Howdy. This evening I set up to test a transformer for a coming build. What I want is +24 and -10 volt rails.
The toroid has dual 18 v and dual 9 v secondaries so I set it up for full wave centre tap. With no rectification or load I made center taps of the two middle of the 18 and the 9 v windings and fired it up unloaded with a bulb tester inbetween. I did not tie the two centertaps together.
Usually if a winding is backwards in this test the lamp shines bright and the tranny hums. But there were no lamp and no hum or getting warm.
Great I thought, found them on first try.
Then I set up rectification, filtercaps and a very easy dummyload of 100 ohm.
Ding, lamp shines and the trafo hums and gets warm😡
I tried 2 different couplings and gets the same results. What am I doing wrong?
best
Staffan
The toroid has dual 18 v and dual 9 v secondaries so I set it up for full wave centre tap. With no rectification or load I made center taps of the two middle of the 18 and the 9 v windings and fired it up unloaded with a bulb tester inbetween. I did not tie the two centertaps together.
Usually if a winding is backwards in this test the lamp shines bright and the tranny hums. But there were no lamp and no hum or getting warm.
Great I thought, found them on first try.
Then I set up rectification, filtercaps and a very easy dummyload of 100 ohm.
Ding, lamp shines and the trafo hums and gets warm😡
I tried 2 different couplings and gets the same results. What am I doing wrong?
best
Staffan
Attachments
Thanks Bwana
I did that test before rectifying. What I did not do was connected the dual 9 volters with the dual 18 volters in any way. I guess one test would be to connect all 4 windings in serie which would get me 54+ VAC right?
I am off to the ancient birthlands today so I'm not sure I have time for a test. We'll see.
Mtumwa
I did that test before rectifying. What I did not do was connected the dual 9 volters with the dual 18 volters in any way. I guess one test would be to connect all 4 windings in serie which would get me 54+ VAC right?
I am off to the ancient birthlands today so I'm not sure I have time for a test. We'll see.
Mtumwa
Just looking at my drawings over breakfast. Does the upper 9 V winding work against the lower 18 V winding as it is now? Especially the left (1) drawing looks suspicious like that to me.
If so I cant find out how to full wave rectify with center taps one positive and one negative and a common ground.
If so I cant find out how to full wave rectify with center taps one positive and one negative and a common ground.
They should give the same results, they should both work.
If they don't, the first thing I can think of is a "phantom" connection between the 9V and 18V sections. Did you make sure with an ohmmeter that the 4 windings are independent?
If you connect just the CT's, without rectifiers or anything, does the light bulb light?
Second possibility is a shorted diode or cap: if the previous test passes, and the problem appears when you connect the rest of the circuit, that must be it
Thanks Elvee
I have two "should be identical" trafos and I quickly measured the one not in circuit.
The windings are separate, ie they show -> 0 ohm measured between winding taps and about 0.8 Mohm between taps on separate windings (interwinding capacistance?)
I will try change rectifiers/caps when I get the moment. Right now I use RURG3020C and some Rifa 10 mF.
But its super to know that I didnt think wrong when making the couplings.
/Staffan
I have two "should be identical" trafos and I quickly measured the one not in circuit.
The windings are separate, ie they show -> 0 ohm measured between winding taps and about 0.8 Mohm between taps on separate windings (interwinding capacistance?)
I will try change rectifiers/caps when I get the moment. Right now I use RURG3020C and some Rifa 10 mF.
But its super to know that I didnt think wrong when making the couplings.
/Staffan
Note that even if the light bulb doesn't react and you get the correct DC output, there might still be an error in the coupling: if you connect the windings in phase, nothing will smoke but the rectification will be half wave
Ok. I did those tests now:
1. Windings in series
2,3. 9 and 18 V center tapped
4,5 and 6. Center taps connected
Does this mean it is correct as I have drawn it (Except for that the unloaded voltage is a little low. Thats what the test was about to investigate from the start.)
best
Staffan
1. Windings in series
2,3. 9 and 18 V center tapped
4,5 and 6. Center taps connected
Does this mean it is correct as I have drawn it (Except for that the unloaded voltage is a little low. Thats what the test was about to investigate from the start.)
best
Staffan
Attachments
Stop the presses! I found me a "late evening soldering mistake"😱. Coupling no 2 now gets me 24,6 and -12,1 VDC with 100 ohm load on each. And I will now test what it gives with 6 respectively 2,5 ohm loads😱.
So for the poll, is any of them to prefer?
So for the poll, is any of them to prefer?
Good. Note that the measurement you made in #8 are only valid if you first checked with an ohmmeter the continuity between the measurement points, otherwise you see capacitively induced voltages between windings, which are useless and misleading.
I prefer the first option, all the windings are referenced to ground.
Thanks Elvee,
In this configuration, is it correct that my DC out should be (AC x 1,41) - Vdrop over one diode?
What I measured before I went on holiday was that I had 18,3 VAC on the positive that gave me 23,45 on the DC rail.
18,3 x 1,41 = 25,8. That means that my diodes drop 2,35 volts. According to spec they should drop 0,3-0,6 depending on temp.
In this configuration, is it correct that my DC out should be (AC x 1,41) - Vdrop over one diode?
What I measured before I went on holiday was that I had 18,3 VAC on the positive that gave me 23,45 on the DC rail.
18,3 x 1,41 = 25,8. That means that my diodes drop 2,35 volts. According to spec they should drop 0,3-0,6 depending on temp.
The discrepancy may be due to the distortion of the mains waveform.
The typical "flat topped" wave can only charge the caps to the peak of the flat top. The DMM measures the average voltage and is then converted to an equivalent sinewave reading. This will over-read the sinewave since it assumes that an undistorted sinewave is what is being measured.
Check the Vdrop of the diode/s.
The typical "flat topped" wave can only charge the caps to the peak of the flat top. The DMM measures the average voltage and is then converted to an equivalent sinewave reading. This will over-read the sinewave since it assumes that an undistorted sinewave is what is being measured.
Check the Vdrop of the diode/s.
Thanks Andrew.
That is done by sending DC thru one?
Might it also be the old Elna 10 mF 63 V longlife caps that I found in the closit and used for the test?
/Staffan
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To my simple knowledge there is AC on one side and DC on the other side of the diode and that is what I just measured. Shall I just set the DMM over the diode and measure?
PS. I am not there now so I cant trial'n'error. Excuse my blunt knowledge.
/Staffan
No, it will not work. But don't worry, you're in the ballpark, and rectifier diodes cannot have a Vf very different from 1V unless they really have a problem.
😀
Is this some kind of cluedo?🙂 How do I measure Vf in circuit if not the way I did?
Heres Vf according to paper:
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put DMM to Vdc and measure DC drop across diode , same as resistor is in question
over-engineering it , again ?
over-engineering it , again ?
Dude🙂 you always tell me that when I'm trying to learn something.
Me: I have this large drop on low drop resistors according to my measurement and calculations
Andrew: measure Vf in circuit
Me: Wasnt that what I just did?
Ballpark, overengineering 🙂
Me: I have this large drop on low drop resistors according to my measurement and calculations
Andrew: measure Vf in circuit
Me: Wasnt that what I just did?
Ballpark, overengineering 🙂
basic learning is for silent mode , at home
not because is waste of time asking simple answers to simple questions , but because of already established praxis that simple questions are resulting in damn confusing threads , which is resulting in further confusing of future greenhorns
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