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Dr_EM 23rd January 2009 09:59 AM

How hot do E-cores get?
Hi! I usually use toroids in my amplifiers but recently built a PSU box to run a small PC system amplifier which incorporates a 75VA E-core type transformer. This is for reasons of safety isolation and hence not requiring troublesome secondary earth (ground loops).

Anyhow, I never had much heat from my toroids at all, warm at most, but the transformer in this gets what I'd call "hot". After an hour or so of use the laminates were too hot to keep a hand on for more than a couple of seconds. The box housing it was just warm. I checked the voltages and resistances of the coils and there were no shorts. A fuse is fitted on the mains inlet and on both secondary outputs (2x12v). The transformer still heated unloaded, with the amplifier unconnected.

Can anyone confirm that this is normal? Most brick type PSU's do get quite warm on the outside and I know that E-cores are less efficient but was a bit suprised at the heat from the transformer itself.

wg_ski 23rd January 2009 01:52 PM

Are you using a 60 Hz transformer at 50 Hz? That will make it run closer to saturation and increase core loss. For that matter, a lot of low cost far-east made EI core trafos are made out of really crappy iron compared to the toriods you buy from major manufacturers. And then run up as high on the BH curve as they can get away with.

Dr_EM 23rd January 2009 02:15 PM

Thanks for replying :)

The unit is the 75VA 12v+12v unit of this range:

it says both 50/60Hz operation and is made exclusively for Rapid (a British distributor) but am unsure where it is manufactured.

Is the sort of temp I'm experiencing unusual or anything to be concerned about?

AndrewT 23rd January 2009 02:45 PM

@ 75VA a toroid will typically run at >=90% efficiency at full power.
This leaves <=8.3W to be dissipated as heat. Input power = 83.3W, output power = 75W.

@ 75VA an EI could be much less than 90% efficient. Let's suppose it's 75% efficient. Input power = 100W, output power = 75W
heat to be dissipated = 25W.

A big toroid can be 98% efficient. 1kVA and only 20W of heat to be dissipated.

About half of these heat values will still be dissipated when the transformer is just idling along with zero output current.

Geek 24th January 2009 03:05 AM

Rule of thumb (or burned thumb as it may be): If you have to pull back in 10 seconds or less, it's too hot.


MondyT 26th January 2009 10:13 PM

Hi Doc

Andy is about right except that small toroids and EI transformers in the range of 75 to 80VA or so, generally have around the same efficiencies at full load (the full load bit is quite important with efficiency) and are typically in the region of 85 to 88% for both styles.

The difference between EI and toroids *of this size* is that toroids generally have a lower no-load loss (iron loss) and higher copper loss and EI have lower copper loss and higher no-load loss.

50/60Hz EI transformers generally use thicker laminations of lower grade steel than toroids and so an EI 75 to 80VA would have typically 7 to 9 watts of iron loss where as a toroid would have typically less than a 10th of this iron loss. So a toroid feels pretty cool off load to it's equivalent EI unit.

Also for European standard EN61558-1, which is the norm for most off the shelf transformers, permits winding temperatures up to 100deg.C for Class A (105 deg.C) rated insulation and the steel core of an EI unit is permitted to run at 70deg.C or 85deg.C depending on if the core can be touched or not in the application (by the dreaded test finger!) The transformer's rated ambient temperature is assumed in the above figures. These temperatures feel pretty hot to touch!

I hope this helps


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