Electronics > Projects, Designs, and Technical Stuff
Trasformer rating for bridge rectifier
taydin:
So I wanted to build a 5.2V, 5.25A DC linear power supply, and without doing any detailed calculations, I figured:
1 bridge rectifier, 10A, 600V
1 choke after the bridge rectifiers to help with ripple (didn't have a particular inductance in mind)
2 4700 µF, 25V electrolytic caps in parallel
And for the transformer secondary voltage, I added the output voltage, the diode voltage drops and 2V margin for the linear regulator: 5.2 + 1.4 + 2 = 8.6, or 9V
And for the transformer secondary current, I just arbitrarily chose a current of 7 Amps, just to have a margin.
But then I thought I should do a simulation of this circuit in TINA. Here is what it looks like:
taydin:
I already had the 4700 µF caps available, so I measured their ESR to be about 10 mΩ. I also had a 5V, 7A transformer lying around, and I measured its winding resistance to be about 24 mΩ. That's the figure I used for my simulated transformer.
The simulated transformer ratio is such that the input is 220Vrms, and output is about 9.2Vrms. I did transient simulation with 0 initial conditions, so the initial high current surge makes sense, and the rectifier should be able to deal with it. But then, during normal operation, the total current coming out of the bridge rectifier, AM3, is peaking at 18 Amps. I measured the average of that waveform, it's about 7.57A, and the RMS of it is 11.74A.
Now I'm not sure how to rate the secondary current of the transformer ... Should I use the average current, or the RMS, or the peak 18A?
taydin:
If I look at this document:
http://www.hammondmfg.com/pdf/5c007.pdf
It says that the secondary voltage should be 9.55V (it doesn't account for the diode drops and the regulation overhead, I added them), while my simulation says 9.2V is enough.
It says that the secondary current should be 5.25 / 0.94 = 5.58 Amps! But this result doesn't make any sense to me. How can a transformer with 5.58A capability periodically deliver 18A peaks without heating up?
Zero999:
Yes the peaks increase power dissipation. The transformer doesn't need to be rated to 18A continuously, but it certainly needs to be rated to more than 5.25A. Assuming no losses and a huge filter capacitor, the RMS input current to the rectifier, will be 1.414 times the output current, so that's just over 7.4A.
The transformer needs to be a higher voltage, as the voltage will fall, when the filter discharges and the mains voltage could be on the low end of the tolerance range. Go for a 12V 100VA transformer.
Why not just buy a 5V switched mode power supply with an adjustable output voltage? 5V is readily available and getting one with an adjustment potentiometer so the voltage can be set to 5.2V, is no problem. Building a linear regulator is obsolete.
taydin:
--- Quote from: Zero999 on January 18, 2019, 02:30:14 pm ---Yes the peaks increase power dissipation. The transformer doesn't need to be rated to 18A continuously, but it certainly needs to be rated to more than 5.25A. Assuming no losses and a huge filter capacitor, the RMS input current to the rectifier, will be 1.414 times the output current, so that's just over 7.4A.
The transformer needs to be a higher voltage, as the voltage will fall, when the filter discharges and the mains voltage could be on the low end of the tolerance range. Go for a 12V 100VA transformer.
Why not just buy a 5V switched mode power supply with an adjustable output voltage? 5V is readily available and getting one with an adjustment potentiometer so the voltage can be set to 5.2V, is no problem. Building a linear regulator is obsolete.
--- End quote ---
Thanks for the response. A switching power supply certainly makes more sense. And in fact, here is what I'm REALLY trying to do:
I have this HP8592 spectrum analyzer with a dead SMPS power supply. It has the following outputs:
5.2V 5.25A x1
15V 2.2A x1
15V 2.0A x1
12V 1.0A x1
I tried to fix this power supply, but wasn't successful. The PCB's had extensive electrolyte leakage, tried to clean it up, but it just keeps exploding the main inverter MOSFET's. And with every explosion, the PCB got charred and those locations are also a source of conductance. I want to revisit that and rebuild that power supply from scratch in SMPS topology, but until then, but I'm eager to use the SA, and so I want to build a linear power supply in the meantime.
I tried looking for adjustable SMPS with the necessary capacity but wasn't able to find any locally. But maybe I didn't look hard enough...
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