Transformer Secondary Voltage Overshoot at Low Loads - Page 1
Forum section: electronics / general

author: beaker353
author_rank: Unknown
timestamp: March 21, 2013, 05:50:56 am
content: I understand that the secondary voltage in standard step down power transformer it is typically spec'ed at full secondary load current. When the secondary has a much smaller load attached, the secondary voltage can be significantly higher than it should be. I've been trying to nail down exactly why this happens and I have a couple of theories that both seem have some certain amount of credence, but I was hoping the group can help me clarify.Theory 1: Increase of current levels on both coils increases the inherent voltage (I*R) drop.With any piece of conductor there will be a voltage drop associated with the current going a resistance. At a lower secondary load there will be less current on both the primary and secondary side, thus less voltage drop across the conductors and coils meaning a higher voltage. A coil inherently has both a Xc and a R component, so in theory the higher the current across the same R, the higher the voltage drop.Theory 2: Primary to secondary loss ratio changes with secondary load.Transformers, like all components, are not perfect and have losses. In particular the loss of magnetic flux between the two coils. Manufacturers take into account the inherent power loss between the primary and secondary coils at a single load point (max rated load). As the load drops, the efficiency (coefficient of coupling) increases between the two coils and thus the secondary voltage goes up.Are both of my theories correct, or does one or both have a fatal flaw I overlooked. Thanks!-EM

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2013, 06:04:19 am
content: Both are true, but the first has the most effect, as the primary resistance is so much higher so the losses at full load are higher, as it is generally wound with the thinnest possible wire to cut cost. You might have an effective voltage drop of 15% on the primary between no load and full load current, which will be reflected on the secondary. The second theory will be much smaller, as the core losses and coupling are going to be reasonably constant, only changing slightly with load.

author: smashedProton
author_rank: Frequent Contributor
timestamp: March 21, 2013, 06:06:10 am
content: I am willing to bet number 1. Go ahead and measure the secondary voltage , current, and resistance. See if ohms law stays linear, else there MIT be some funny buisness happening. After all, this is the scientific method

author: amspire
author_rank: Super Contributor
timestamp: March 21, 2013, 06:22:32 am
content: Theory 1 is correct.Theory 2 is probably right, but I am just not sure what you mean. Any lack of coupling ends up looking like a series inductance in each core - ie a series impedance. As the load increases, so does the voltage drop due to this inductive impedance. Inductive losses do not waste any power, but they do lower output voltages.Part of it is economics. If you are making a 5W transformer, you would have a choice of making a larger transformer that was fairly efficient, or a much smaller transformer that used half the steel and a third the copper with 3 Watts of losses. 3W just means the transformer is warm, and who even notices 3W of wasted mains power? The cost benefits and small size and weight win almost universally over good efficient design when it comes to small cheap mains transformers. Everyone wants a transformer that has a minimal size, minimal weight to make PCB mounting easier and minimal price.Because the manufacturer of the small transformer has skimped on the copper and core size, the marginal primary inductance means there is a fairly large magnetizing (inductive) current, and because the wire is a bit thin, a lot of this current turns to heat. The core may be so marginal that it has significant magnetic saturation each cycle increasing the peak magnetizing current and hence even more resistive losses. Again with the marginal design, there will be core losses and this shows up as energy from the primary that never reaches the secondary.When you make larger transformer, you just have to build them better as you cannot afford the heat loss that you get in a small transformer. The transformer would just get too hot.Richard.