Dual channel PSU transformer question

[belzrebuth]

I was wondering if I can use two of those banggood psu kits together to make a dual channel power supply with only one transformer.
Here is a link of the psu in question:

http://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html
I know the parts aren't the best for the claimed specs etc but I won't be using this for any critical or current hungry tasks and I thought It'd be a good a idea to chain two of these together.

So I have fairly big transformer with many taps.

Measuring the tap voltages (no label on the traffo itself) I can get  24V (2x12 center tapped ) and from another secondary I can get 50V (2x24).
That's also 24V when probing the center tap and one of the 50V terminals.
The two secondaries are isolated from each other (I get infinite resistance between any of their pins)

Given the size/weight of the transformer I think that both of its secondaries can handle the amperage I plan to use the PSU for.
(I think about 1.5A max ,only for powering small projects,low current electronic devices,testing things etc)

I've already bought one kit and it's working nicely with the 24V outputs of the trafo.

So ,my question is:
What would be the best way to connect these kits together with this transformer?

A} Connect the two of them in parallel with the 24V taps?
I think this should forbid connecting the two supplies in series though because I would essentially be shorting the transformer's secondaries, no?

B} Connect one kit to the 24V tap and the other with the 50V tap?
I think that could be the safest bet since the 24 and 50V secondaries are isolated from each other.
Is that the closest to having two separate power supplies?
Any hidden traps?

or
C}
Connect them in series?( one 24V to one AC input of the first supply and then the other to the same input of the second supply and then the two remaining AC  inputs together?

I'm not sure if I have thought or even understand C correctly so I don't know if it's even a valid option..

[Kleinstein]

If you don't need a high current, the best way would be using separate windings for 2 kits. This way one could also have the two kit's wired in series to give something like a +-25 V, though individual adjusted. For more current with one channel one might opt for using a switch to use the 12 V tab if needed, for 0-10 V use.

It is a good idea to limit the current of this kit to less than 3 A anyway. It gets more viable at 2 A max. With a 24 V transformer one might have to look for OPs, that can stand a higher supply voltage and maybe modify the negative supply a little (e.g. lower voltage zener). Still the maximum output voltage might be a little less than 30 V (e.g. 28 V), reaching 30 V only at low current and high grid voltage.

It might be good idea to a stronger diodes for revers protection if you have 2 channels capable of more than 800 mA.

p.s. It's a good idea to measure winding resistance: the 2x12 V winding might be weak, if the transformer is from an old audio amplifier.

[belzrebuth]

How much resistance should I be getting from the 2x12 windings in order to have about 1-2A available in your experience?
I have a toroidal here rated for 3.75A and I'm getting the same exact resistance figures (0.5ohms between the 24V taps and about half (0.2) between the one 24 and the tap (12V))

The transformer I've got has only a Sony part number written on it:
1-429-435-11
A friend gave it to me and it was indeed salvaged from an old Sony hi-fi system.
Power consumption of that model was 80 watt (from what I've just found online.)

I think I've also downloaded the service manual for the device it came from some time ago but I don't think I've seen any amp ratings for the transformer.
It's quite hefty though.

I think I'll order a second kit and try the dual psu build powered from the 2 independent secondaries.

[David Hess]

Wire the 24VCT with a full wave bridge rectifier to get 24V * 1.414 - 2 * 0.8Vdiode = 32 volts DC and wire the 50VCT with a full wave center tap to get 24V * 1.414 - 1 * 0.8Vdiode = 33 volts DC at twice the current.  Now you have two floating outputs of roughly 32 volts DC each so the regulator outputs can be connected however you want but it would be a good idea to add protection diodes.

Measure the wire diameter of each transformer winding to get an idea of its maximum current or at least the ratio of currents.

[Kleinstein]

For an 2 A AC capacity I would consider something up to about 1-1.5 Ohms resistance reasonable. This would be good for something like 1.2 A DC after rectifier / filter without PFC.

The ratio of resistance is also a good measure, since the total power of the transformer can be estimated from the size / weight and former use.

Using the 2x24 V windings and just 2 diodes for rectification will not allow twice the current compared to just using one 24 V half and a bridge rectifier. It is more like 10% (up to 40% at most) more, since it is only spreading the loss, not reducing it.

[David Hess]

Depending on who you ask, it is like 50% to 62% greater.  From Hammond who should know:

http://www.hammondmfg.com/pdf/5c007.pdf

For full wave bridge, DC current = 0.62 X secondary AC current.
For full wave center tap, DC current = 1.00 x secondary AC current.

Using both in this case to get equal output voltages is better than nothing and and we do not know the exact specifications for the transformer anyway.

I have occasionally seen 60 Hz power transformer isolated switching regulators where PFC was used to get the most out of the transformer.

[Kleinstein]

The numbers from the article
http://www.hammondmfg.com/pdf/5c007.pdf
give the ratio of AC current to DC current. With the center taped version they only assume the current on one side. The article is still interesting to show how much AC (RMS) current is needed for a given DC current. However keep in mind the numbers depend on the size of the filter caps and the impedance of the transformer - so they are are not fixed constants: with a large (and thus low impedance) transformer and large caps the AC current will be higher and with small transformer it will be lower.

This is different from the current you can draw from a given transformer. This is limited by the heat produced. Here using the center taped winding is not reducing the losses significantly but only splitting them to both windings. So it is only the local heating that is avoided. But loss is also from the primary and much of the thermal resistance is usually from the transformer surface to the environment, not so much inside the transformer.