How do I source transformers?

[jdurango]

I'm looking for a replacement xformer for an amp. I can't find it based on the markings on the xformer itself, but it's used in a simple rectifier circuit to create 12v dc for a relay that passes power through the main power xformer. The xformer I currently have is only producing about 8.5vac, which is not enough to close the 12v relay fully, which results in problems, heat, etc. I've tried to find a similar xformer with roughly 9:1 (or 1:9?) ratio that will convert 115v to roughly 13v, yielding about 12v after voltage drop through rectifier, but I'm unable to do so. I'm not even sure what specs I'd need to enter in @ Mouser to find this one....there are sooooooo many!

Can someone help? The amp itself in about 700W RMS, claimed 1400W peak, so the xformer needs to be able to hold the 12vdc/120vac relay closed at that type of load. Thanks in advance!

 

[Brian Roth]

Your pictures seem to show the xfmr you're attempting to use...correct? Does this application require something that fits into the same footprint?

What is the DC resistance of the relay coil in the amplifier?

Bri

 

[jdurango]

Your pictures seem to show the xfmr you're attempting to use...correct? Does this application require something that fits into the same footprint?

What is the DC resistance of the relay coil in the amplifier?

Bri

Correct that is the xformer I'm trying to source. Of course I'd like it to fit that exact footprint, but if that's not possible, all good. Right now I'm using a Variac turned down to about 13.3vac with an AC power cord soldered to the secondar pads, yields exactly 12vdc at the relay. I could even wedge a SMPS power brick in there if I really needed to, but again, not ideal. I suppose I could use any xformer in the 12-15vac range. The data sheet for the relay doesn't spec a min/max voltage, but I'm sure a volt one way or the other isn't gunna hurt anything.

Anyway, to answer your question, DC resistance is 270ohms, power consumption is 540mW

This is the relay in question
https://www.mouser.com/ProductDetail/TE-Connectivity-OEG/OZ-SS-112L200?qs=8wHch9UpSvaOeIGVJNZOlw==

 

[Brian Roth]

It appears that xfmr in the pix is PC board mounted. It will be tricky to find something in the exact physical package and pinout. You might have to resort to using a chassis mount replacement with "flying leads" for the electrical connections.

Bri

 

[jdurango]

It appears that xfmr in the pix is PC board mounted. It will be tricky to find something in the exact physical package and pinout. You might have to resort to using a chassis mount replacement with "flying leads" for the electrical connections.

Bri

That would work. There's plenty of space to mount it. But again, I'm not even sure how to filter the results for what I need. I need to knock 115v down to around 13v, so I guess I'm looking for 9:1 or 1:9, and 540mW = about 45mA @12v, so doesn't need to be too big, maybe 100mA rating (don't mind something bigger). I can find a ton of "12v" xformers, and I know with tolerances, they can often be as much as 14v....so might just order a few off Amazon, keep the one with voltage closest to 13.3v and return the rest. But yeah, I'd love any suggestions of how to filter these things out at either Mouser or Digikey. Thanks a ton for the help!

 

[Brian Roth]

You mentioned the relay requires DC for the coil so there is at least one rectifier and likely a filter cap following the rectifier. How are you measuring the DCV feeding the coil? A simple single diode/half-wave rectifier with minimal filtering will result in a "lumpy" waveform that will result in a measurement error with a "minimalistic" meter.

12.6 VAC was (is?) a common secondary voltage since it was used for tube filament applications. Also be aware that a lightly loaded xfmr secondary will provide a higher voltage.

There is a bit of "seat of the pants" when selecting transformers.

Bri

 

[jdurango]

You mentioned the relay requires DC for the coil so there is at least one rectifier and likely a filter cap following the rectifier. How are you measuring the DCV feeding the coil? A simple single diode/half-wave rectifier with minimal filtering will result in a "lumpy" waveform that will result in a measurement error with a "minimalistic" meter.

12.6 VAC was (is?) a common secondary voltage since it was used for tube filament applications. Also be aware that a lightly loaded xfmr secondary will provide a higher voltage.

There is a bit of "seat of the pants" when selecting transformers.

Bri

Yeah after xformer, typical bridge diodes, cap, voltage reg, reverse polarity protection diode right before relay. Measuring right at relay with a Fluke 87V.

A little higher voltage is fine, 13-14v isn't gunna hurt that relay. Again, I'm just not sure how to filter these results for what I need. I thought there'd be some options for winding ratio, and that would of course determine voltage on secondary.

With some Google searching and dumb luck I was able to find this, would should do the trick....should yield about 12.7vdc at the relay....close e-damn-nuff!

https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166L14?qs=PRAnc%2BlXlHhRT7edqZEYAQ==

 

[jdurango]

Like you said, there are a bunch of 12.6v xformers too. If I got one rated for higher amps (2a?) maybe it would actually supply closer to 13v? Giving me closer to 12v at the relay? I feel like it's better to go a little over voltage than under....all the components can handle it. I'm sure either is fine.

