I have a circuit that includes a bridge rectifier and a transformer.

Values are,

Transformer secondary 4.7v x 2 9.4v AC

Diodes 1n5408 (3A Rectified Output Current)

Rectifier connections are right and i measure from anode anode junction to cathode cathode junction. Multimeter reads around 15V DC

When i put an amp meter between one of the outputs of the transformer and bridge rectifier input it reads 1.7 Amps.

In the continuity test there are no short circuits.

Can you think of a reason why is this happening? I would expect around 6.5 or 7 volts DC at the rectifier out and next to none amps in the amp meter.

Thank you very much. (By the way this case happens when a reservoir capacitor connected or a load connected also. Transformer itself without the rectifier works as expected.)


Rectifier Output Power On

Rectifier Output Power Off

  • 2
    \$\begingroup\$ You will need to post a schematic and photo of the device to get useful help. You should not be getting 1.7A on the secondary, but we have no idea why without more information. \$\endgroup\$ Commented Jan 29, 2018 at 18:14
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    \$\begingroup\$ Perhaps you have the transformer wires connected to the bridge rectifier incorrectly, or one (or more) of the diodes are incorrectly placed \$\endgroup\$ Commented Jan 29, 2018 at 18:18
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    \$\begingroup\$ Do you have the multimeter set to AC current? \$\endgroup\$ Commented Jan 29, 2018 at 18:36
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    \$\begingroup\$ make a DC current measurement, one of the diodes (or both) that oppose that DC current have failed, if it comes out as an AC current one diode (or both) each way has failed. it might be easier to just replace all the diodes. \$\endgroup\$ Commented Jan 29, 2018 at 18:47
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    \$\begingroup\$ 9.4VAC rectified and fed to a capacitor will approach the peak value of the AC, also small transformers output a higher voltage when lightly loaded. when loaded to the rated current the output voltage will be close to the label voltage. \$\endgroup\$ Commented Jan 29, 2018 at 19:31

1 Answer 1

When i put an amp meter between one of the outputs of the transformer and bridge rectifier input it reads 1.7 Amps.

Randomly probing around with a ammeter is not a good idea. Remember that a ideal ammeter is dead short. It's not clear what you are doing here since both outputs of the transformer are also inputs to the rectifier.

If you are getting such currents with nothing connected, then one of the diodes is blown or connected backwards.

The open circuit output voltage seems a little high, but maybe your 9.4 V transformer output voltage is the rated voltage under load.

With a little capacitance on the output of the rectifier, it will go to the peak voltages of the input waveform. For a sine, the peaks are sqrt(2) higher than the RMS value. 9.4 V RMS would therefore mean 13.4 V peak.

There are two diodes in series between that and the output. Under normal use, each silicon diode drops about 700 mV, so the output should be about 1.4 V less than the peaks of the input, or about 12 V. However, that's with some reasonable load. At no load, the transformer is probably putting out a few volts higher, so the output is also a few volts higher.

Basically, there doesn't seem to be anything wrong here.

  • \$\begingroup\$ Thank you very much for the clear explanation. I think the diodes are blowing. Because i changed the diode quartet, then the current draw stopped. Then after turning on and off couple of times the current problem started again. What could be the reason making my diodes blow? \$\endgroup\$
    – Ali Somay
    Commented Jan 29, 2018 at 23:57
  • \$\begingroup\$ I also tested the diodes with the scopes component tester. They show a good L shape. This situation comes a bit cryptic to me. Then i changed the diodes again to 6A ones. First everything was ok, then i turned the power on and off. The current draw started again. \$\endgroup\$
    – Ali Somay
    Commented Jan 30, 2018 at 12:11
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    \$\begingroup\$ @Ali: It sounds like the flyback pulse is destroying the diodes when the transformer is switched off while there is significant current thru the primary. Put a TVS, varistor, or the like across the primary that triggers a little above the normal input voltage peaks. \$\endgroup\$ Commented Jan 30, 2018 at 12:56
  • \$\begingroup\$ I found this link "Protection Of Linear Power Supply From Over voltages". Thank you for pointing me to the right direction. I will put varistors, try with fresh diodes and give feedback. books.google.com.tr/… \$\endgroup\$
    – Ali Somay
    Commented Jan 30, 2018 at 13:17
  • \$\begingroup\$ I put three varistors to the primary. Also i bought a new set of diodes i tested them, i turned on the power and there is the current draw again. I had a big transformer before i took it out from an old vacuum tube radio i have never lived through this kind of problem with it. What can i trace else to debug this issue? (2 varistors are from live wires to neutral and one is from L1 to L2 I could find 460V clamping ones my AC peak is around 310 - 320) \$\endgroup\$
    – Ali Somay
    Commented Jan 30, 2018 at 16:46

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