I'm building a power monitor (so I get notifications if the power is lost in one of the inputs). To monitor it safely, I used these 220/12 0.35VA transformers (http://www.hahn-trafo.com/english/pcb-transformers-bv20.php), I have them in a row one next to another. The transformers each are connected like this:
simulate this circuit – Schematic created using CircuitLab
The LED is for indication and the optocoupler is for connecting to Raspberry Pi.
The transformer is rated for 29mA and the actual load is lower (but not by much).
The transformers get quite hot (sticking a temperature probe between two transformers I get 70C). The manufacturers website says that the transformer is rated for "ta 70C/B". Am I still OK or do I need to add a fan?
EDIT: I connected both LEDs in series with total 2K series resistance. The temperature dropped to 58C.
EDIT2: Connected a 390R resistor in series with the diode (the PSU designer software showed that it would reduce the peak current), didn't help. Probably the 1.2W no-load dissipation as specified by the manufacturer adds quite a lot to the temperature. Added a fan to circuate the air (this is in a 1U case). Hopefully it will reduce the tempaerature.
What is a "normal temperature" for a transformer? I know that some bigger transformers get quite hot when working at specified load (no rectifier, just transformer and vacuum tube heater), don't know about the little ones – I do not have a lot of experience with very low power devices such as this.
EDIT3: With a fan the temperature is ~43C on the transformer that is furthest away from the fan. Is that considered good enough? The 58C with no fan looks to me too high, but should be below the 70C spec for the transformer (in case the fan fails).
Best Answer
Small transformers like this often run hotter than one might hope, it is a economising symptom. Operating them at 70 degC will cause long term reliability issues with nearby solder joints. A fan that is not monitored in some way is also a long term reliability issue. If you have space then also spread the transformers a bit apart if you can to help with cooling. Place holes in the PCB between the transformers to allow air convection.
I would suggest you use the lowest voltage output version to get the largest secondary current rating.
I would suggest that you use the largest primary voltage to get the least core losses at your rated mains voltage.
You waste half of the transformer secondary current rating by using a halfwave rectifier. Using the centre tapped secondary lets you get fullwave with two diodes or using a full bridge does the same with the single secondary.
I would use the 400V input transformer BV 202 0172 with the centre tapped 9V secondary. You should get about 4.4V on the capacitor with two diodes, with your indicator LED and OPTO in series you might not need much of a resistor, if the current limiting and voltage regulation with a small resistor value are a bit hard to characterise across part tolerances then the 12V secondary might be the way to go if the 6V secondary has enough voltage it would have the thickest secondary winding and current rating. You should be able to get a good OPTO signal with 2 to 20mA current Design for 10mA and you should be pretty safe with any of them.
The small value peak current limiting resistor will not have much effect on temperature in your transformer.
EDIT:
If you are stuck with that model of transformer you might be able to move some of the heat out of the transformer with a series resistor on the input side if you can prevent it saturating the core (keep the magnetising function linear) the iron losses will be reduced.
Using the fullwave bridge, as shown, and series connecting the LED and OPTO and reducing the current as much as possible will all help with reducing the copper losses.