Full wave center tapped power supply defies the laws of physics

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#22
I see two red wires connected to the cathode of the diodes; how is the connection physically made, why two wires, why didn't you use the center pin of the diodes?
Remember that heatsinks are anodized, and do not conduct current very well.

Something else: in theory 100V Vrrm should be (just) sufficient for 24V rectification, and schottky's don't like the transients that can appear with a 50/60Hz center-tapped transformer, especially when the windings do not share the same core. It would be preferable to replace these diodes with something more suitable, because on the long term they will end damaged. They could already be: try lifting the three connections and substitute any pair of diodes you have lying around, 1N4007, 1N5404 or similar. This will also test your transformers arrangement.
 
#24
Elvee, the case and the center pin are internally connected and it was easier to use a ring terminal on the screw that holds the TO220 to the heat sink than use the center terminal. I actually cut the center terminal off to give me more room to get at the anodes.
 
#27
powerbob said:
OK, it looks like we are done here. I connected both primary's in parallel, powered it up and in a few seconds burned out one of the primary's.
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That suggests that you were using two separate 115V transformers with primaries in series on a 230V supply, which can't be expected to work properly. Then swapped to two 115V transformers in parallel on a 230V supply, which can be expected to damage the transformers.

It might help us if we know where you are, and how much experience you have in building mains-powered PSUs.
 
#29
These transformers were unmarked and are 35 to 40 years old. When I originally experimented with them I connected the primary’s to 120 vac and found the idle current to be 320 mA, very high for a 60W transformer. I also needed more than 60W. So I decided to connect them in series and see how that worked out. The voltages looked OK so I gave it a try.

When I rewired for parallel operation I had the connections to the low voltage primary terminals, just guessing 100 to 105 vac. I turned on the power strip and the scope started beeping which I never heard before. This distracted me for 10 to 15 seconds while I watched all the controls light up and the beeping continue. I finally realized it was probably a low voltage alarm and I put a voltmeter in the outlet strip and measured 99 vac, probably another 20 seconds. I went back to the transformers and felt them getting hot. And that is all it took. One transformer fried.
 
#33
everything matters draw your actual schematic as built include any and all part numbers!!!!!

IDK maybe it's not a power 60Hz transformer* and your attached schematic pictures don't depict what your actually doing. don't hide stuff, the primary is part of the circuit otherwise youre just tricking folks trying to help you

*if it doesn't run properly off the mains voltage it might not be what you want it to be.
 
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#35
powerbob said:
infinia, regardless of the primary's, 29 vac on each secondary is 29 volts. Why 29 x 1.414 does not work is what this thread is about.
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That is your first mistake: believing what a meter tells you without understanding what you were actually doing and what the meter is doing.

Were the voltages measured under identical conditions? If a transformer has a strange power source (e.g. two separate transformers in series) then what you measure may depend on exactly what load it sees. You can't simply measure off-load AC voltage and expect to see 1.414 times that as a DC voltage - you could see more or less, depending on the exact conditions. At least two reasons for that:
1. meters on AC ranges may measure average voltage after full-wave rectification (but no reservoir) or peak voltage (with reservoir); in either case they will tell you 'RMS voltage' which assumes a clean sine wave. You might not have a clean sine wave.
2. the voltage you get from one transformer will depend on what the other transformer is doing at the same point in the AC cycle - this is what you get when you wire separate transformers in series.

Before you rewrite the physics textbooks, it might be helpful to learn what they already say!
 
#38
powerbob said:
These transformers were unmarked and are 35 to 40 years old. When I originally experimented with them I connected the primary’s to 120 vac and found the idle current to be 320 mA, very high for a 60W transformer. I also needed more than 60W. So I decided to connect them in series and see how that worked out. The voltages looked OK so I gave it a try.

When I rewired for parallel operation I had the connections to the low voltage primary terminals, just guessing 100 to 105 vac. I turned on the power strip and the scope started beeping which I never heard before. This distracted me for 10 to 15 seconds while I watched all the controls light up and the beeping continue. I finally realized it was probably a low voltage alarm and I put a voltmeter in the outlet strip and measured 99 vac, probably another 20 seconds. I went back to the transformers and felt them getting hot. And that is all it took. One transformer fried.
Click to expand...


Throw those unknown/unmarked transformers away and look here: Transformers - AnTek Products Corp

I further suggest that in the future you implement a "bulb tester" (do a search here if you don't know what that is) to prevent melt downs in case of wiring errors.

Mike
 
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