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Old 13th Feb 2018, 12:50 pm   #21
kalee20
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Default Re: HT / filament transformers from Aliexpress

Good test plan!

I doubt there will be any issues with insulation resistance, to be honest. Though it is a good thing to check.

Unloaded temperature rise - you may want to check every few minutes at first, the 'Can I put my hand on it' and 'Can I smell it' tests are good indicators.

As for temperature rise itself, you can use the primary winding to do this. Measure resistance cold (assume room temperature is 20 deg C); measure resistance after soak test, immediately after switching off. Work out the percentage increase. Then, temperature rise = % resistance increase x 2.54. Add the 20 deg C room temperature starting point and you'll get the final temperature in the heart of the transformer, which could be several degrees hotter than the outside.
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Old 13th Feb 2018, 1:47 pm   #22
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Default Re: HT / filament transformers from Aliexpress

Thanks - will do that. Rather neat trick using the primary winding. Beats working out where to wedge a thermocouple!
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Old 13th Feb 2018, 2:35 pm   #23
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Default Re: HT / filament transformers from Aliexpress

An expectant public awaits your endeavours

B
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Old 13th Feb 2018, 2:43 pm   #24
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Default Re: HT / filament transformers from Aliexpress

Yes I'm rather excited about this. If they aren't total rubbish, it opens a whole world of possibilities. I have two dead end projects which this may kick into action as well: VTVM and oscilloscope. Both are currently shot down awaiting suitable HT transformers. Not sure they have enough grunt to drive the VCR139A tube I have (with voltage tripler) but may be.
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Old 13th Feb 2018, 2:46 pm   #25
Bazz4CQJ
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Default Re: HT / filament transformers from Aliexpress

Maybe I could use one in my Acorn valve gdo? only needs 120V @ 5mA and 6.3V @ ~300mA. I have a transformer lined up but it is bigger than needs be.
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Last edited by Bazz4CQJ; 13th Feb 2018 at 2:53 pm.
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Old 13th Feb 2018, 2:54 pm   #26
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Default Re: HT / filament transformers from Aliexpress

Yes if you could drop the voltage down a bit. They are specified at 150 and 200v respectively. No idea what that will turn out at after rectification and smoothing under load. Possibly another experiment to do...
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Old 13th Feb 2018, 3:17 pm   #27
Bazz4CQJ
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Default Re: HT / filament transformers from Aliexpress

The original design (1940's) actually uses a 240V transformer, with a valve rectifier and then a voltage regulator tube to get down to 150V (the required anode voltage is only ~105V). I'll replace the regulator with zeners just to save space.

My 450MHz digital frequency counter (~8 inc p&p) arrived from China a few days ago and the quick tests I've done so far are all good. Mostly, my budget-buy Chinese cheapies have been very satisfactory.

B
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Old 13th Feb 2018, 3:24 pm   #28
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Default Re: HT / filament transformers from Aliexpress

I wonder if the Chinese still label things 220V as that was what the continent used before the whole EU became nominally 230V. As has been previously stated, the US hasn't been 110V, at least in theory, for a long time.

I see there's also a 150-0-150 version: https://www.aliexpress.com/item/Tran...fa7daf9a&tpp=1

I too await Mr B's report with interest.
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Old 13th Feb 2018, 3:26 pm   #29
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Default Re: HT / filament transformers from Aliexpress

Quote:
Originally Posted by Bazz4CQJ View Post
The original design (1940's) actually uses a 240V transformer, with a valve rectifier and then a voltage regulator tube to get down to 150V (the required anode voltage is only ~105V). I'll replace the regulator with zeners just to save space.

My 450MHz digital frequency counter (~8 inc p&p) arrived from China a few days ago and the quick tests I've done so far are all good. Mostly, my budget-buy Chinese cheapies have been very satisfactory.

B
Zeners also probably consume a lot less power than the valve rectifier and regulator tubes!

I bought one of those but I couldn't get it to stop turning off every 30 seconds or so. This turned into a journey of despair buying three different second hand brand name counters until I found one I really liked.
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Old 14th Feb 2018, 10:52 am   #30
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Default Re: HT / filament transformers from Aliexpress

There is a nice little 17VA transformer made by T.Giatras in Italy.
https://www.giatras.com/home
Their transformer IPOO CLF has a copper shielding to minimise magnetic effects with such things as RIAA amplifiers.
I have been quoted 12 Euros + VAT 24%, Post & Pack extra.
It has dual 115v primary windings and two separate secondary windings. One is 240V 10mA and the other is 14V 1 Amp
Although this model is not shown on their website, there is a nice little kit that shows the transformer in use:-
https://www.ebay.co.uk/itm/RADI0KIT-....c100675.m4236
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Old 14th Feb 2018, 11:13 am   #31
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Default Re: HT / filament transformers from Aliexpress

That's not bad actually. They look very well made.

I came across this as well yesterday: http://primarywindings.com/product/small-200v-50ma/

Works out at 26.20 including delivery. They also sell other transformers with a bit more grunt.
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