torroid transformer wiring

[m8kingit]

i recently purchased a really nice transformer from ebay, however, i'm just wanting to make sure i'm correct in my thoughts before i wire this for my power suppply.

This transformer is in ther neighborhood of 100ova i have the spec sheet here in front of me, but , like i sadi, I'm not an electrician, so i need a little advice.
It shows to sets of primary leads and secondary leads. However they are all sharing a ground ( 3 wires on each tap)

I'm wanting to use one secondary tap, which states it is @29v ac and @19 amp. This is all i need, but i have an idea later to use the other tap for another machine.

My question is about this third wire in each bundle. I'm thinking on the primary side that i can wire the two coils together with the two 110 and just use the two grounds to get my 110 on both coils, but i guess it's not necessay if i only use one coil and secondary tap for now.

Can someone tell me if i'm even in the ballpark of what is correct.
Also, can someone tell me the different combinations of currents i can achieve with this . The gentleman that sold it to me said i could wire both secondary taps in series to acchieve 2-54vac taps, i don't see that possible becasue it's only a 1000va. I only need 1-29 volt 19 amp tap for now, but would like to know if i'll have the same on the other for later on for another machine.

PLEASE SEE THE ATTACHED SHCEMATIC!
Thanks in advance
Mark

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[Al_The_Man]

If I read the print right, a bit faint in places, the primary has two separate windings 0-100-120v?
If so you could either wire them in series for 240vac input or parallel them for 120v, make sure you use the same wire numbers for each connection, otherwise it will be out of phase, obviously do not connect the 100v wires when hooking up for 120v..
The secondaries appear to be two 27-0-27 or 54v centre tapped.
You can use this one of two ways for 27vac, either put a full wave bridge from a 0 cenre tap to one 27v out.
Or use the centre tap as common and put one Rectifier in each of the 27v sides for one set of windings, this is full-wave centre tap arrangement.
Any parallelling of windings must use the same numbered wire to connect together.
Al.

[m8kingit]

If I read the print right, a bit faint in places, the primary has two separate windings 0-100-120v?
If so you could either wire them in series for 240vac input or parallel them for 120v, make sure you use the same wire numbers for each connection, otherwise it will be out of phase, obviously do not connect the 100v wires when hooking up for 120v..
The secondaries appear to be two 27-0-27 or 54v centre tapped.
You can use this one of two ways for 27vac, either put a full wave bridge from a 0 cenre tap to one 27v out.
Or use the centre tap as common and put one Rectifier in each of the 27v sides for one set of windings, this is full-wave centre tap arrangement.
Any parallelling of windings must use the same numbered wire to connect together.
Al.

Al, is it necessary to paralell the two 120 along with the 2 center taps on the primary side. to get 120 input for both coils?

if i'm only wanting 120v input, do I just cap the 100v input wires?

When you say keep the wires in sequence do you mean as in from top to bottom as they read on the schematic?

Also, if i chose not paralell the secondary side, will i have two separte taps at 27 volts and 19 amps if i only connect each to a rectifier and a center tap?

[irving2008]

If you zoom it to 500% you can read it OK...

The primaries are indeed 0-100-120
The secondaries are 27.2V onload. 29v off load

The small print at the bottom says 'Calculated temperature rise at 1240VA cont load: 80degC'

Wire as follows for 120V primary:
Black + White (0v)
Brown + Grey (120V)
Isolate blue and orange wires

I think there's some mistake in the datasheet or this is more than 1240VA... not all secondaries can be 27.2 @19A simultaneously. I think the 19A refers to the whole secondary when used as a 1/2 bridge configuration

so thats 2 x 27v @ 19A with a bit to spare.

[edit]seems a quick google shows these transformers are well liked by the DIY audio crowd and they are indeed spec'd as above[/edit]

[irving2008]

Mark,

Following on from your PM...

This transformer is fine for your needs and the SB254 rectifier will do the job. You can certainly expect to get two 27v @ 12A out of it using those rectifiers and you may well go higher but you may find it doesnt get to 19A. this is because only 1/2 the winding is used over the full AC cycle so the average current density in the winding is higher and it may either get too hot or the magnetic core may saturate limiting the current output. I cant guarantee this will happen and it may be you can get close to 19A... just dont be surprised if you can't. It all depends on the way the windings are put on the core...

With a half-bridge each 1/2 of the winding is used on alternate half cycles so the average current density and core magnetic density is less.

As to the difference between full and half bridge, see the attached pic...

Incidentally you can still use the SB254 in the 1/2 bridge arrangement. The outer leg of the windings connect to the ~ inputs on the bridge but you only take the + output as your common output from the rectifier, the - side of the DC coming from the winding centre tap instead of the rectifier.

[jalessi]

Mark,

Easy to read PDF for this transformer.

Jeff...

[irving2008]

Mark,

See attached as to how to use the SB254 full-bridge as a half bridge. This will work fine.

I may be teaching grandmothers etc, but for safety I would insulate the - terminal of the bridge with a push-on connector and sleeve as it will be -80v relative to the + terminal and with the available current that would be potentially life threatening (50v is usually considered safe max). In any case you should never work on these sorts of voltages/currents when they are live and if you must (i.e. to measure volts), only with one hand at a time (and no jewellery/rings - I once saw someone burn their finger through when their ring shorted a 50v 100A buss in a phone exchange)

regards,
Irving...

[irving2008]

...i planned on using 33,000mfd caps for each tap, do you think that will be sufficient.

mark

I'll reply here Mark as your mailbox is full...

Capacitor in mFd = 80000 * I/V =80000*15/40= 30,000mFd for 0.1v ripple. or 10,000mFd for 0.3V ripple

You also need to consider inrush current... crudely:
C x V = I x t so:

I = C x V/t = .03(F) x 40 (v) / .016(s) = 75A (for 30,000mfd)

I = C x V/t = .01(F) x 40 (v) / .016(s) = 25A (for 10,000mfd)

SB254 good for 300A surge so no probs there but with 30,000mFd the primary current will be 20A surge against 5A running so need to fuse accordingly.

1% ripple is good enough so I'd probably go lower than 30,000mFd, but no lower than 10,000mFd

[Al_The_Man]

I generally design for 5% ripple, an increase in capacitance for lower ripple results in a higher transformer VA required at rated current.
Al.

[m8kingit]

I generally design for 5% ripple, an increase in capacitance for lower ripple results in a higher transformer VA required at rated current.
Al.

okay, now I im into another dilema. Someone, please tell me what type of capacitance in going to work best here.
I'm using gecko 201's and 3 motors rated at 5 amps.
Thanks
mark

[Al_The_Man]

The numbers I come up with for load of 15A at 40vdc and a 5% ripple at max load would be 25,000µfd the VA rating would be 534VA for 60% duty cycle.
I would go with at least 65vdc voltage rating for the capacitors.
Al.

[m8kingit]

Al, do you think i will blow a 20 amp breaker from the power surge when i power up using your advice on 25000 mfd filter caps. Keep in mind that eventually, I'll want to use both taps at 40v 19 amp at the same time, also, should i use a resistor connected to the caps to bleed them down when powered off?
Thanks
Mark

[Al_The_Man]

If you build in the 60% factor based on the improbability that all three motors will rarely, if ever be drawing max current at the same time, this drops it to 15,000µfd.
And this will lower your inrush.
The drain down resistor is optional at that voltage.
Al.