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No output from σ24

sigma25, sigma26, sigma24

No output from σ24

Postby brian92fs » December 2nd, 2017, 2:34 pm

This is a pretty basic question. I must be doing something very simple incorrectly. I built my first ε24 powered by a σ24, but it's not working. I think the problem is with the σ24 transformer board. I connected the σ24 to a power strip and placed a 82ohm 3 watt resistor across the molex pin secondary output. I can't measure any VAC output. My DMM reads 0.0.

I'm using a Zettler Magentics model BV301S12020 with a single 115VAC 50/60Hz primary and single 12VAC 2VA secondary. I bought several of these and tried swapping to another unit just in case the first was bad. Same result.

Am I doing something trivial incorrectly? Or did I buy an incompatible transformer?

Thanks in advance.
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Re: No output from σ24

Postby amb » December 2nd, 2017, 11:22 pm

The transformer you chose should be compatible with σ24. Did you check to see if the primary winding has the proper line voltage across the solderpads? Be careful with measuring this as you're dealing with AC line voltage. If you're using a fused IEC power inlet module, make sure the fuse holder is in place, and there is a fuse in it and not blown.
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Re: No output from σ24

Postby brian92fs » December 3rd, 2017, 9:09 am

Partially my problem. I have a new multi-meter and after rereading the manual, I realized I was using the VAC mode wrong. I now get 2.0 to 2.1 volts when I test for VAC. Same reading with or without the 82ohm resistor in place. I checked frequency and its right on a 60 Hz. I tried to read current and got zero.

I'm currently testing this on my bench. Its connected to a mains AC line in without a fuse. Something I should correct. I need to get an inline fuse holder for my testing cord. I also tried swapping to a different cord to make sure it wasn't my connection. Same results

Is my approach to testing valid? Without a load or with the 82 ohm dummy load, shouldn't I be reading 12v AC? I'm using a Amprobe AM-510 for testing (if that matters).

I also tried testing the voltage across the solder pads for pin 1 and 5 (transformer primaries) and it arced. I didn't have the 82 Ohm dummy load in place at the time. Without a mains inline fuse in place, I'm hesitant to try this again.

One other question: with the transformer removed from the PCB, I get continuity across the secondaries but not across the primaries. Is this normal?
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Re: No output from σ24

Postby amb » December 3rd, 2017, 10:38 pm

Just to make sure your DMM is working OK, try measuring the AC voltage of the primary side of the transformer. Also, measure the DC voltage across a battery to see if it's correct.
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Re: No output from σ24

Postby brian92fs » December 6th, 2017, 10:16 pm

Problem solved. Two issues: 1) Without a fuse on the AC mains line, I was being especially careful and not applying enough pressure on the pads to get a solid reading on my DMM. Once my inline fuse holder arrived, I added a 100mA fast burn fuse and was able to test with more confidence. 2) Testing revealed proper unloaded 20VAC at the secondary pins, but only 2VAC at the Molex pins. Tested resistance between the secondary solder pads and the Molex pins and found a bad solder joint. Re-soldered this and everything is working now.

Thanks for the help with this!
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Re: No output from σ24

Postby amb » December 6th, 2017, 11:56 pm

Cool.
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