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Old 5th September 2006, 09:58 AM   #1
barendh is offline barendh  Netherlands
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Default Parallel secondaries

Since my (admittedly rather lengthy) question about this remained unanswered , I'm trying an abridged version:

What problems can I expect if I parallel several identical secondary windings of a toroidal transformer?
If it is without penalties, should I check for identical winding direction?

Sorry, but my electronic basic training is so long ago (40 years) that some obvious things are sunken very deep inside my brains...
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Old 5th September 2006, 10:13 AM   #2
Did it Himself
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Just ensure the windings are correctly phased. Put a house light bulb in series with the primary to limit current in the event of connecting wrong, just to check first time.
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Old 5th September 2006, 10:52 AM   #3
AndrewT is offline AndrewT  Scotland
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Yes, that light bulb trick will save you and your equipment from severe damage if you make a mistake with your wiring.

Use it EVERY time you start up/maintain/ modify/build some new piece of equipment.

Build an extension lead :- plug top, bulb holder and socket outlet.
The mains light bulb goes into the live lead only. The neutral and earth leads pass straight through.

It goes between the mains socket outlet and your equipment plug top.
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Old 5th September 2006, 03:19 PM   #4
barendh is offline barendh  Netherlands
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Default Windings

Thanks,
I will try with 2 x 3 parallel windings first + bulb + 8 Ohms/100W loads on the outputs, see what happens.
Suppose my scope is a good indicator of phasing, because it's difficult to see where the wires are going on the toroid.
Or is there any other obvious way I'm overseeing?

I would greatly prefer the parallelling instead of running 28 (!) wires + detachable connector from the PSU to the amp proper.
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Old 5th September 2006, 03:24 PM   #5
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With a scope, you would connect one lead from each winding into a bunch to make a big common, then put the probe on each of the other wires in turn to note polarity.
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Old 6th September 2006, 10:04 AM   #6
AndrewT is offline AndrewT  Scotland
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Lightbulb Checking primary polarity

Hi,
The mains light bulb goes into the mains live feed to the primary NOT in the secondary.

You don't need a scope to check your phasing.

You need:-
A DMM with Vac beyond mains voltage, resistance 0 to 200ohms, Light bulb tester with plug top and socket outlet and a terminal block (with sufficient terminals to match the number of wires in your transformer) .

Wire all the ends separately into the multiway terminal block. The screws ensure you break through the enamel.

Now measure the resistance across all pair combinations to find which ends are connected to the same winding (open circuit confirms a separate winding). Write this down with the wire colour codes (even a dual primary and dual sec with screen adds up to nine terminals to check).

Low resistance indicates low winding voltage and conversely high resistance indicates high voltage windings ( a high VA transformer reads lower on all windings).
You can usually identify the primaries from this first checking process, but unfortunately not the phasing.
It will be worth separating the primaries into a separate terminal block now to help avoid accidental mains contact later.

1. Connect one primary to your plug top live.
2. Connect it's partner to the next primary winding (you have a 50% chance of getting this connection wrong). Use a short insulated wire link to connect between the the two terminals.
3. Connect the last primary wire to the plug top neutral.

As said above this could be wrong so the light bulb MUST be used.
Check you light bulb wiring is correct by testing it on a good piece of equipment (a working amplifier is ideal for this ). The bulb should be OFF when the socket outlet is empty.
The bulb should flash BRIEFLY as you switch on the power at either the mains supply or at the amplifier. The bulb glow should extinguish within a few tenths of a second or possibly upto a few seconds if the amp has a big capacitor bank. Is the light bulb working and indicating correctly?

Now switch off at the mains, plug your test transformer into the bulb circuit socket outlet.
Leave ALL the secondaries open circuit. Some might be high voltage so take care you have not shorts/tools etc lying close by (no , not clothing).
Switch on at the mains - if the bulb flashes very briefly as the transformer builds it's magnetic flux and goes out then all is OK.
If the bulb glows dim or bright then switch off immediately. This indicates that the primary phasing is wrong.

Go back to 3. above and disconnect the primary from the neutral.
Go back to 2. above and take the first winding to the primary wire that was 3.
4. connect the alternative second primary spare end to neutral.

Repeat the light bulb switch on procedure.
This time the bulb should flash and extinguish immediately.

Success, no blown fuses and no damage to the transformer.

Sorry this took so long to read. The wiring up and testing is almost quicker than the reading.

You will be working with Mains Voltage TAKE CARE.
and be wide awake/alert. A mistake here is potentially lethal.
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Old 11th September 2006, 02:54 PM   #7
dowser is offline dowser  Switzerland
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Kind of related, kind of off-topic - I recently bought a NAC 62 with a home-made PSU. Basic bridge, smoothing caps and LM series 24v regulator. I plan on making this a little better.

It currently has the transformer secondaries paralleled up - after a recent try using my bench supply to provide the 24v to the 62 (and disastrous sonic degredation [base and presence basically disappeared]) I am going to try removing the one of the secondaries to see if it makes an audible difference. Will it?

And regarding phase of secondaries - I ultimately plan 4 secondaries with a decent regulator for each to feed the 62's stages. What are the sonic issues with making sure secondary phase is matched to each bridge? Something I never thought of until reading this.....I've been a long time out of electronics!

Thanks, Richard
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Old 14th April 2007, 07:28 PM   #8
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Default Please help another noob or (boob)

This is the first I have seen the lightbulb trick.

I am building a BrianGT Stereo chipamp, which has one power

supply board and two amp boards. All boards are assembled.

I don't understand the light bulb trick, is it installing a light bulb

in series with the primary power lead and observing the caps

charge?

I am also stuck trying to understand how to wire the transformer

to the power board and the amp boards to

output, RCA terminals to speakers.


Thank You
Attached Files
File Type: pdf wiring .pdf (77.2 KB, 29 views)
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Old 14th April 2007, 09:50 PM   #9
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I don't think connecting transformer output windings in parallel is a good idea. If there is a small voltage mismatch, the higher voltage winding will drive a lot of current through the low resistance of the lower voltage winding.

I_F
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Old 15th April 2007, 04:31 AM   #10
Tweeker is offline Tweeker  United States
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Is a pair of antiparallel diodes on each winding an answer to this?

The toroids I have are all closely matched, but dual secondary, not several.

Some EIs had concentric windings with notably different resistances.

Hi Richard,
Quote:
What are the sonic issues with making sure secondary phase is matched to each bridge?
Are you feeding a separate bridge and filter for each regulator? In this case, it will all take care of itself if hooked up correctly.

If they are all paralleled into one bridge and its not matched, you have serious problems.

Also, paralleling regulators is involved. Are you feeding different channels and/or stages of the amp with each reg?
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