Mains transformer primary winding(s) query - UK Vintage Radio Repair and Restoration Discussion Forum

buggies · 7th Sep 2018, 7:51 pm

Bit of a long initial explanation - skip to query at **

I have a NOS Farnell 5 volt 40 amp SMPS which I was going to advertise. Thought it would be as well to check it first and after a brief flash of the output LED it died. Dismantled and checked that 340 volts on the main capacitor but nothing on the output or the control board (5 and 15 volt auxiliary supplies).
Found the small transformer too hot to touch and very low output. Eventually traced to s/c 22uF 25v tantalum on the 15 volt rail.
The supply then worked ok but I was worried about the overheated transformer as the 15 volt rail was about 22 volts.

** There is a centre-tapped primary for 115/230 volt operation with a link change. I measured one half about 390 ohms and the other 680 ohms. Unsoldered transformer from pcb and got same readings (both DVM and Avo 8). Applying 115 volts to either primary gave the same secondary voltage. Eventually looked with magnifier and found that one half of the primary is wound with wire about double the thickness of the other.
I assume this is because when linked for 115 volts, only one half of the primary is used. Is this a normal transformer feature as I have always seen equal resistances previously?
TX301 in the circuit extract - right hand half 390 ohms.

Ed_Dinning · 7th Sep 2018, 9:39 pm

Hi George, probably a better quality transformer this one, as many dual voltage types just series/ parallel the windings. That is fine, but if only one winding is used for 110v then it will need to be rated for twice the current. This can give problems if the transformer is a "tight" design to fit all the windings in.

Ed

broadgage · 7th Sep 2018, 10:45 pm

If a transformer is to accept both 110/120 volt mains and 220/240 volt mains there are two different designs.

1) build the transformer with two identical windings that are connected in parallel for 110/120 volt operation, and in series for 220/240 volt operation.

2) build the transformer with a single winding for 220/240 volt operation with a centre tap for 110/120 volt operation. That half of the winding intended for 110/120 volt operation must be wound with thicker wire as it will carry twice the current.

The first way has the merit of minimum copper used and minimum overall size, but has the drawback that switching between the voltages is slightly more complex.

The second way simplifies the switching but on either voltage wastes about one third of the copper used in manufacture.
On 220/240 volt input, one half of the winding is of twice the needed cross section.
On 110/120 volt input, the thiner half of the winding is unused.

The first approach is favoured for very small transformers of a very few watts capacity, the waste of a few cents worth of copper is offset by the simpler changeover switching.

The second approach is favoured for slightly larger transformers when the copper cost is more significant.

buggies · 8th Sep 2018, 6:17 pm

Thanks Ed and Broadgage - I was completely unaware of the distinction - despite having spent most of my working life servicing equipment to component level.
I shall have to check my various transformers before use although I can't see me having to build anything for low voltage mains.

Argus25 · 9th Sep 2018, 12:17 pm

Post #3 misses out on option number "3" which is a way SMPS designers have solved this problem since at least the early 1980's.

To have a dual voltage SMPS with a small transformer to supply the low voltage/start up circuits, you do not need a dual primary winding or center tapped primary winding transformer for dual voltage operation. You just need a plain old 115V primary transformer for both 115V and 230V mains operation.

Why ? Because the bridge rectifier on the incoming mains voltage (and the filter caps) provides a low impedance virtual split incoming AC power supply rail supply. It is explained on page 4 of this article:

http://worldphaco.com/uploads/The_IB...WER_SUPPLY.pdf

So it surprises me that the designers had to resort to split winding or dual voltage transformer.So perhaps that circuit is a fine adjustment option, since the center tap is connected to the junction of the two filter caps. Actually it looks to me that the centre tap is just used to help keep the junction of the two filter caps at mid voltage.

daviddeakin · 11th Sep 2018, 8:26 am

Quote:

Originally Posted by Argus25

It is explained on page 4 of this article:
http://worldphaco.com/uploads/The_IB...WER_SUPPLY.pdf

When I simulate this and monitor the voltage across the transformer (modelled with a choke and resistor) I get a strange crossover in 240V mode, but not in 120V (doubler) mode?

