Redesign transformer for 1:1. Bigger primary or smaller secondary?

[mkiijam]

Hi Guys, thinking about winding my own version of a transformer that has a slight stepup into a version without the gain. Right now, I believe it's something on the order of 900 turns on the primary and 1100 turns on the secondary. How do I decide if I should add more turns to the primary or remove turns from the secondary all other things being equal?

 

[Chandra]

Do you want isolation?
If not, get a standard low voltage transformer, say 18 volts sec. You can connect it in series with primary to get supply voltage+18 volts (boost) or supply voltage-18 volts (buck)
If this is not what you require, please tell us the application and how you arrived 900 or 1100 turns.

Regards.

 

[Whoops]

Although this is the Drawing board, as your post is related to transformers I think you should post it in the "Magnetics" section of this forum, that's where the transformers gurus of this forum walk around

 

[Chandra]

I think the transformer core you are using is 1 1/2" square core.
The ratio you mentioned is (1100/900) is 1.22. With primary voltage at 230 volts, the secondary voltage will be 281 volts. Sufficient to blow any device connected to it and do a permanent damage. Most electronic equipment are rated to 230 + 10%. If the TV or computer or a audio system has SMPS power supply, it blow with an explosion.
If you are using to rectify to DC, the smoothing capacitors must have a working voltage of about 450 volts DC.
So keep it to 1:1 or use my above post to vary the voltage if required. If you are boosting, always watch the output voltage before connecting anything to it.
Regards.

 

[andia]

don't think this is about power transformer.

 

[gyraf]

the question is what inductance you need to get the correct low-frequency response with the source impedance you have at hand

if the transformer is otherwise right, perhaps just use it the other way around?

/Jakob E.

(moved to /Magnetics)

 

[Chandra]

Since Sept 4th, OP has not answered anything. Probably he found a solution.

 

[mkiijam]

This is an output transformer. If I use it the "otherway" around, won't it then become a "step down."? I would just like to make it a 1:1 and therefore could wind it 900:900 or 1100:1100.

But I think Gyraf's inductance idea is the correct way to think... The source impedance is very low. The output of a discrete opamp, like a JH990 or something

 

[Chandra]

Trying to explain in simple way...
Before doing anything, you need the following information.
Maximum output voltage from source (primary voltage),
Required voltage to the destination (secondary voltage),
Destination impedance (ohms).
Lowest frequency (Hz).

Input voltage to output voltage is the voltage ratio = the turns ratio.
Since this is an output transformer, the primary impedance (reflected) is dependent on the secondary impedance (load).
Example 1: Source is 10 volts, destination is 1 volt. The voltage ratio = turns ratio will be 10:1 = 10
Load is 1000 ohms (for 1 volt side), Reflected load on primary will be 1000 * (10 ratio)^2 = 1000 * 100= 100 kilo ohms
Example 2 : Source is 10 volts, destination is 4 volts, The voltage ratio = turns ratio will be 10:4 = 2.5
Load is 1000 ohms (for 2.5 volt side), Reflected load on primary will be 1000 * (2.5 ratio)^2 = 1000 * 6.25= 6,250 ohms
The transformer core and turns must be selected for the lowest frequency, the turns ratio must be maintained for impedance ratio.
You can have multiple primaries or secondaries that are connected in parallel to improve high frequency response, less leakage and good coupling. Means smaller wires and same turns each, same ratios. You may have to wind all primaries and secondaries at the same time, using a few bobbins
Regards.

 

[abbey road d enfer]

This is an output transformer. If I use it the "otherway" around, won't it then become a "step down."? I would just like to make it a 1:1 and therefore could wind it 900:900 or 1100:1100.

But I think Gyraf's inductance idea is the correct way to think... The source impedance is very low. The output of a discrete opamp, like a JH990 or something

Following gyraf's comments, inductance has two main consequences.
It must be high enough to provide good LF response in conjunction with the source that drives it.
It must be high enough to limit LF distortion at an acceptable value.
In both respects, one has to decide what LF is and what deviation of frequency response and what distortion level is acceptable.
A transformer designed for 40Hz LF response is very different than one designed for 20Hz.
If you designed the transformer from scratch, you would have to choose the core first and then determine the turn numbers, but since you have an existing transformer, you have to measure it first.
Using the circuit that will drive it, what is the LF frequency response? What is the LF THD level? Note that driving the 1100 turns side will give slightly better results than driving the 900 turns side.
Depending on the results, you may decide that the LF performance is adequate from both sides, so you can use 900 turns as the reference, or if both sides do not give good performance, you may have to increase the numer of turns, until you get proper performance, but remember that increasing the number of turns may result in HF loss. All xfmrs are a compromise between LF and HF response.
Now you may find that driving the 1100 side is good, but the 900 is not, then you would choose 1100 as the reference.