Transformer blind pin and other questions

Thread Starter

sollap

Joined Sep 24, 2022
11
I have an older spitznagel PCB transformer that has gone bad on an AC board for a grid tie inverter.

I cannot locate an exact replacement, but before I install something "close", I have questions about how this transformer was originally being used, because the circuitry does not seem to match the markings on the transformer.

The transformer says the 13.5v secondary pins are positions 5&6, and there are blind pins at 9&10. This is where the confusion lies. The circuit has a resistor placed across pins 9&10. If they are blind (no connection internally), why?

Here are images of the transformer:
Trans1.jpg
Trans2.jpg
Here is a picture of the board showing the secondary output side:
Trans-Board.jpg
Here is a schematic I put together from what I see on the board:
Trans-Schematic.jpg

The board is multilayer with a wide plane on the backside, but doing the best I can with a light, I am not seeing traces extend beyond the 9&10 resistor. I have also done some continuity scouting with a multimeter and not found that these pins are connected to anything else.

I'm not able to get much from the transformer since it seems cooked. The case is swollen and slightly browned on top, and the epoxy is swollen on the bottom with a small crack across it. The primary does not show any resistance to 20Mohms. The secondary shows 88 ohms. The blind pins show open. When I apply 240 to the primary I see nothing across the secondary or the blind pins. However, I do see about 3 volts from the secondary to either of the blind pins. Either 5 to (9 or 10), or 6 to (9 or 10) produce the same ~3v. This doesn't make sense, so I'm thinking the windings are shorted.

Here is info from Spitsnagel on this type of transformer. My older transformer (014 1200) is not listed.
http://www.spitznagel-gmbh.de/pcb-transformers-spk014.html
http://www.spitznagel-gmbh.de/userfiles/file/Spitznagel_B014_09.pdf

I have a second inverter board that is working. The voltage on pins 5&6 under load (at the resistors) is 20v. In the board photo, you can see a 2 pin header. That's on the load side of the resistors - maybe a TP. Anyway, it's 1.3v on the load side of those resistors.

So here are my questions for anyone that wants to take a stab at one or more of these:

1. Why was the resistor added across pins 9&10?

2. Do you know of a source for this older Spitznagel transformer or a functional equivalent?
I don't care about it's form or size, but this transformer is part of an 240v line power quality sensing circuit. I need to match it as closely as possible.

3. On the working unit, how did the secondary get to be 20v? Is it possible the transformer is very lightly loaded, and therefore I'm seeing a no load voltage? According to the Spitznagel pdf above, the no load voltage of a 12v transformer is 18.5v, so it would follow that a 13.5 transformer would show 20v no load. Or, is it something else?
 

Ian0

Joined Aug 7, 2020
6,294
At a guess, it is so that the board can accommodate a single 230V primary or a transformer with 0-115V 0-115V primaries.
(I've used the small Spitznagel transformers before - they run hot and fail just like yours - I'd never use them again)
There are plenty of suppliers making encapsulated transformer in that size. Block, I think is one, though 13.5V is an unusual secondary voltage.
The regulation percentage is huge - a transformer rated at 13.5V on load is probably 20V off load.
 

Thread Starter

sollap

Joined Sep 24, 2022
11
Thanks Ian0.

I also found Myrra, Hahn and some older Pulse transformers of this type. I went with a Block replacement due to availability in the U.S. and details on their website.

To replace the 13.5v 50ma Spitznagel transformer, I went with a 2x15v 33/66ma Block transformer. I would have preferred the 15v 1.2va transformer but I could not source it.

The reason for the voltage difference is the no load performance of the transformers. Because I measured 20.03v on the operational unit, I think this voltage is more important than the load voltage.

Block specifies the model I picked with a "No-load voltage (app. x factor)" of 1.4. The 15v x 1.4 is 21v.
The newer spitznagel transformer specs suggest a rating of 1.54, which I derived by dividing the rated voltage and no load voltage. Assuming my older transformer is the same, it's no load output would be 20.79v. The fact that I measured 20.03v on the operational unit is consistent with that calculation since the circuit is providing some kind of load, even if it's very light.

I'm hoping the extra 21/100 of a volt doesn't impact much. I don't think it will once it gets through the resistors. I'm also hoping the "sag" rate from no load to light load on the new transformer brings me close to the desired 20v under load. The new transformer is 16ma larger which may work against that, although I may be splitting hairs here.

My plan is to closely monitor the pins on the board that I previously saw ~1.3v on. I'm going to get a baseline over time from the working unit while I wait for parts, then I'll know what to aim for with the replacement.
 
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