 

[Brian Roth]

Yeah after xformer, typical bridge diodes, cap, voltage reg, reverse polarity protection diode right before relay. Measuring right at relay with a Fluke 87V.

A little higher voltage is fine, 13-14v isn't gunna hurt that relay. Again, I'm just not sure how to filter these results for what I need. I thought there'd be some options for winding ratio, and that would of course determine voltage on secondary.

With some Google searching and dumb luck I was able to find this, would should do the trick....should yield about 12.7vdc at the relay....close e-damn-nuff!

https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166L14?qs=PRAnc%2BlXlHhRT7edqZEYAQ==

Wow....that DC supply is far more complex than I had imagined! Diodes .... two in a fullwave or four in a fullwave bridge configuration? What voltage regulator?

With a fullwave rectifier (2 diodes) you'll need a transformer with a center tapped secondary.

The regulator should "nail" the voltage to the relay at 12 VDC as long as the "raw" DC voltage is comfortably above the dropout voltage of the regulator.

That transformer you linked from Mouser is insanely oversized in terms of current if all you need is 45 mA (ish) for the relay.

Does this power supply do anything else besides driving the relay?

Bri

 

[ruffrecords]

It would help you could post a schematic of the power supply section that includes this transformer.

Cheers

Ian

 

[jdurango]

Wow....that DC supply is far more complex than I had imagined! Diodes .... two in a fullwave or four in a fullwave bridge configuration? What voltage regulator?

With a fullwave rectifier (2 diodes) you'll need a transformer with a center tapped secondary.

There are 4 diodes, so I assume full bridge? But no center tap on secondary. See pic. Only 2 of the secondary pads on the PCB are actually connected to anything. You can see I've got my AC input from my Variac plugged into those, supplying about 13.3v and the circuit is happy with 12vdc @ the relay (you can see little TP probe wires I hooked up there).

The regulator should "nail" the voltage to the relay at 12 VDC as long as the "raw" DC voltage is comfortably above the dropout voltage of the regulator.

That's a good point! The regulator is an LM340 which can handle up to 35v....of course I don't want it to work that hard if I can avoid it.

That transformer you linked from Mouser is insanely oversized in terms of current if all you need is 45 mA (ish) for the relay.

Does this power supply do anything else besides driving the relay?

It does supply a little standby power button board, with has a on/off indicator diode and a large 14 or 16 leg flip-flop IC (I think it's some kind of logic circuit for the power up/power down process). But yeah, 2a was the only 14v xformer they make.

I found this one, which is PCB mount, 16v, 260mA, and looks like it would fit in the footprint (can't believe they don't show pin spacing/pitch or layout in the data sheet???!??!)

If it mounts in the current place, I'm sure pinout will be wrong, but I can always cut traces and rewire correctly.

This would probably work too, 75mA, I doubt that diode and flip-flop pull much amperage....but probably better to have a little overhead with the 260mA one.

Bri

 

[Brian Roth]

There are 4 diodes, so I assume full bridge? But no center tap on secondary. See pic. Only 2 of the secondary pads on the PCB are actually connected to anything. You can see I've got my AC input from my Variac plugged into those, supplying about 13.3v and the circuit is happy with 12vdc @ the relay (you can see little TP probe wires I hooked up there).

Yes....4 diodes = FW bridge. Not sure what you are referencing re. Variac connection and probe wires. All I see are two pix of the transformer.

That's a good point! The regulator is an LM340 which can handle up to 35v....of course I don't want it to work that hard if I can avoid it.

That would be a LM340-12...functionally the same as a 7812. What package is it in....any heat sink?

Also, what value is the filter cap (uF and voltage) that follows the FW bridge?

It does supply a little standby power button board, with has a on/off indicator diode and a large 14 or 16 leg flip-flop IC (I think it's some kind of logic circuit for the power up/power down process). But yeah, 2a was the only 14v xformer they make.

I found this one, which is PCB mount, 16v, 260mA, and looks like it would fit in the footprint (can't believe they don't show pin spacing/pitch or layout in the data sheet???!??!)

The data sheet shows the dimensions between pins, etc.

If it mounts in the current place, I'm sure pinout will be wrong, but I can always cut traces and rewire correctly.

This would probably work too, 75mA, I doubt that diode and flip-flop pull much amperage....but probably better to have a little overhead with the 260mA one.

I may seem to be a PITA <lol> but there's many small details that goes into even a simple power supply. I want to gather as much info as possible to tweak everything. For grins, I punched in some preliminary values into the Duncan Amps PSU2 app and attached a screen shot. Looks like 12.6 VAC may be too low to avoid the regulator dropping out.

