Electronics conundrum.

[ajlelectronics]

Bog standard transformer, 28V measured common to red either side, 28V which is expected.

Red to red however shows zero V? I'm missing something obvious, clearly.

 

[MetalMonkey]

What's the resistance of red to red and reds to black?
Perhaps it is not centre tapped, just two red leads connected to one end of the coil?

 

[Screwdriver]

Two separate coils aren't they? Voltage will be induced in each with respect to common. Wouldn't expect to find a voltage developing between the two. You should be able to short the two red ends together and still get the same 28v albeit with higher power rating.

 

[ajlelectronics]

Two separate coils aren't they? Voltage will be induced in each with respect to common. Wouldn't expect to find a voltage developing between the two. You should be able to short the two red ends together and still get the same 28v albeit with higher power rating.

So what you are saying is that the two coils are in antiphase and cancelling?

 

[ajlelectronics]

What's the resistance of red to red and reds to black?
Perhaps it is not centre tapped, just two red leads connected to one end of the coil?

It is meant to be a CT. 4r each winding and 8r total.

 

[voipio]

Two same voltage secondaries, but sharing one common, perhaps used to reduce the crosstalk between two circuits? It's certainly not centre tapped at the moment.

 

[Red'n'Black]

I've never seen a transformer with two commoned secondaries that wasn't centre tapped, but it looks like that's what we have here.

A 2-channel scope would confirm it, if you have one handy.

Does the circuit it was powering offer any clues?

 

[MoreWellie]

this sounds ridiculous even as I write it but....

the black sleeve appears to contain the expected connections from two windings. Is it possible that when you are measuring between the two reds that the wires in the black sleeve are not making contact?

I appreciate they are tinned but the wire in the sleeve will likely be enamel coated

 

[ajlelectronics]

I've never seen a transformer with two commoned secondaries that wasn't centre tapped, but it looks like that's what we have here.

A 2-channel scope would confirm it, if you have one handy.

Does the circuit it was powering offer any clues?

This is a rotator controller with mashed plastic gears, I am rebuilding it with electronics. It works by applying AC 18V ish from the common to one or the other windings so there is no reason why it would be "special".

 

[ajlelectronics]

DC voltages are all as expected.

 

[ajlelectronics]

This is what is going into the original case...

 

[Redsailor]

Centre tapped tx's are usually labelled as Xvolts - 0v - Xvolts. From the labelling on yours it looks like two separate windings with a common 0v

 

[Red'n'Black]

DC voltages are all as expected.

View attachment 451464

Was that with any load on the DC? If the windings are not centre tapped then what you have is two half wave rectifiers in parallel, which will give you a similar DC output to the centre-tapped version with no load.

Scope the voltage on the big electrolytic, with a few hundred ohms across it, and see if the ripple is 50 Hz or 100 Hz.

 

[ajlelectronics]

560r on the DC 37v output from the circuit shown. 50Hz ripple.

 

[Red'n'Black]

560r on the DC 37v output from the circuit shown. 50Hz ripple.

View attachment 451467

Ok that confirms it. If it was centre tapped you'd have 100Hz ripple.

My inclination would be to connect the two red wires together and use a 4 diode bridge. But I can't quite follow the 3 wire connection to the motor - is the new controller your own design, or a published one?

 

[ajlelectronics]

is the new controller your own design, or a published one?

https://televideo.ws/images/docs/rotor/Rotator_QST_1_2021_Copyright.pdf

 

[ajlelectronics]

Was that with any load on the DC? If the windings are not centre tapped then what you have is two half

I have got it sorted out in my head now, thank you very much. A decade ago that would have been such a triviality, but as I am getting older I find that my brain is getting far less useful!

 

[Barking Mat]

Over my head, like an aeroplane.

 

[ajlelectronics]

Over my head, like an aeroplane.

I have a (used to be) cheap and quite nasty aerial rotator. The controller to activate the motor is a triumph of cost saving and uses plastic gears on plastic shafts. Fairly quickly it all wears out and I have already printed a couple of new gears for it. This time it's terminal and so as they are impossible to find, I decided to build an electronic version in the old cabinet using the original transformer.

The original method of operation is very dirty... The motor in the rotator head is synchronous, meaning that it is locked to the frequency of the mains. There is a matching motor in the controller head and it is geared to drive a pointer.

To calibrate, one drives the motor until it stalls, then drive it the other way until it stalls. That is "synchronisation". It works but it is very much chewing gum and string. There is no actual feedback of any sort.

 

[ajlelectronics]

But I can't quite follow the 3 wire connection to the motor

Detailed in the attachment.

What I think might be going on with the tranny, is that one winding drives the rotator head, the other winding drives the local motor. Sadly I went in there with gay abandon cutting it up, without sketching out the original circuit.