[Help] Identify toroidal transformer output current

Thread Starter

muno

Joined May 17, 2022
38
hi! Please help me identify the current output of this toroidal transformer. There is no marking indicating how much amps on the output. The only marking is BX1201, the primary is 230VAC/50Hz, and secondary is 28Vx2-0-28Vx2, it has a 1.5A fuse in the primary. I got this transformer from a speaker with built-in amplifier. The speaker is MG Audio Box-1201 550W. I'm planning to use this transformer to build my own linear bench power supply, maybe 0-30VDC 5A if it can. I'm attaching the actual picture of the transformer below for reference. Thank you!
 

Attachments

MrSalts

Joined Apr 2, 2020
1,762
No good way to know. Many Toroids are custom wound. Note that the model number on the transformer 1201 matches the number on your device. This points to a custom wound unit. You can try looking up other cores with similar size, voltage ratings to see if you find something in the range. Based on a 1.5A fuse primary, I would guess 7 amps range on the secondary and assume the amp was rated 550 "peak" watts, not RMS, with the peak somewhat enhanced by a large capacitor for short bursts at "peak". A 1.5a fuse points to 250-325W actual.
 

Thread Starter

muno

Joined May 17, 2022
38
hi thanks for the reply i really appreciate it, im just still learning electronics, and am not so familiar with toroid specs like windings, turn, cores... etc., only basics,. anyway, if its capable of 7A in secondary, that would mean that if i hook up or use the 28V-0 winding then i can extract upto 7A of current and it will be perfect for my 30VDC 5A DIY bench PSU?
 

Ian0

Joined Aug 7, 2020
5,082
Transformer maximum current is a thermal rating. It's down to how hot you dare run it!
Have a look through a transformer supplier's website and find one the same size
https://airlinktransformers.com/category/chassis-mounting-toroidal-transformers-standard-range
it will have the same power rating.
Divide power by voltage to get maximum current.
Most transformers will take an overload of x2 for a few minutes.

Another way you can estimate it from the amplifier power (the speaker power is irrelevant)
Measure the speaker impedance.
A 28V transformer will give a ±40V supply. The output power will be 40x40/(2xZ) where Z is the speaker impedance.
The transformer power will be similar to the amplifier power.
 

Thread Starter

muno

Joined May 17, 2022
38
i cant find the exact 230V primary and 28-0V-28V on the link you gave, but there is one 230V primary and 28V secondary, which is rated at 300VA 10.7A, do you thinks this is the same as mine? thank u
 

Ian0

Joined Aug 7, 2020
5,082
Just look for one the same size (and/or weight), don't worry about the voltages.
If your transformer is the same size and/or weight as a 300VA, then it's probably about 300VA.
If it's 300VA then you can get 300/28 Amps out of it (AC).
28V AC will give you 40V DC when rectified. In that case you can get 300/40 A DC before it gets a bit too warm.
 

Thread Starter

muno

Joined May 17, 2022
38
thank you very much. I will look on the link again. and weight my transformer to confirm. I will feedback soon.
 

Thread Starter

muno

Joined May 17, 2022
38
Just look for one the same size (and/or weight), don't worry about the voltages.
If your transformer is the same size and/or weight as a 300VA, then it's probably about 300VA.
If it's 300VA then you can get 300/28 Amps out of it (AC).
28V AC will give you 40V DC when rectified. In that case you can get 300/40 A DC before it gets a bit too warm.
hi again, my transformer weighs 1.25kg, its outer diameter is 94mm, height 39mm, inner diameter is 45mm.
 

Thread Starter

muno

Joined May 17, 2022
38
thank you. atleast i have an idea now. seems like the marking BX1201 is related to 120VA, what do you think?
 
Last edited:

MrSalts

Joined Apr 2, 2020
1,762
As you load a transformer towards is limit, you also start to see voltage sag. Sometimes before temp limits are reached before you see any voltage sag and sometimes you start to see voltage sag before the specified current limit. Each manufacturer tends to specific transformers differently.
 

MaxHeadRoom

Joined Jul 18, 2013
25,190
The gauge - voltage of the secondary conductors can give you an idea of the current of the secondary in order to roughly calculate the Va.
 

Ian0

Joined Aug 7, 2020
5,082
There's no correct answer. Put 120W load on it and see how hot it gets. If it's about as hot as you think it should run, then the @muno rating is 120VA.
If you think it could run warmer, then it might be 150VA. If you burn your fingers on it then perhaps 90VA.
 

MrSalts

Joined Apr 2, 2020
1,762
It's a lot of steel so it may take some time to warm up to a steady-state temperature. You could also place the whole thing in the enclosure you plan to use so you can estimate how much fan power you need to avoid a thermal runaway condition.
 

MaxHeadRoom

Joined Jul 18, 2013
25,190
hi thanks for the reply i really appreciate it, im just still learning electronics, and am not so familiar with toroid specs like windings, turn, cores... etc., only basics,. anyway, if its capable of 7A in secondary, that would mean that if i hook up or use the 28V-0 winding then i can extract upto 7A of current and it will be perfect for my 30VDC 5A DIY bench PSU?
28V at 7A would be 200Va which by the picture, looks about right.
If you do place it in an enclosure as suggested, ensure the Centre bolt does not also go through the enclosure, top and bottom. it creates a shorted turn!!
 
Top