question about multiple transformer wiring on primary side

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have some harvested amp boards and transformers that I would like to make use of. The transformers have a single 120VAC primary and multiple secondaries that are used to power multiple amp modules. All these parts were obtained from a multi-channel Crestron CNAMPX-16x60 power amp.

The question I have is how I might be able to use (or not) the same transformers on 220/230VAC mains (European standard). If I will be using two of these transformers, can I simply connect their primaries in series so that each one sees half the AC mains voltage? If I do that, can I leave some secondaries "unused" on one or both transformers?
 
In principle yes with the two primary windings connected in series. BUT, there are some very important restrictions! In practice it is not advisable.
The voltages across each primary winding are divided according to how much the secondary windings are loaded. This meaning that if one transformer is loaded less than the other, it has a higher primary voltage, and accordingly also secondary voltage, than the transformer with the highest loading. Then it starts getting really difficult to balance the voltage sharing of the input voltage.
If you have two identical transformers and you connect the primary windings in series (they are assumed identical) and also the two secondary windings in series (they are also assumed identical), then the loading of the two transformers is the same and they share the input voltage pretty well. With two different transformers it is very difficult to control.
So in conclusion - don't do it.
 
Last edited:
I think FauxFrench is getting at what I am concerned about: current flow.

So, yes I have multiple identical transformers of around 800VA-1kVA each. Each transformer has a single 120VAC primary and four (eight wires) identical secondaries which are around 45VAC each or so IIRC. The secondaries are completely independent of each other.

The amp that these were taken from has eight two-channel amp boards. Four of these are powered by one transformer, and the other four by another (identical) transformer. I have both the two transformers and eight amp boards from an amp that I disassembled a couple of years ago, and I also have two other intact amps of this model.

So, here is my dilemma. Let's say I want to use these in Europe on 220/230VAC mains. Apart from getting a step-down autoformer, can I use the two identical transformers, with their primaries wired in series? Each would see 120VAC, half of the mains AC voltage. That seems to pass the sniff test.

What I don't understand for that arrangement is how current is flowing on the primary side of these transformers when current is drawn from the secondary side(s). Since the transformer primaries would be in series, current flow would be identical for both, but not necessarily the current draw from the secondaries. In fact, how does this even work for a single transformer of this type, where there are multiple secondaries which might draw vastly different current at any given time? This I cannot quite figure out in my mind, so I am looking for some background and info about it. Saying "dont' do that" is good, but I am always interested in WHY. I might, you know, learn something. :)
 
The primary voltages in series will split according to the secondary loads. (Like any series string.) Have you addressed that?

A-Ha. I didn't know about that. I'm not clear about how magnetic circuits function under various operating scenarios so this is a new info nugget for me.

So for example if the primaries of the two identical transformers are connected in series and the first transformer's secondaries are bearing all the load(s), the first transformer would also get more of the total primary voltage drop? And this would drive up the voltage across its primary (or would it be the other transformer's primary?), which would cause it to exceed the 120VAC rating?

I'm starting to see the "issues". But would like clarification on the above hypothetical scenario.
 
If you are still in the USA try it. Put the primaries in series and load the secondary's separately and monitor the primary. Depending on your application try putting the primaries in series and the secondaries in parallel (phases must be correct). If the secondary voltages are not exactly the same you may pop a fuse. Use 2 bridges and connect them in parallel after the rectification.
 
Its all about impedance. If the secondary load impedance is lowered thus trying to draw more current the primary impedance will also reduce to supply more current and normally depends on a voltage source with low impedance like normal mains. If there is a second transformer primary in series with no load its impedance is unchanged, so the 2 impedances in series will reduce in total but not enough to allow the first secondary voltage to be maintained and the increased primary current in the second will force its secondary output to increase. Perhaps disastrously.
So dont do it.
 
scottjoplin was trying to help you, and you got insulting. That's it from here.

Is it that hard to see my frustration? OK, let me make an analogy for you: If someone posted a specific question about loudspeakers and my response was simply a link to the "Loudspeakers Design Cookbook", with nothing else, I think they might feel the same kind of frustration. And bonus points if that topic was not actually covered in the Cookbook! That's a good burn, eh? :mad:

In an effort to find some elusive nugget of info that ScottJoplin was tossing my way, I read thru the link and its subsequent parts but I found nothing related to what will occur when TWO transformer primaries were connected in series with different draws on the secondaries (that WAS the nature of my question). This is in stark contrast to the other posters, yourself included, who tried to explain the issue with the specific question at hand. They were helpful in clearing up my confusion, and I learned something, too. See the difference there?
 
There was no need to answer your specific question, all you needed to know to answer it for yourself was in section 3 Transformers Part 1 - Beginners' Guide to Electronics

Sure, but in assuming I would read INTO that part to get my answer when it is just part of one paragraph buried in multiple web pages of info on transformers is really not helping me find the answer. Look, I don't think you intended to be unhelpful or the like, but it has a shove-off-you-idiot kind of feel when the link title is "Transformers Part 1 - Beginners' Guide to Electronics". You could have said "hey check out section 3" or something.

That's all I will say about it. Let's just move on.
 
Yes, I learned something, but not from you. I already knew "how transformers work", just perhaps not in depth, and if anything I felt insulted by your post. Maybe this is because you have trolled me before in other threads. Even the section you later pointed me to wasn't of much help me until I read the textual explanations posted by others.

If you want to see how I try to help people, read this recent thread:
How can I make my ALSA card to use the fixed number?
I didn't just post a link to "Beginners guide to ALSA"!
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.