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.

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!

Re: HT / filament transformers from Aliexpress
 

An expectant public awaits your endeavours ;)

B

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.

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.

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...

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

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.

Re: HT / filament transformers from Aliexpress
 

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.

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

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.

Re: HT / filament transformers from Aliexpress
 

They have finally shipped. Chinese new year delays :-)

Re: HT / filament transformers from Aliexpress
 

About time!!! It is the year of The Dog.

Re: HT / filament transformers from Aliexpress
 

I would not trust one of these for continual or long term unattended connection to the mains.
The markings clearly suggest that it is not intended for 240 volts input, which our mains still is in reality most of the time in most places.

As well as the risk of failure, remember that the mode of failure may be more dangerous than expected. The insulating materials used may be flammable, resulting in it burning like a firelighter rather than emitting a bit of smoke and then blowing a fuse.

Possibly OK for short term use for tests and experiments, but I would be uneasy regarding unattended operation, as for say a pantry transmitter.

Re: HT / filament transformers from Aliexpress
 

If I was running a permanent pantry transmitter I’d probably do it solid state myself. This is more wire and fire experimentation toy territory than permanent fixture.

I will run them up continuously during the test cycle to see of course.

Re: HT / filament transformers from Aliexpress
 

Well transformer number one arrived this morning finally along with a bunch of other toys from the far East :)

Came in a large well packed and padded box. Looks well made and the taps are actually neatly tinned stranded copper wires. It's approximately 2x larger and considerably weightier than the rather underspecified transformers used in heathkit VTVMs. Rates in quality and appearance similarly to the Vigortronix transformers I have been buying for a few years now.

There is no time to test it this week so I will wait for the other one to arrive first. At first glimpse though, it looks good.

Attachment 158641

Obviously it's going to be run at 50Hz and 240V so we'll see how that goes :)

Re: HT / filament transformers from Aliexpress
 

Yes; based simply on what I can see of the gauge/insulation of those red leads on the primaries, that looks like quite a viable component :thumbsup:.

B

Re: HT / filament transformers from Aliexpress
 

And specimen number two arrived today...

Attachment 158735

Notably this is also pretty well made. This one is rated 220V 50Hz with 150v and 6.3v secondaries so is the most viable candidate for success so far. This is likely to go in my VTVM project.

Also noted that both have 105oC PVC rated cabling and the high voltages taps have double insulated wire on both transformers. Construction is typical EI stacked laminations, copper windings and plastic bobbin. All neatly taped.

Strangely it was shipped from Southall, not that far from me via Royal Mail tracked 48 despite being shipped from China. Must be a drop shipping operation. Good show either way.

Ordered some choc blocks and some power resistors to hook up and will start testing soon (when I get some time which might be a couple of weeks off yet)

Re: HT / filament transformers from Aliexpress
 

Quick update on this. More than a couple of weeks have elapsed now, due to a combination of work concerns and sorting out my intermediate license. All sorted now. I've got a two week queue of stuff in front of me, like gardening and other uninteresting things, before I'm going to get to look at these transformers however. Boo hiss. Will post back as soon as I have something :)

Re: HT / filament transformers from Aliexpress
 

Finally some progress.

Firstly specimen A, as sold here: https://www.aliexpress.com/item/Tran...712131800.html

Test equipment:

1. calibrated Keysight U1241C DMM
2. calibrated Fluke 62Max IR thermometer.
3. Loads consisting of a selection of wirewound resistors connected to a choc block

Unloaded voltage on the LV winding is 7.4V RMS

Loaded voltage on the LV winding pulling 600mA (60% rating approx) is 6.3V RMS on the dot.

Unloaded voltage on the HV winding is 162V RMS.

Loaded voltage on the HV winding pulling 17mA (85% rating approx) is 146V RMS

Left for two hours under load, the temperature of the transformer windings measured with the IR thermometer peaked at 41.6 degrees C. Chassis at 38.1 degrees C.

No sign of any problems, degradation or breakdown.

Next one coming up in a few days ...