How do I source transformers?

[jdurango]

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

Yeah it's 25v 470uF....Nichi or Panasonic, that's pretty much all I buy unless I need some weird value or package.

 

[Brian Roth]

Yeah it's 25v 470uF....Nichi or Panasonic, that's pretty much all I buy unless I need some weird value or package.

OK...I did some "what iffing" with PSU Designer (a great simulator IMO...at a great price...FREE!).

I attached the latest and greatest simulation. Not trying to over think this, but....using a 14VAC secondary, 470uF filter and 100 mA load.....

1. You can see the ripple waveform that will appear at the input of the LM340-12. It peaks at 18V and droops to around 16.5V. The regulator data sheet says dropout voltage is 2 Volts TYPICAL (which would be 14V) , so we have approx. 2.5V margin. The 18V peak is safely within the 25V rating of your filter cap.

2. I looked through the Triad and Hammond Mfg. catalogs to see what were standard secondary voltages in smallish current ratings and there were a number of options. I stuck with those brands rather that trusting potential fire hazards made by MyChowHung <g> as sold on Epay or Crapazon.
Triad and Hammond are reputable and available from multiple USA distributors.

3. Wildcards in the sim. The actual load current. Hopefully 100mA is good. I did a wild guess of the resistance of the xfmr secondary at 0.5 Ohms. That spec is never found in the spec sheets and I don't have anything similar lying around to do an actual measurement. Maybe someone on the forum has something similar to take an actual resistance reading?

4. AC mains voltage and Triad/Hammond primary specs. Interestingly, both companies use 115VAC, and if the spec sheet is accurate will result in a slightly higher secondary voltage.....a good thing in this situation

5. Allowance for low/high mains voltages from the power company. A big sag will cause the LM340 to drop out of regulation and a high will cause the voltage across the filter cap to exceed the 25V rating.

Probably other Partridges in this Pear Tree, but I'll stop now!

Next item on the agenda is the physical size and mounting of the replacement.

I attached sims with a 14VAC and 15V secondary.

Triad and Hammond xfmr links:

https://catalog.triadmagnetics.com/category/power-transformers/

https://www.hammfg.com/electronics/transformers/power

Bri

 

[Cqwet Dbdfte]

Relays do not have very close tolerances for coil voltage. Test with variable power supply for pull-in and drop-out voltage. Max voltage is just about heat, that can be tolerated. A simple resistor is enough to drop out voltage, if needed.
The circuit that switches the coil may need a "catch" diode as the voltage on opening the circuit can be quite high.

 

[Brian Roth]

Relays do not have very close tolerances for voltage. Test with variable power supply for pull-in and drop-out voltage. Max voltage is just about heat, that can be tolerated. A simple resistor is enough to drop out voltage, if needed.
The circuit that switches the coil may need a "catch" diode as the voltage on opening the circuit can be quite high.

I agree that there is some "slop" in relay coil voltage pull-in tolerances, but with a quality manufacturer that spec should be the guaranteed voltage for the relay to meet all specs.

In this case, the OP says the 12VDC regulated supply appears to also feed a control circuit with a "logic" IC and associated components. Hence no need to dance around with tweaking the coil voltage. Just make the 12V supply work as designed by the amplifier's manufacturer, and that seems to be replacing a faulty xfmr.

In the pic in post #15, D105 appears to be a "catch" (I call it a flyback) diode.

Bri

 

[Khron]

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)

I don't suppose you had any way to monitor the current draw? Or noticed any remarkable heating of the LM340?

How about feeding some DC through those wires, from a lab / adjustable supply (or at least with a DC ammeter in series)?

I'm just thinking, since that stock transformer was so dinky, something downstream might just be sucking more current than it should, thereby dragging down the voltage.

I don't suppose you've considered removing the LM340, and measuring the DC after the bridge rectifier, without a load?

Let's face it, chunks of iron & copper fail faaaaaaaaaaar more rarely than silicon or other more "dainty" components...

 

[Disco Volante]

Relays fail waaay more often than transformers...