Hello,

Sorry for posting a problem which is violating the rules previously. For now I will try to re-phrase it as much as I can as I am not powering LEDs from the main directly.

I am currently doing a project which needs me to convert AC to 20-30Vdc.

As the whole project have to be small. I am thinking of rectifying the AC then chopping it to a higher frequency and from there I will use a transformer, which I think will be smaller to bring the voltage down. Just want to know if anyone have any experience of bringing a voltage down with a smaller transformer?

 

You can use a transformer to bring the voltage down to 24VAC and then rectify it to DC
or you can use an AC-to-DC adapter that delivers 24VDC.

 

Hello,

You could be looking for something like this:
http://www.digikey.com/product-detail/en/RACT20-700/945-1631-ND/2766438
Or this:
http://www.digikey.com/product-detail/en/RACD20-700/277/945-1707-ND/3844917

Bertus

 

AC 230V->Transformer AC 240V/24V DC->Graetz bridge->Capacitor->Voltage regulator LM3xx->Capacitor->Output=31-32VDC

LED goes after the capacitor not before!

 

The whole point of, "switching" power supplies is to use a higher frequency to make them smaller. In my opinion, no amateur can match the size and quality of a retail product (and that includes me). Most of the people here know how to do a 50Hz or 60Hz power supply from scratch, but the question is, "How small does it need to be?" and, "How much current-voltage-power- do you really need?" You need to come up with some hard numbers before anybody can do a calculation.

 

In my opinion, no amateur can match the size and quality of a retail product...

Or cost.

 

Hello,

Here is a quote from his original post:

powering a string of LEDs which have the output 24Vdc and approx 700mA

This is taken from this closed thread:
http://forum.allaboutcircuits.com/threads/ac-from-main-to-dc.101279/

Bertus

 

24V 1A transformer should be enough.

This one: http://uk.rs-online.com/web/p/pcb-transformers/2017454/

 

Thank you all for replying. I understand that it is possible to step down the AC through the transformer. But the project I was given specified that it must be small. Therefore I am looking if there is alternative instead of applying the big transformer at the beginning. I read online about fly back transformer. Using MOSFET to create the cycle and similarly stepping down the voltage.

 

Thank you Bertus for helping answer some questions. The output is 24Vdc 700mA. Output voltage will be vary from 20-30 and will be monitored be Infineon micro controller (XMC1200)

 

Hello,

You can not regulate the leds with controlling the voltage.
Leds are current controlled devices and need a regulated current source.

Bertus

 

Hi Bertus,

The voltage that my project will then go to another DC-DC converter known as a LED driver to me till date. I do not know what is there from the LED driver. My supervisor was not willing to show the driver. Therefore the main aim is to have a AC DC which should have a output of 20-30Vdc and approx 700mA. I read up of all the configuration like buck topology and something called SMPS.
I am really confused now as I am a fresh undergrad who do not really have the skills to do this which a requirement of this project is to keep it small. I was thinking about having transformer and rectifying the AC but was rejected by him as he says that a 50Hz transformer is too big. @@

 

My supervisor was not willing to show the driver.

I am really confused now as I am a fresh undergrad who do not really have the skills to do this ...

I'd be looking for a new job if I was treated this way. Perhaps you are being tested to see if you will openly admit that you don't have the skills to do this task. Humility is a valuable trait and none of us know everything. I have no problem admitting it quickly if I don't know something. If you fear losing your job as a result, well, consider what is going to happen anyway.

A fair question for your supervisor is why you don't just buy the power supply you need, perhaps as a module to incorporate into a larger design.

The type of power supply you need to design is not allowed for discussion here.

 

I'd be looking for a new job if I was treated this way. Perhaps you are being tested to see if you will openly admit that you don't have the skills to do this task. Humility is a valuable trait and none of us know everything. I have no problem admitting it quickly if I don't know something. If you fear losing your job as a result, well, consider what is going to happen anyway.

A fair question for your supervisor is why you don't just buy the power supply you need, perhaps as a module to incorporate into a larger design.

The type of power supply you need to design is not allowed for discussion here.

Ok. I will look into it again and see what I can do to get this working. Thank you for the time.
I wish to leave too! But I was under a contact for a year.

Thank you all for replying and sorry if caused any inconvenience for posting something that is not allowed for discussion.

 

Don't let me run you off. That's the Mods' job.
Seriously, there are plenty of discussions around here about switch-mode power supplies (SMPS) and other things that stretch the interpretation of the forum rules. LEDs to mains and automotive modifications will get you tossed quickly, but other things are usually more case-by-case.

 

Hello,

You could have a look at this product:
http://www.onsemi.com/PowerSolutions/product.do?id=NCL30000

Here is the product overview with a block diagram in it:
www.onsemi.com/PowerSolutions/product.do?id=NCL30000&pdf=Y

It will give you an isolated led driver using a smps.

Bertus

 

Hi Bertus,

From the block diagram, it shows that they are using a flyback topology and can I ask what is the name of the transformer on the block diagram. I believe it uses the MOSFET to switch the primary side to charge it up and pass it to secondary side when the switch is off right?

I tried to look for transformer for this type and I believe that such transformer is small in size too.

 

Hello,

In the links of the following page of the EDUCYPEDIA, you will find a lot more info on SMPS circuits:
http://educypedia.karadimov.info/electronics/powerelectronicssmps.htm

Again said, you will need quite some experience to be able to build one.

Bertus