Isolation transformer from 2 toroids with multiple secondaries

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tizman

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Joined 2012
#1
Hi All.

A quick question. I built an amp recently that sources its 120 AC from 2 EI frame 0-35 volt power transformers with their secondaries connected. This is working fine with respect to the electrical operation and sound of the amp, but the two tranformers create a mechanical buzz that is easily audible from the listening position, and therefore annoying and distracting. I have on hand two toroid transformers that I could use instead. These transformers have multiple secondaries. They are as follows:
0-24V @ 48VA (green-red)
0-24V @ 48VA (brown-blue)
0-10V @24VA (white-grey)
9-0-9V @18VA (orange-yellow-orange)

While the connection of the EI transformers is very straightforward, the toroids have multiple secondaries. Should I connect all like colored secondaries wires together except for the yellow center tap wire on the 9 volt? Should I only use some of them? I am hoping to have as much capacity as possible for the circuit, although it will only draw about 70 milliamps.

Thanks in advance for your help with this.
 
#2
Existing transfomers have secondaries connected in series or parallel? What voltage does the circuit expect to see?

A 24V 48VA secondary can supply 2A RMS AC, which equates to a bit less than 1A continuous DC after rectification. This is significantly greater than 70mA so current rating is fine - overkill, actually. You sound like you are considering wiring the two toroids in parallel. If so, why?

I am hoping to have as much capacity as possible for the circuit, although it will only draw about 70 milliamps.
Click to expand...
What do you mean? If a circuit uses 70mA there is no advantage in taking a transformer which can supply 1A and doubling up to get 2A rating.
 
#3
Considering the thread title (rather than what you say in your post) it occurs to me that you could be asking about using two toroids back-to-back as an isolation transformer. Your use of the word "instead" could have sent me off down the wrong path, if by "instead" you actually mean 'as well as'.

So what are you proposing: replacement or isolation?
 
#4
if that is mechanical humming, that can be fixed by
wedging in Popsicle sticks in between cores and coil,
tightening bolts and nuts, dipping in electrical varnish,
a combination of these can help....
good luck and let us know if that helps...
 
#8
I'm scratching my head, too. If an isolation transformer is needed (what the thread title says), consisting of two individual trannys back-to-back, all corresponding secondaries should be connected, with respect to minimize loss.


Best regards!
 
#10
Sorry if I was not clear. Kay Pirinha answered my question though. I want to connect the two toroid power transformers back to back so that together they produce an isolation transformer with an output the same as the input (minus any losses). I am looking to get 120 AC or so out of them as that is what the circuit requires . I happen to have a few pairs of sizeable power transformers that I can use for this purpose, and used one of the EI pairs first, but they buzzed mechanically, so I switched to a pair of toroids with multiple secondaries. Thanks to everyone for your help. I'm going to connect the rest of the secondaries together. Should I connect the one centre tap as well or not?
 
#17
My question relates to how to best connect the secondaries of two step-down (or step-up) transformers to get the most current capacity at the output when the goal is to use them to create a one-to-one ratio transformer. I guess the power transformer that I have created by putting two step-down transformers back to back also serves the function of isolating the line voltage from the circuit. In a sense, every transformer, be it step-up, step-down or one-to-one also isolates the circuit from line voltage, and is also an isolation transformer. In the case of this amp, it required a 120 volt AC power transformer, and my local voltage is 120 volts AC. I am used to transformers of this sort being referred to as isolation transformers, but perhaps that isn't the most accurate description in this case.
 
#18
Use a 120:24Vac transformer with an 8% regulation and you will find that the open circuit voltage is actually 25.92Vac.
The turns ratio is not 5:1 but really 5:1.08

Now series connect that into a second 8% regulation transformer also with a 5:1.08 turns ratio and you will find that the final output voltage, when fully loaded, is not 120Vac but ~ 111Vac.

Using two back to back transformers results in (approximately) the addition of the two regulation values.

Find the lowest current rated secondary.
That becomes your new current rating for the whole series connected secondaries.
You have two 24Vac, one 10Vac and one 18Vac (split into two centre tapped valves).
That gives you 76Vac plus regulation.
You can connect all the secondaries together on both transformers and get ~ 120:111 as your completed back to back isolation transformer.
You can omit one or more tappings on the second to adjust the final output voltage. Maybe just use one 9Vac winding.

If you use any of the 9Vac secondaries then you are limited to 1Aac in the low voltage secondary windings.
That gives the first transformer a 76VA rating.
The final rating at the output will be slightly less than this.
If you don't use any of the 9Vac secondaries, your rating increases to ~116VA
 
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#19
Thanks Andrew T. That really answered my question. I have it hooked it up with just the two 24 volt windings connected to each other, and my voltages are a little bit higher than with the previous, buzzing, EI transformers, so I'll just leave it that way.
 
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