AC Step Down Voltage form 230V to 18V

Thread Starter

milind

Joined Apr 27, 2006
1
Hi Everyone!
Can I do like AC Step down voltage from 230V to 18V without Transformer.
Actually I want to measre the AC Line Voltage with the help of Microcontroller's ADC so I want to Step Down the volatge.
Thanks
 

Papabravo

Joined Feb 24, 2006
12,767
Originally posted by milind@Apr 27 2006, 07:44 AM
Hi Everyone!
Can I do like AC Step down voltage from 230V to 18V without Transformer.
Actually I want to measre the AC Line Voltage with the help of Microcontroller's ADC so I want to Step Down the volatge.
Thanks
[post=16602]Quoted post[/post]​
From that level it is very hard. The safest way to detect the AC voltage it is with a half wave rectifier and an optoisolator. The problem is making a voltage divider that produces enough current to turn on the LED of the opto without creating a boatload of heat in the divider.

To actually measure the voltage, safely, without a transformer is going to be a challenge. I can think of ways to approach the problem, but I'm not sure any of them would actually work.

What is magic about 18V? Don't microcontroller ADC's work from 0V to Vdd? Last microcontroller with an 18V supply was the RCA 1802, which went to microcontroller heaven some time ago.
 

beenthere

Joined Apr 20, 2004
15,819
Hi,

As pappabravoo says, there is no safe way to do this without a transformer. If you really have to, though, you could make a voltage divider with a 112K resistor attached to the line and an 18K resistor in series attahed to neutral. This will develop 18 volts across the 18K resistor. To save yourself timing problems, use a diode to half wave rectify the AC, and a capacitor to make smooth DC. Otherwise, you have to make a peak voltage detector to trigger your A to D to get sensible data.

It's really not smart to fool around with line voltages without isolation.
 

Papabravo

Joined Feb 24, 2006
12,767
Originally posted by beenthere@Apr 27 2006, 12:45 PM
Hi,

As pappabravoo says, there is no safe way to do this without a transformer. If you really have to, though, you could make a voltage divider with a 112K resistor attached to the line and an 18K resistor in series attahed to neutral. This will develop 18 volts across the 18K resistor. To save yourself timing problems, use a diode to half wave rectify the AC, and a capacitor to make smooth DC. Otherwise, you have to make a peak voltage detector to trigger your A to D to get sensible data.

It's really not smart to fool around with line voltages without isolation.
[post=16619]Quoted post[/post]​
The peak power disipation in the 18K will be 112 mW so a quarter watt will do.
The peak power in the 112K will be 701 mW. You need a 1 Watt minimum or 2 Watt just to be safe for this one. The current will be about 2.5 mA.

Please be careful and don't let the magic smoke out of any of your components or yourself.
 

windoze killa

Joined Feb 23, 2006
605
Originally posted by Papabravo@Apr 28 2006, 04:42 AM
The peak power disipation in the 18K will be 112 mW so a quarter watt will do.
The peak power in the 112K will be 701 mW. You need a 1 Watt minimum or 2 Watt just to be safe for this one. The current will be about 2.5 mA.

Please be careful and don't let the magic smoke out of any of your components or yourself.
[post=16625]Quoted post[/post]​
Digital multimeters have the circuitry to do just that internally. If you can find the schematic for one of them you could tap of the low DC voltage and feed to your micro.
 

SparkMan

Joined Apr 27, 2006
8
Originally posted by beenthere
...

It's really not smart to fool around with line voltages without isolation.
[post=16619]Quoted post[/post]​
Absolutely correct. It would be pretty crazy to have no isolation. They never could stop talking about it in classes.

Honestly, I think a transformer is the easiest approach. You don't need a lot of current with an A-D convertor, your only sampling the voltages to convert them to logic, so a 230V to 18V or 12V transformer with small current shouldnt be that bulky, and that's probably what I'd use.
 
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