|
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. |
|
Thread Tools |
29th Jun 2020, 12:02 pm | #1 |
Octode
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 1,567
|
Transformers what for?
I will post this in here but not sure if correct place.
I picked up a couple of transformers recently, one is an old Radiospares 4 Volt "Standard" mains transf. "Normal". This has as the name suggests LT of 4 Volt 4 amp CT Rec of 4 V 2.5 amp. as well as the HT side of 250-0-350 at 80mA. So a couple of questions, what radio, amp or other would have been 4 Volt heaters? The second question relates to this and a second transformer I have:- When this says 350-0-350, 80mA, I am guessing that it would have been for a valve rectifier with two diodes and that each diode would have only done a max of 40mA hence 80 mA total, therefore I believe the windings could only deal with 40mA average current? Or am I wrong in the above assumption and could you draw 80mA average from each anode of the Rec. valve Could I in fact neglect the CT and bridge rectify the 350-350 to give 700 Volt at 80 mA which would mean the winding can take 80mA average? The second transformer is 250-0-250 at 75mA with LT of 6.3 at 2A and 5V at 2A or 6.3 at 1 A this being labelled as a later RS transformer Econ. Mains. Dumb question day. Adrian |
29th Jun 2020, 12:11 pm | #2 | |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
|
Re: Transformers what for?
Quote:
Chris
__________________
What's going on in the workshop? http://martin-jones.com/ |
|
29th Jun 2020, 12:14 pm | #3 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
|
Re: Transformers what for?
4V heaters were ubiquitous here in early/mid-30's domestic radios, after that date they started to be supplanted by the US-originated 6.3V series. Often, the HT secondary voltage would have been quite high to allow for the voltage drop of an energised speaker field winding that doubled as a smoothing choke with the low value reservoirs/smoothers of the era.
The convention was that the "80mA" referred to the DC output capability with a full-wave rectifier and capacitor-input filter- sometimes, there was also a figure for the higher current but lower voltage DC HT capability with choke-input smoothing. Not dumb at all! |
29th Jun 2020, 2:03 pm | #4 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
|
Re: Transformers what for?
4v was chosen as a heater voltage because it suited gauges of wire easy to work with and gave good life at the wanted temperature. Plenty of surface area for emission from directly heated valves.
4v was a 2-cell lead acid accumulator not on charge. You took it to the bicycle or radio shop and the man there charged it for you. You picked it up the following day and paid him. 6.3v is because the yanks were relatively wealthy and car radios became a thing over there. 6.3v was the voltage a car's 6v battery ran at with the dynamo doing its best. Having a 6v battery crank a huge straight-8 engine is an unbelievable strain on it, but they did it. And some of the posh ones of those monster automobiles that moved Mr Capone and his violin case toting pals around had glass partitions between the passengers and driver, and the passengers had the radio in the back. So, American valves converged on 6.3v and then they came over here. Cars went to 12v for more efficient starters and wiring. Radios stuck with 6.3v valves, but wired sinilar heaters in series pairs. Valves stayed at 6.3v quite happily. The next change was to series string 300mA or 100mA heaters for AC/DC sets. But 6.3v versions survived that as well. The two diode HT arrangement gave 80mA DC output rating. The current from the transformer was in pulses near the peak voltage times and peaked well over 80mA, but averaged 80mA over a half-cycle. While one side of the HT winding, the other one was resting and cooling. So an average of 40mA in each side, but it was really fiercer than that and alternating from side to side. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
29th Jun 2020, 2:28 pm | #5 |
Octode
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 1,567
|
Re: Transformers what for?
Thank you Chris, turretslug(?) and David.
I did not think about the old 2 Volt accumulators or the old valve radios although I have read about the era, just never put 2 + 2 together. David you confirmed the average 40mA per winding side of the HT thanks, that clears up a few anomalies in my head. turretslug you mention different types of input filtering, I have noticed some things in the rectifier spec sheets and this is something I need to study a bit more, I was thinking Cap Choke and Cap again for an amp as I have a 10H choke to use. Again thanks all. Adrian |
29th Jun 2020, 3:35 pm | #6 | |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
|
Re: Transformers what for?
Quote:
Each half of the winding can maintain the 80mA as it is only doing so for half of the time. Now if you use it in a 4 diode bridge rectifier circuit (700 volts AC) both windings are conducting all of the time so there is no half cycle resting / cooling period. In this case is it rule of thumb to rate it at 50-60% of the current value, so your 80mA transformer will be okay at 40 to 50mA... PS. Is there a confusion that the current from the two windings /rectifiers (centre tap full wave) add together? This is not correct, each winding and diode work consecutively and provide all the rated current, if needed, for each half cycle.
__________________
Never Leave Well Enough Alone... Last edited by snowman_al; 29th Jun 2020 at 3:45 pm. Reason: Added PS |
|
29th Jun 2020, 5:22 pm | #7 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
|
Re: Transformers what for?
"Dumb question day." Were here to share knowledge, if you've tried searching online or here, (search box up top) and can't find the answer, ask the experts : )
RS made a wide range of tfmr's for lots of applications, am no expert but they did an Economy line and a De Luxe line of valve amp tfmr's etc . The RS Deluxe PP OPT sells for mad money, it being used in the famous amp that Eric Clapton used to record the "Beano" album, guitarists will saw off their legs for one, it being possessed of The Tone. Andy.
__________________
Curiosity hasn't killed this cat...so far. |
29th Jun 2020, 5:32 pm | #8 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
|
Re: Transformers what for?
It's one of those holy grail things. People think they only have to get the same gaer a s Stevie Ray Clapton and they'll instantly have his fame and wealth. Skill doesn't seem to enter into their aspirations.
Yeah, they were only so successful because they happened on the right strings and output transformer. Anyone wanting great success and wealth would be better advised to work on what's coming next. If they really managed to duplicate Carlos Hendrix, and got it all just right, they'd have assembled the perfect tribute band. Frightfully derivative, old bean David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
29th Jun 2020, 6:15 pm | #9 | |
Octode
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 1,567
|
Re: Transformers what for?
Quote:
In my head I have it that as each diode only conducts for half the cycle, I appreciate that the current out of the transformer will peak higher as it will only conduct for a small fraction of the half cycle once the Cap has charged, but, over a full cycle one half of the transformer it is only doing 40mA Ave. On the other half cycle, the other half of the winding is doing another 40mA average, so therefore over the cycle there is 80mA average available. That is how my head is working. I also tried looking at it another way in terms of VA. As in, if the transformer was rated to give an output of 350 Volts at 80mA that is a VA rating of 28VA, therefore if I was to use the windings from end to end to get 700 Volts and maintain the same VA I could only draw 40mA through the winding. I looked at the Mullard EZ80 rectifier where it says 90mA output but with each anode having a rating of 45 mA and guessed it to be a similar operation to that. But wanted to see if I was understanding it correctly as I would not wish to overload a transformer. Adrian ps if only an amp could make us all famous. |
|
30th Jun 2020, 10:29 am | #10 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
|
Re: Transformers what for?
There are two ponts to be considered.
1. The primary works over the full cycle and this may be a power limitation. 2. The dissipation in the windings is proportional to the current squared so it is much better to run the windings continuously rather than in spurts. In order to smooth the current in higher power supplies, a choke input filter is used. This enables a smaller mains transformer to be fitted. I have not seen a plot of a transformer primary current when a choke input filter is used but expect it to be close to a square wave. |