why.
How do I properly size my power transformers? How do I properly size my isolation transformer?
First, we've to know what is the distinction between VA (Volt-Amp) and Watt. For equipment, the power consumption, or the actual power drawn by the equipment, is usually expressed in Watts. For transformers, usually they are rated in VA, which is also known as "apparent power", and is simply Vac x Iac.
For resistive loads, such as bulbs with filament, heater with heating elements, and many more, the VA rating and Watt rating are the same - this is because the "Power Factor" is 1.0. For power supplies with capacitor-input, choke input supply, switch mode power supply (SMPS) and etc, the "Power Factor" is <1.0, such as 0.6 - 0.8, and therefore the VA rating and Watt rating will be different. Google for "power factor" if you're unsure what it is - we will postpone this for another article - hold on first.
The Watt rating has to be divided by the "Power Factor" to get the actual VA rating required for the equipment in use. Or, the VA rating must be multiplied by the "Power Factor" to get the Watt rating.
Alright, enough with the theory, cut the crap, how do I order the d*mn transformer?
Ok, ok, here's the simplified version, easy for non-technical people to follow. It's not super accurate, but is good enough for ordering use:
Example #1: Consider the case of a equipment that consumes 260W from the wall on average.
If we conservatively assume (yes, assume, since we do not know what's inside, we will take the worst case) 0.6 Power Factor, then the calculation would be as follow for the transformer VA to order:
VA = 260W / 0.6 = 433VA
(Some will use rule of thumb of doubling the VA required versus the power consumption in Watts.)
I would expect users to do some additional de-rating as well since transformer is not a perfect device - there may be losses. Also, if it is an amplifier, then the power consumption may actually be more since amplifier is dynamic, drawing different power at different point of time due to the volume versus music nature, which is dynamic and unpredictable. Therefore, double or triple of that is nothing surprising for amplifiers.
*** If you prefer cool-running transformers, de-rate it further - do note that de-rating requires readjustment of the primary/secondary ratio since the secondary voltage will be higher than normal when under-load. *** Inform us if so when you're ordering, so that we can set the right voltage for you.
Example 2: Order power transformer for an amplifier. Assuming you need Vdc of 400V at Idc of 200mA, using a bridge with capacitor input power supply (important note: different circuit calls for different formula).
Vac = V-average-dc x 0.9 = 360Vac
Iac = Idc x 1.61 = 0.322A
VA = Vac x Iac = 116VA
If you do not know the calculation method, you can go for simple calculation:
P in Watts = Idc x Vdc = 0.2 x 400 = 80W
Power factor = 0.6
VA = 80W / 0.6 = 133VA
Above is just for a single winding. If there are multiple windings in a power transformer, add them all up, and add additional margins for multiple windings since efficiency will drop further with more windings in a single transformer.
Some will use rule of thumb (again) of doubling the power (Watt) required for the VA rating, for a quick check.
Do not forget to take losses into consideration, which is not included in this guide, especially tube rectifier / diode drops. D.C. regulator requires voltage margin for stable operations too and is dependent on the type of circuit used. Also, consider mains voltage drop/hike too in your calculation. Some will add additional buffer for lower operating temperature since there shall be copper losses and core losses that will generate heat and cause temperature to rise.
Alright, here's your calculation example for how to order a power transformer, and how to differentiate VA and Watts during transformer ordering. Keep this in mind when you order. If you do not know how to calculate, let us know and we can help you estimate.
J&K Audio Design
26/4/2020
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