Power Transformer Construction

[Steve_Bell]

I have a 200VA transformer kit with a ready-wound 230v primary and a separate space for winding of my own secondaries.

I need an HT sec of 240v tapped at 220v at 330mA and a heater winding of 6.3v, centre tapped, at 3.3A. I am only using about 100VA. Any hints for working out the turns per volt and the wire gauges I need?

thanks.

[retailer]

Try this web site for starters it's a good guide - once you have decided on the turns/volt and wire sizes you should do a calculation on the wire build to check that your design (wire + insulation) will fit into the winding window, if your calculations show a fill of approx 85% or lower it should all comfortably fit.

https://www.giangrandi.org/electroni...tml#core_table

[Diabolical Artificer]

Wind on 10 turns of wire, measure the voltage. 240/1.35 = 177, so say you need 180v RMS to get 240v HT, divide that by whatever voltage result you got with 10 turns of wire. Then use attached table to find the right gauge, you'll need a thinner wire than you think. Do the same for the other windings.

At the same time you have to take into account the winding window,this is the tricky part. Or you could try this program - https://sklaic.info/electronics/transformer/ this works everything out for you. There are several other calculators out there too, but the one I linked to is the easiest to use.

There is also this article - http://ludens.cl/Electron/trafos/trafos.html and this PDF - https://www.***********/doc/31254622...G-Wolpert-2004

Andy.

[dougietamson]

Originally Posted by Steve_Bell

I have a 200VA transformer kit with a ready-wound 230v primary and a separate space for winding of my own secondaries.

I need an HT sec of 240v tapped at 220v at 330mA and a heater winding of 6.3v, centre tapped, at 3.3A. I am only using about 100VA. Any hints for working out the turns per volt and the wire gauges I need?

thanks.

Do you have details of the kit?

It would be useful to have the core area in mm2. Most transformers use waste-free EI laminations.
Measure the middle prong of the E aka tongue, this is one side of the core. You can also measure the height of the E or length of the I, these are the same length. You can then divide this by 3 to get the tongue width. The second side is the stack height, measure the the height of the lamination stack, eg if the the laminations are 0.5mm thick and there are 50 of them, the stack height should measure 25mm. You can just directly measure the stack instead of counting the number.
As an example, if the height of the E or I is 75mm then the tongue width will be 25mm, With a stack height of 25mm then we have a square core with area 25x25mm = 625mm2


You said the primary is already wound, do you have the number of turns?

[Ed_Dinning]

Hi Steve, if this is a conventional kit with the windings side by side and a central insulation barrier (to meet safety regs) then the have poor regulation and you will probably need additional compensating turns to get the voltages you need.
I take it you are using solid state rectification?
Valve rects have a bigger volt drop and again need additional turns.

The 3.3A winding needs to be 1.1mm^2 wire; 1.25 dia or 18 SWG

330mA needs to be 0.4mm dia

Note that these are typical sizes, if you go thinner the winding will get warmer and regulation will be poorer.

Ed

[M0FYA Andy]

Is this one of the RS Transformer kits? The data sheet quotes the TPV for the 200VA kit as 2.1

Andy

[Steve_Bell]

M0FYA Andy - it looks like an RS kit but has no stock number to verify. I have no further info and don't know the number of primary turns. I did a quick test and came up with a figure of 1.9 Turns/Volt.

Core cross sectional area is 1443mm^2. Width of the E section is 31.5mm. I have 20mm height available on the former for the secondary windings.

Ed - Windings are as you describe and I am using solid-state (bridge) rectifier. I have 1.25mm and 0.46mm dia. wire.

Thanks for the links.

[dougietamson]

Originally Posted by Steve_Bell

M0FYA Andy - it looks like an RS kit but has no stock number to verify. I have no further info and don't know the number of primary turns. I did a quick test and came up with a figure of 1.9 Turns/Volt.

Core cross sectional area is 1443mm^2. Width of the E section is 31.5mm. I have 20mm height available on the former for the secondary windings.

Ed - Windings are as you describe and I am using solid-state (bridge) rectifier. I have 1.25mm and 0.46mm dia. wire.

Thanks for the links.

There's some info about the RS kits in this thread:
https://vintage-radio.net/forum/showthread.php?t=55926

the attached RS Kit datasheet pdf only has info for 20/50/100VA though but still useful.

Does anyone remember how much these kits cost compared to a similar VA rated complete transformer which could be rewound for custom winds?

Doug

[Steve_Bell]

I have finally wound my transformer but am missing the correct terminals to connect the secondary windings to. I have attached a picture of what the primary terminals look like and have drawn a crude representation of the terminal type. Does anyone know what these are called and where to get them?

thanks,
Steve.

[Diabolical Artificer]

Not sure but you could use a round ground lug wrapped around a bit of 2.5mm copper earth strap.

Andy.