Bri

 

[Whoops]

I'm looking for a replacement xformer for an amp. I can't find it based on the markings on the xformer itself, but it's used in a simple rectifier circuit to create 12v dc for a relay that passes power through the main power xformer. The xformer I currently have is only producing about 8.5vac, which is not enough to close the 12v relay fully

You are making a wrong assumption.

8,5 VAC after Full Wave rectification will be 12VDC.

8,5VAC x 1,414 = 12,019 VDC

So if the transformer is outputing 8,5VAC and the rectifier and filtering circuit after it are working probably you will have the correct 12VDC after the rectifier.

I really don't see what your problems is, you didn't explain the problem and why do you want to replace the transformer when all seems good.
So whats the problem you're having?

, which results in problems, heat, etc.

Heat is normal, if the temperature the transformer reaches didn't burn the transformer until now then it means all is fine, it can withstand the operating temperature

 

[Brian Roth]

You are making a wrong assumption.

8,5 VAC after Full Wave rectification will be 12VDC.

8,5VAC x 1,414 = 12,019 VDC

So if the transformer is outputing 8,5VAC and the rectifier and filtering circuit after it are working probably you will have the correct 12VDC after the rectifier.

I really don't see what your problems is, you didn't explain the problem and why do you want to replace the transformer when all seems good.
So whats the problem you're having?



Heat is normal, if the temperature the transformer reaches didn't burn the transformer until now then it means all is fine, it can withstand the operating temperature

The OP says that the xfmr/rectifier/filter cap feeds a LM340 (likely a LM340-12 aka 7812) and then the relay through some control circuitry. The regulator will need several volts more than 12 VDC to avoid dropping out of regulation.

In addition the VAC x 1.414 formula doesn't account for diode drops in the rectifier or the resistance of the xfmr secondary winding.

Ian requested the schematic of that part of the amplifier for inspection and I concurred.

Bri

 

[jdurango]

Man guys, I really wish I had the schems for this thing. I've been flying blind, fixing one problem, then finding another pop up. I've already replaced both the caps in this picture. It seems it'll need a complete recap, or at least the PSU's. I'm really hoping I don't need to recap all the amp cards, but the way it's been going, lots of bad caps so far. It's a 7 channel Cary Audio Cinema 7 amplifier.

Yes the voltage reg is an LM340T-12. It's my understanding that it'll need more than 12v in order the regulate the voltage down to 12v, but I've certainly been wrong before. It does seem odd that the xformer is putting out about 8.4vac. If it indeed failed, this is an odd failure state....still working, just putting out low voltage. Again, I wish I had schems....or could find a datasheet on this xformer (it's labeled "xb bdg-1013-019").

This pic may help. This is what I tried to post previously. You can see the TP leads at the relay, and the AC line I'm running into the xformer secondary via my Variac, set to about 13.3v yields 12v @ the relay. I could try lowering the voltage and see what happens at the relay, but pretty sure it will drop once the Variac output is below about 13v (leaving below about 12v after diodes for the LM340)

 

[Whoops]

The OP says that the xfmr/rectifier/filter cap feeds a LM340 (likely a LM340-12 aka 7812)

My fault then, I didn’t understand that.
Yes, with a regulator higher than 12v is needed

In addition the VAC x 1.414 formula doesn't account for diode drops in the rectifier or the resistance of the xfmr secondary winding.

It works well enough for an approximate value 98% of the time

 

[Brian Roth]

@jdurango What is the value of that filter cap following the rectifiers?

Bri

 

[jdurango]

@jdurango What is the value of that filter cap following the rectifiers?

Bri

It's 470uF now. I can't remember what it was before I replaced the bad one, I think it was actually 470 originally too. It was somewhere in that ballpark anyway.

 

[Brian Roth]

It's 470uF now. I can't remember what it was before I replaced the bad one, I think it was actually 470 originally too. It was somewhere in that ballpark anyway.

I'll revise my PSUD numbers (I had guessed 1000 uF) and post the results here this evening. I'm ASSuming the 470 you installed is rated at least 25 VDC; that will determine "how high we can go" re. the xfmr secondary voltage.

Another project for you <g>. Measure and draw out the physical dimensions of the outer footprint of the old xfmr and add in the dims. of the pin locations. You had one dim. in the pic in post #1.

That will be useful on the slim chance an off-the-shelf unit will match up.

Bri

 

[Brian Roth]

It works well enough for an approximate value 98% of the time

Diode drops are more of an issue at lower voltages. Two diode drops (approx. 1.4 Volts total) is nothing in a 250 VDC PSU. It matters much more at low voltages.

Bri