# Varying transformer voltage

Discussion in 'General Electronics Chat' started by Vasanthkini, Jun 14, 2010.

1. ### Vasanthkini Thread Starter Member

Jun 9, 2010
40
2
Greetings !

I am working on a power supply design and i have a question pertaining to the transformer.

The primary or the input side of the transformer has a varying AC voltage between 90Vac to 305Vac.I need a out put varing from 12Vac to 18Vac.Selecting the turns ratio mathematically does not help me manage getting the required o/p.How would i manage achieving this,strictly for the i/p and o/p range?

2. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
With the wide variation on the primary and the limited variation on the secondary that is going to be a very tall order. Best bet would be a fixed ratio transformer for the brute force reduction being fed from a motorized variac to the primary.

3. ### Norfindel Active Member

Mar 6, 2008
235
9
Why do you need the voltage exactly between 12 to 18v for that input voltages? The more info you can give us, the better.

4. ### Vasanthkini Thread Starter Member

Jun 9, 2010
40
2
@Norfindel

Hey,i need the out put to be between 12V to 18V mainly because,this is rectified and the resulting voltage is fed to a IC.The IC stops working or gets killed if the range is exceeded.

Thanks !

5. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
What IC are you trying to supply power to?

And what current are you dealing with?

You have mentioned that the transformer has a variable input spec, but what is YOUR actual available voltage? Are you running this from mains power?

Are you in need of a mains transformer that will take your 120vac or 240vac and drop it to 12vac ?

Depending on the current(amperage) requirement, these transformers are readily available.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
This doesn't make sense if you are going to rectify the secondary voltage.

Why do you need a variable secondary alternating voltage?

Pose the problem properly.

Wiring for 50 - 150 volts AC has different standards and specifications from

Wiring from 150 -250 volts

and different from

Wiring from 250 - 450 volts

So what will your supply actually be and how will it be connected?

What current do you require at the output?

If your supply varies do you expect the device to compensate automatically or will a switch suffice?

7. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Since you are using the system to power a DC IC circuit, my best bet would be to get a universal input switching regulator type power supply. Many on the market will accept 90 to 240VAC as their input and output is regulated and reliable. Probably less expensive than starting from scratch, also.

Vasanthkini likes this.
8. ### Vasanthkini Thread Starter Member

Jun 9, 2010
40
2
Hey guys,thanks a lot for the responses.Many seem to be still wondering about what exactly am i trying to do.Let me pose the problem very perfectly and try explaining the problem i am facing.

Basically i am looking for a power supply design whose input is varying over a large range from 90VAC to 305 VAC.This comes from the mains directly.Required "transient voltage protection" is taken care of by using the necessary components.Once this "protection" part is taken care,i then would rectify the voltage to get the equivalent DC voltage.Now i have the supply varying from 135VDC to 430VDC.This is then "voltage divided" and given to a APFC IC,where the power factor correction is taken care of.But as the "safety regulation" reads,voltage division is replaced by a isoltion transformer.Now,the input to the transformer is a pulsating DC and i need the out put to be within the specified range of 12VDC to 18VDC for the power factor correction IC to be safe and working.Here is where the problem pops up.Achieving a secondary voltage of very small range is proving to be more tougher than it seemed.And i would like to also add this information that,i am designing the transformer and am not buying it for some reasons.Even if not,theorotically,how can this be achieved?I hope i would get more answers like before.

Thanks All !

9. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
It can be done by using PWM (Pulse Width Modulation) of the DC feeding the isolation transformer. That is the standard setup for a Switching Power supply and how they achieve such a wide input range for a given output range. Feedback from the output is used to control the pulse width of the power going to the input of the transformer. Closed loop system provides very accurate regulation when set up properly. Look into switching power supply theory for a lot more info.

On edit.... Where do you live that the "mains power" has such a wide variation in voltage??????????

Last edited: Jun 15, 2010
10. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
I am guessing that this is for offgrid generation? A windpowered alternator or similar supply source could explain your wild swings in input voltage? More details would be a very good idea.

Forced to guess I would wonder about the basics of your power source.
Is there a mechanical system like the governor on a wind turbine that could affect the system maybe providing a +- 20% tolerance on the frequency for the AC then it would be a good idea to let us know. That would make it much more appropriate to use a switcher because you can eliminate the transformer and any problems it could cause with changing frequency.

These are just examples of the questions that might need to be asked, but without more details I don't really have any hopes that these are the right questions.

11. ### Norfindel Active Member

Mar 6, 2008
235
9
I recommend buying a voltage stabilizer of those that can be used to protect any AC equipment. If you have such an horrible mains voltage that can vary as that, i bet someone sells some stabilizers that can take that maximum input.

In fact, i would buy one to protect the whole house, if i were you. How do things keep working under such conditions? Most power supplies are designed to 240Vac max input + some safety margin.

12. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
I agree. Voltage fluctuations like that would fry 99% of appliances out there.

And you might want to buy stock in fuse companies.

13. ### Vasanthkini Thread Starter Member

Jun 9, 2010
40
2
Hey!

This is a simple power supply application for a particular application.The idea behind having the input varying over a large range is to make the unit universal(110V in USA and 230V in India).Answering about a question that was shot some where above about where do we live in a place that has got such a variation,this variation is essential to be considered as the project needs to be approved by many regulatory agencies.

SOLUTION: Guys i would like to thank all of you for your time and patience.I finally could manage to find the solution to this problem.I just implemented a feed back sensing approach where the out put is sensed for power factor and then a supply is given to the IC directly via another IC.Will write more details about this soon

Thanks!