high DC voltage reducer

[ravi prakash]

Hi all
I am making a power supply which takes 135vac three phase and outputs 5voltDC
1. I have used tl783 liner voltage regulator which can take 125 VDC as DC input .and can give me 5volt .
2. My main concern is to provide 125 volt to tl783 because by rectification of 135 I will get 150vDC .
How do I reduce this 150vdc to 125vdc keeping my cost low .

Please suggest me ....

 

[(*steve*)]

Use a transformer.

 

[ravi prakash]

Thanks sir ...but that's not I want because i don't have that much space and my load current will be 500mA only ...
Is possible by using zener diode

 

[Bluejets]

3 phase input for 500mA 5V?

 

[ravi prakash]

I have 3 phase 130 volt in my system . Which is used to run a Moto .
My objective is to add some control to the same system.
This is what I did...
1. Rectified 3 phase by adding a the phase rectifier.
2.I got 150 vdc .
3. Now I need to supply this 150v to tl783 but the maximum allowed voltage is 125vDc .
4. How do I get 125 volt by 150 vdc.
5. Since the tl783 is used to run a small circuit the load is not more then 500mA on 125vDC bus ....

 

[(*steve*)]

Get a usb charger designed for "universal" input (generally 90 to 265VAC) and use that.

If you user a linear regulator you will need to dissipate about 65W of heat. If you have no room for a transformer then you won't have room for the massive heatsink either.

 

[ravi prakash]

Thanks sir I will see that circuit ...but will that run on three phase or two phase as I dont have neutral supply

 

[(*steve*)]

Run it between two phases (as long as the voltage is ok for the adapter)

Assuming you don't know how to calculate this... Its 270VAC.

But that raises the question of how you got 150VDC. Are you measuring 135VAC between the phases or between the phase and the neutral you don't have?

 

[ravi prakash]

Yes sir its delta voltage

 

[(*steve*)]

Ok, you should be fine. I had not assumed they were delta voltages. Perhaps I missed something earlier.

 

[(*steve*)]

In retrospect I should have assumed they were delta voltages. My fault

 

[hevans1944]

Be afraid, be very afraid, of cheap universal USB power supplies assembled in Asia. Some do not adequately isolate the power-line input from the 5 V DC output. This has allegedly led to lethal consequences. See this article for more information. Personally, I would not want to operate anything other than a three-phase motor or an enclosed ceiling lighting fixture from a three-phase mains supply without a transformer for isolation, neutral or no neutral.

The USB "wall wart" chargers provide isolation by rectifying the AC line voltage and using this to drive a high-frequency (kilohertz) transformer connected in a switch-mode power supply (SMPS) configuration. Circuits to accomplish this feat vary widely in performance and efficiency. Corners are cut and compromises are made to produce a device that sells for two or three dollars. If aware of the possible consequences of isolation failure, then perhaps a wall-wart 5 V USB "charger" is right for you. Ask your doctor... er, electronics professional, before proceeding.

Edit: I found this blog post on genuine Apple iPhone chargers. It shows how to do USB chargers right.

 

[(*steve*)]

In any case, it's still way safer than a linear regulator running from the rectified power line.

I will agree that a "good" usb charger is worth it's weight in defibrillators (or something). These days it's getting harder and harder to define "good" in a way that is useful :-(

 

[hevans1944]

A "good" design offers high quality, low price, and quick delivery. Choose two out of three.