7th Sep 2018, 7:51 pm	#1
buggies Heptode Join Date: Jan 2009 Location: West Lothian, UK. Posts: 762	Mains transformer primary winding(s) query Bit of a long initial explanation - skip to query at I have a NOS Farnell 5 volt 40 amp SMPS which I was going to advertise. Thought it would be as well to check it first and after a brief flash of the output LED it died. Dismantled and checked that 340 volts on the main capacitor but nothing on the output or the control board (5 and 15 volt auxiliary supplies). Found the small transformer too hot to touch and very low output. Eventually traced to s/c 22uF 25v tantalum on the 15 volt rail. The supply then worked ok but I was worried about the overheated transformer as the 15 volt rail was about 22 volts. There is a centre-tapped primary for 115/230 volt operation with a link change. I measured one half about 390 ohms and the other 680 ohms. Unsoldered transformer from pcb and got same readings (both DVM and Avo 8). Applying 115 volts to either primary gave the same secondary voltage. Eventually looked with magnifier and found that one half of the primary is wound with wire about double the thickness of the other. I assume this is because when linked for 115 volts, only one half of the primary is used. Is this a normal transformer feature as I have always seen equal resistances previously? TX301 in the circuit extract - right hand half 390 ohms. Attached Thumbnails __________________ George

7th Sep 2018, 9:39 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Mains transformer primary winding(s) query Hi George, probably a better quality transformer this one, as many dual voltage types just series/ parallel the windings. That is fine, but if only one winding is used for 110v then it will need to be rated for twice the current. This can give problems if the transformer is a "tight" design to fit all the windings in. Ed

8th Sep 2018, 6:17 pm	#4
buggies Heptode Join Date: Jan 2009 Location: West Lothian, UK. Posts: 762	Re: Mains transformer primary winding(s) query Thanks Ed and Broadgage - I was completely unaware of the distinction - despite having spent most of my working life servicing equipment to component level. I shall have to check my various transformers before use although I can't see me having to build anything for low voltage mains. __________________ George

9th Sep 2018, 12:17 pm	#5
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Mains transformer primary winding(s) query Post #3 misses out on option number "3" which is a way SMPS designers have solved this problem since at least the early 1980's. To have a dual voltage SMPS with a small transformer to supply the low voltage/start up circuits, you do not need a dual primary winding or center tapped primary winding transformer for dual voltage operation. You just need a plain old 115V primary transformer for both 115V and 230V mains operation. Why ? Because the bridge rectifier on the incoming mains voltage (and the filter caps) provides a low impedance virtual split incoming AC power supply rail supply. It is explained on page 4 of this article: http://worldphaco.com/uploads/The_IB...WER_SUPPLY.pdf So it surprises me that the designers had to resort to split winding or dual voltage transformer.So perhaps that circuit is a fine adjustment option, since the center tap is connected to the junction of the two filter caps. Actually it looks to me that the centre tap is just used to help keep the junction of the two filter caps at mid voltage. Last edited by Argus25; 9th Sep 2018 at 12:27 pm.

7th Sep 2018, 10:45 pm	#3
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,130	Re: Mains transformer primary winding(s) query If a transformer is to accept both 110/120 volt mains and 220/240 volt mains there are two different designs. 1) build the transformer with two identical windings that are connected in parallel for 110/120 volt operation, and in series for 220/240 volt operation. 2) build the transformer with a single winding for 220/240 volt operation with a centre tap for 110/120 volt operation. That half of the winding intended for 110/120 volt operation must be wound with thicker wire as it will carry twice the current. The first way has the merit of minimum copper used and minimum overall size, but has the drawback that switching between the voltages is slightly more complex. The second way simplifies the switching but on either voltage wastes about one third of the copper used in manufacture. On 220/240 volt input, one half of the winding is of twice the needed cross section. On 110/120 volt input, the thiner half of the winding is unused. The first approach is favoured for very small transformers of a very few watts capacity, the waste of a few cents worth of copper is offset by the simpler changeover switching. The second approach is favoured for slightly larger transformers when the copper cost is more significant.