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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 26th Feb 2015, 5:57 pm   #1
stuart_morgan_64
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Default Transformer off load

Just fitted a transformer that's been rewound. Sure all is fine. I received sometime ago, but due to various health reasons have only just refitted.

The primary had different colours so have used multimeter to work out which is which. Transformer is now fitted, powered up, no load reading, ie valves removed.

LT 7V circuit says 6.3V
HT 310-0-310 circuit says 265-0-265

Does this seem normal? Thanks Stuart
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Old 26th Feb 2015, 6:11 pm   #2
paulsherwin
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Default Re: Transformer off load

Seems plausible to me.
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Old 26th Feb 2015, 6:15 pm   #3
stuart_morgan_64
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Default Re: Transformer off load

Thought perhaps. There are a few valves to power up, I shall assume all is well, reassemble and fingers crossed. Thanks
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Old 26th Feb 2015, 6:18 pm   #4
ukcol
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Default Re: Transformer off load

Ditto.

There will be a small drop in voltage on load due to transformer losses.

Someone like Ed Dinning will give you the definitive answer.
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Old 26th Feb 2015, 6:59 pm   #5
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Default Re: Transformer off load

A rough rule of thumb is that voltages drop 10% or so between no load and full load. So your 7V would drop by 0.7V down to 6.3V (exactly what your circuit says) and the 310-0-310 would drop by 31-0-31 to 279-0-279. This is a bit more than you might have expected but it could be that the rewind has resulted in a slightly higher voltage than you previously had. And there can also be fluctuations caused by mains voltage changes.

In short I don't think the new transformer is far adrift.

Cheers,

GJ
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Old 26th Feb 2015, 7:12 pm   #6
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Default Re: Transformer off load

Well within normal engineering tolerances for such a thing, all will be well.
 
Old 26th Feb 2015, 9:06 pm   #7
broadgage
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Default Re: Transformer off load

Both the LT and the HT are a bit high, not by that much though and as others post the voltage will fall a bit when loaded.

Might be worth double checking that the correct primary tapping is in use, presuming that more than one is provided.
Could be a 220 volt or 230 volt mains tapping in use on a supply that is actually still about 240 most of the time in most places.
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Old 26th Feb 2015, 9:57 pm   #8
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Default Re: Transformer off load

Hi Stuart, it could also be you mains supply. If it is high (say 245) and the pri is wound for 240 or less you will get a higher output.
This is why there is always a tolerance applied when measuring a transformer . Best bet is to check open circuit and if reasonably close connect up and re measure on load, taking into account actual applied pri volts.

Ed
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Old 27th Feb 2015, 12:03 am   #9
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Default Re: Transformer off load

It's close enough to plug in the valves and switch on.

You might want to hang a meter off the HT line to check this is right, after all you don't want the HT electrolytics to go bang. If this is OK, then measure the heater voltage.

With the voltages 10% high, nothing will die in the amount of time it takes to do these measurements.
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Old 27th Feb 2015, 9:49 am   #10
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Default Re: Transformer off load

Surely the reading depends on the type of multimeter used? A cheapo meter may be reading anywhere between RMS and peak? Off load transformer voltages will normally read high but the readings you have should be ok for valve equipment
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Old 27th Feb 2015, 11:37 am   #11
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Default Re: Transformer off load

Quote:
Originally Posted by saxmaniac
A cheapo meter may be reading anywhere between RMS and peak?
Only a faulty cheapo meter could be that random. Most meters measure average or peak after full-wave rectifiying AC but then indicate the corresponding RMS assuming a sine wave. This is good enough for most purposes.
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Old 27th Feb 2015, 2:14 pm   #12
kalee20
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Default Re: Transformer off load

Yes - an AC meter, whether average-reading or true RMS, will be fine as we are measuring AC voltages straight from a transformer.

And for DC HT voltage, again, smooth DC is exactly what it would be calibrated for on the DC ranges.
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Old 27th Feb 2015, 5:12 pm   #13
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Default Re: Transformer off load

Thank you, I think the meter reads ok, I have a Avo 8 which I will compare to. With the above info I will plug up tomorrow morning. Thank you as always for the info and comments
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