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Unlabeled Transformer's Parameter issues

rajaram04

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Hello Everyone !

I got this transformer in picture from my pal . . . It was installed in an old model cassette playing music system with a cassette loader carrying a stereo head , a 12v old motor , couple of leaf switches and an audio amplifier board with IC 4440 . . . Now Transformer inside has no label as you are able to see in figure below, but here with 250v AC input the unit is showing AC output of about 14v and the primary coil resistance value showing 85 Ohms & Secondary Coil approximately shows 20 Ohms, all on a digital scale . . . . . . . .

Issue here is the unknown power rating of the unit . . . . I mean to say that when I usually go for a purchase, I use to ask like => Sir, Please get me a 12 volt 1.5 Amp transformer or get me a 9v 250 Amp transformer and so on like that . . . . . . So now how to determine the actual rating of this local made device ????

Please help me finding it out for power rating !!

Thanks !





20210504_212501.jpg
20210504_212627.jpg
 

KlausST

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Hi,

What size is the transformer?
Is 20 Ohms / 85 Ohms the DC resistance without being powered?

Klaus
 

betwixt

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Pure guess based on experience with similar units - about 1.5A.
Transformers were one of the more expensive parts in old audio equipment so they were rarely over-rated (and often under-rated!) for the job. It would have to supply maybe 200mA for the tape mechanism and enough to keep the amplifiers happy.

Brian.
 

rajaram04

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Ya ya with no power nothing. . . . Just a direct measurement between 2 terminals . . Anything I need to do other other this ??
Size I hv to check for each side . . . I didn't take .
Hi,

What size is the transformer?
Is 20 Ohms / 85 Ohms the DC resistance without being powered?

Klaus
Power
--- Updated ---

Pure guess based on experience with similar units - about 1.5A.
Transformers were one of the more expensive parts in old audio equipment so they were rarely over-rated (and often under-rated!) for the job. It would have to supply maybe 200mA for the tape mechanism and enough to keep the amplifiers happy.

Brian.
Yes that 250 mA generally installed in small tape recorders where 6v supply were also there in some units and those transformers very very small size . . .
Here it's a lil big one seems more than a 1 or 1.5 amp . . . .
 

rajaram04

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Let me go through the stuff !! thanks
--- Updated ---

But
if the LV secondary is a true 20 ohms, then for a dissipation of 3 watt the max rms current allowed would be 350mA.
But size here is big enough and not satisfying the condition of 350 mA physically . . . . Here its like 1.5 or 2 A . .. then how come ?? Please elaborate ! thanks
--- Updated ---

We normally see a winding resistance ratio R2/R1 = (n2/n1)^2. The secondary resistance is unusually high
Ya but here its showing 20 ohms . . . May be multimeter or some other hardware issue is there , can't say !! As we are here under lockdown so no related shops are opening for any kind of maintenance or repairs . . . . Need to check it again . . let's see what can I do !!
 
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betwixt

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Rajaram04, you have to understand how the ratings of a transformer work. For example, 1 A transformer doesn't limit the current it can produce to 1A. As the load current is increased, the secondary voltage drops and the heat losses in the core and windings increases. The rating a manufacturer quotes is based on a permittible temperature rise and the secondary voltage at a particular load current.

You say the secondary voltage is 14V, presumably this is measured by applying 250V across the primary and you connected your voltmeter directly across the secondary wires. To me, that suggests it is a 12V transformer and would produce 12V under rated load, in other words it is designed so it's output would drop 2V at it's
design current rating. You could connect a 12 Ohm resistor across the secondary and see if the voltage dropped to 12V, if it did, the transformer is probably rated at 1A.

Brian.
 

treez

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Can you tell us
Primary resistance =
Secondary resistance =
Primary inductance =
Secondary inductance =

You can use eg an LCR40 meter to measure inductance

Now NP/NS = sqrt (LP/LS)

NP/NS = VP/VS

LCR40
https://docs.rs-online.com/7fc9/0900766b80e3ca9f.pdf
--- Updated ---

Also, attached is an LTspice sim (LTspice is free download) showing your mains transformer....open it up and put in your parameters, to check your workings out...or just play with the sim and work it out by trial and error
 

Attachments

  • Mains Transformer.zip
    724 bytes · Views: 0
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