I have a circuit on PCB with a transformer that generates a lot of heat.

It is actually an energy meter, and I'm getting increasing error as time goes on. I observe that the transformer heats up considerably, and I was wondering if this can increase the noise in the circuit, and if so, by how much.

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EDIT: I made some measurements on the power supply:

Input Ac voltage: 220VAC

Voltage at the transformer secondary (with bridge rectifier disconnected): 19VAC

Voltage at the transformer secondary (with bridge rectifier and the rest of the power supply connected): 9VAC

Current after bridge: 220mA

Current at the end of the power supply entering the main circuit (from the point where you see the +sign on the schematic): 10mA

Someone suggested I put a resistor after the bridge to limit the current, saying it was way too high. I need some input here please.

One more thing, can anyone tell me the function of 130Ω R7 resistor between the two capacitors just after the zener diode?

  • \$\begingroup\$ well from my understanding if the winds heat up, the resistance changes, which could fluctuate the output voltage. But that why you would have a voltage regulator after it or a "smoothing cap" Besides- it should not heat up "that" much. What sort of errors do you experience though? wireless data out of synch or what? \$\endgroup\$
    – Piotr Kula
    May 3 '12 at 14:45
  • \$\begingroup\$ Thermal noise goes with temperature (squared, IIRC), but it will likely be a very small part of the total noise, and the real cause will likely be another one. \$\endgroup\$
    – clabacchio
    May 3 '12 at 14:53
  • 1
    \$\begingroup\$ @clabacchio - Thermal noise energy is linear with temperature \$\endgroup\$
    – stevenvh
    May 3 '12 at 15:07
  • \$\begingroup\$ I think he is referring to a hum, hum is not a noise. Just filter for ripple. \$\endgroup\$ May 3 '12 at 17:42
  • \$\begingroup\$ I just answered to your question about that noise. I taught that noise is some kind of noise that visualize through a sound or on VDU. \$\endgroup\$ Sep 24 '12 at 23:01

OK, you have 20VAC from the transformer, which is then full-bridge rectified to give (1.414*20) - (0.7*2)=26.9V (peak volts - diode drops), and passed through D2 (now down to 26.2V) into a Zener diode D3, which is a 9.1V diode! D3 has to pull enough current for T1.1's winding resistance to drop 17 volts. (some of this droop is provided by R118 and L8 as well.)


  • \$\begingroup\$ from measurements, i get 10VAC from the transformer secondary, 7.84VDC after the bridger rectifier, and 7.68VDC across the Zener diode. U3, the main regulator is the TL431 shunt regulator. can i elimiate the Zener diode? \$\endgroup\$
    – TiOLUWA
    May 4 '12 at 8:58
  • \$\begingroup\$ Actually 1N4736A is a 6.8V 1W zener. \$\endgroup\$
    – Lundin
    May 7 '12 at 13:00
  • \$\begingroup\$ @Lundin yes it is, but the value i measure with a meter is 7.68 \$\endgroup\$
    – TiOLUWA
    May 7 '12 at 13:07

Maybe the effects can be explained by some other kind of error caused by heat (not every kind of error caused by heat is noise), e.g. unbalanced drift in some device parameters that are supposed to be balanced.


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