Help identifying transformer windings

[SurreyNick]

Mod Edit: When testing a mains transformer, especially an unknown one, great care is required. Although not stated in the opening post, Nick tested this transformer using a low voltage (10V), and scaled up the measured voltages accordingly. This is an excellent approach, and one which should be considered if you need to do a similar thing. If in any doubt about how to safely test a transformer, or indeed have any safety-related concerns about topic, please ask for help - that's what the forum is here for


I am confused. (It doesn't take much it seems!)

I have been given this salvaged transformer (see pics) and I want to label up the various windings and tap voltages. The difficulty I have is that I have no idea which winding was what, nor what the values of the various taps should have been and my naff DMM is giving me some rather odd values. I have attached a PDF with the various readings (ohms and volts) that I have obtained.

As you can see from the two pictures the transformer has four tag strips and there are five windings.

Tag strip A with its two tags I have named winding 1 and I have assumed this is the primary winding.

Tag strip B has one winding with five tags. I have named this winding 2. I get 240v across the first and last tag, which suggests a 1:1 ratio with the primary. The other values are shown in the attached PDF.

Tag strip C is two windings, one with three tags (winding 3) and one with two tags (winding 4). With winding 3 I get 1248v across the first and last tag with 624v between the centre tap and each end tag. This seems absurdly high. I get 10v between the two tags of winding 4.

Tag strip D is one winding with three tags. I have named this winding 5. It has continuity but I get zero volts. When I got the transformer the centre tap of this winding was strapped to the centre tap of winding 3 and when strapped I get continuity across the two windings but still no voltage! The ohms reading is very small and I'm wondering if this is a low voltage filament or rectifier winding and my naff DMM, which won't zero on AC Volts just can't get a low enough reading.

I guess my questions are:

1. Have I assumed correctly that winding one is the primary?
2. Do the readings seem reasonable - even the 1248v reading?

The benefit of your knowledge and experience would be most helpful.

Thanks

Nick
       
  Philips Power Transformer schematic.pdf (Size: 223.7 KB / Downloads: 47)

[AlanBeckett]

Evening Nick,

My inclination would be to put the Mains across Winding 2 as that has taps possibly for Voltage Selection.
Winding 4 and 5 may turn out to be Heater Windings. 
Winding 3 looks positively dangerous so be careful.
Winding one may be a straightforward HT Winding.

Any idea what it came from? It looks like a piece of Test Gear. Scope?

Which Windings have Thick Wires and which ones are thin?

Alan

[Mark Hennessy]

The clue on the PDF leads me to this: http://www.radiomuseum.org/r/philips_795a_795_a.html

If true, I'm not sure why it has the kV winding. It doesn't look entirely like the schematic on RM...

[Mark Hennessy]

Nick, have you measured the "magnetising current" in the primary yet? That is, how much current is this transformer drawing with nothing connected to the outputs?

That can be a good way of establishing if the primary really is what you think. If the current is small (less than 20 or 30mA, perhaps), then that's a good start. If it suddenly starts to rise as you reach 250-260V, that tells you that it's a 240V winding.

[pwdrive]

Philips Sonata (796a) possibly tapped primary here:

http://www.oldradio.cz/philips/ph796.pdf

Lawrence.

[SurreyNick]

It did indeed come off the chassis of a Philips 795A mark, but I don't know if it was an original item or a later replacement.
Alan, I think you must be right that winding 2 is the primary. I had a look at the link Mark posted to the 795A on RadioMuseum and it mentions Power type and voltage as (AC) 100, 125, 145, 200, 220, 245 volt. I don't know enough about power transformers to work out how each voltage would be accommodated from winding two, but I'm guessing it can be.
If winding two is the primary would this influence the voltages off winding 3? I'm guessing it almost certainly would if 100v was selected. The ratio would be the same right? But with 100v being at 41% of the 240v that would give circa. 500v and 250v off winding three I think.
Nick

[SurreyNick]

Nick, have you measured the "magnetising current" in the primary yet?

I haven't Mark, but I will give it a go tomorrow and let you know.
Nick

[Mark Hennessy]

Yes, despite the schematics showing a single primary with no taps, it definitely makes sense for winding 2 to be the primary. Let's see if we can work out the options:

• 100V: That's the two 53V windings in series
  
• 125V: I think that's using just the 122V winding
  
• 145V: Don't immediately spot that. Perhaps the 122+53V combination? That would be about 20% too low...
  
• 200V: I'm thinking the 122 plus 53V, which would be around 15% too high.
  
• 220V: Another mystery...
  
• 245V: All of them
  

So, assuming the numbers found on RM.org are accurate, then I would double-check the voltages across each of the tappings of winding 2. It might make more sense if one of them was lower than ~50V. Perhaps 25V? If we pretend for a moment that the lower tapping - the one that is 6.0 ohms - was actually 25V, then 145 becomes 122+25V, and 220 becomes all of them minus the bottom winding. Also, 200V becomes 25+122+53V.

Still worried about the 1250V winding. That suggests a CRT, as Alan hinted earlier. Was there a version of this set that had a TV (in which case is the transformer anything like powerful enough?), or perhaps a small 'scope CRT to provide a tuning display or something?

Perhaps it was a replacement part that could work in many sets? Any Philips experts who know about sets with CRTs?

[ppppenguin]

If that 1250V bites you it maybe the last thing you do. It's high enough to be lethal without any trouble at all. It's not really the voltage, it's because it can deliver enough current to kill you pretty quickly.

The worst shock I ever had was about 1200V DC on a 'scope. From a transformer rather like that, but rectified. I was lucky as I only made momentary contact and was thrown clear. My arm msucles contacted so violently that they were sore for a few days.

Please be very careful.

[SurreyNick]

Very wise words of caution guys regarding winding 3, thanks. I can't afford to get a shock, even a relatively modest one, so I will just put a few volts into the primary to re-measure this winding and then I'll insulate the terminals as a safety precaution.

Mark I am sure you, Alan and Lawrence have got it nailed insofar as winding two being the primary, and thank you very much for your explanation how the various voltages are achieved - a most informative and useful lesson. I am also pretty certain that when I get my better quality DMM operable again (just a flat battery to sort out) that I will get a different set of readings. I have a sensible disrespect for the inaccuracy of my £2.99 cheapo Chinese DMM

I'll re-measure everything again tomorrow and will then confirm findings.

Nick