Putting Two Transformers in Series

Thread Starter

sjgallagher2

Joined Feb 6, 2013
131
Hey all,
This question is giving me a headache, and I need some practical advice. I need a proof of concept that I can put two separate transformers in series like so:
PRIM1 : SEC1 -> PRIM2 : SEC2 -> output (input x turns ratio1 x turns ratio2)
I quickly ran a spice simulation and got normal results from a single transformer (10:1) and two transformers in series (5:1, 2:1), pictured below:



So the theory obviously checks out, no surprises there. So practical considerations: If both the transformers are rated properly for whatever input/output voltage I have in mind, and they both share the same VA rating, will this really work. Here are some example transformers, a 5:1 and a 1:1 (both rated at 60Hz and 40VA)
http://www.digikey.com/product-detail/en/4-1611453-7/PB1649-ND/1128437
http://www.digikey.com/product-search/en?KeyWords=4000-60E07K999-ND&WT.z_header=search_go
Would you feel comfortable putting these two transformers in series as depicted? Applications aside, as I do not know the application for this configuration but am only researching.
Thanks in advance.
Sam Gallagher

PS I'm having difficulty with pictures so Ill attach them.
Transformer1.PNG Transformer2.PNG Transformer1.PNG Transformer2.PNG
 

dl324

Joined Mar 30, 2015
12,927
Hi,

It's very common to put primary windings in series for 120/240VAC switching. Less common for secondaries, but I've seen it done (e.g. to make complementary supplies). You just need to pay attention to output phase and current rating.
 

MaxHeadRoom

Joined Jul 18, 2013
23,364
A misconnection on series secondaries just results in non or lower output, if in parallel you can short them out, so the detection sequence is different.
The Va rating will be for the lower rated transformer.
Max.
 

Thread Starter

sjgallagher2

Joined Feb 6, 2013
131
Just to be clear, I have two transformers, separate components. The input (say, 120VAC) goes into the primary of TX1, and the secondary of TX1 is connected to the primary of TX2. The output is taken from the secondary of TX2. Just to avoid confusion, because I'm uncertain what Dennis is referring to by 'putting secondaries in series' since I'm not putting the secondaries in series, am I?
Sam Gallageher
 

#12

Joined Nov 30, 2010
18,222
The Power Company does multiple step-downs and so do I.
I use a Variac to run a 120:24 transformer and use that to run a microwave oven transformer, and then rectify that to get DC voltages in the higher ranges, like 400 VDC.

Then, for a different job, I use the 208 volt primary winding of a 24VAC transformer by applying 125 VAC. The result is 14.4 volts which I wire to buck the power line voltage down to 110.6 VAC so my outdoor lights don't burn out every 3 years. When their inductor is fed a few percent LESS voltage than it was designed for, the bulbs often last 10 to 12 years.

You can play mix&match with transformers all day if you are careful to calculate their limits so you don't over-heat them.
 

dl324

Joined Mar 30, 2015
12,927
Just to be clear, I have two transformers, separate components. The input (say, 120VAC) goes into the primary of TX1, and the secondary of TX1 is connected to the primary of TX2. The output is taken from the secondary of TX2.
There was a problem with the pictures when I read your original post. You're using them as multiple step down as #12 referred to. Maxheadroom and I interpreted your query as can you connect the windings in series...
 

crutschow

Joined Mar 14, 2008
27,425
You can certainly feed the secondary output of one transformer to the primary input of another as long as its primary voltage rating and the current rating of either transformer is not exceeded.
 
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