# How to design a control or sensor circuit for following

• posted

Hi There is a transformer which has got different tappings on it for

120,208, 277V. Now I want that whatever the supply voltage is say 120, 208, 277 that switch should turn on from the tappinngs i.e switches are connected at tappings and heating coil should be connected to that so for that I need to sense it but iam not aware of the way I can sense it or can any one guide me with circuit that can be usedto sense it. Thanks Jayesh
• posted

thanks

Can u plz elaborate on wat u mean by peak detect the AC voltage and how much value shall i convert to dc voltage i.e whats the valueof dc voltage . And abt op-amp i have no ideaso iam goin thru it now so will get back to u later with doubts.

Thanks Jayesh

• posted

ok so if its say 120-277V ac then dc will be (120-277 )* 1.4 DC But i cannot supply such large voltage to Op-amp so what do i do

Thanks jayesh

• posted

You would use what's called a 'voltage divider' - two resistors in series, such that the voltage drop is proportional to their resistance.

Maybe if you need to ask this question, you shouldn't be messing with 277 volts!

Can this voltage change unexpectedly? You could simply install a switch, or give the user three input jacks, so they could plug the unit in to either power at the site.

But automatic switching isn't that hard - you could use what's called a "signal transformer" or some such - a 277/12V ordinary transformer, and when the input is 120V, the output will be

12 * (120/277) V. That's way easy territory for an opamp.

Good Luck! Rich

• posted

thanks but can it be done without using a transformer what abt power loss at resistor .

Jayesh

• posted

The approach would be:

use a diode and capacitor to peak detect the AC voltage and convert it to a DC voltage

Use 3 OpAmps or comparitor IC's set to detect a voltage below 120, between

120 and 208, between 208 and 277

Apply logic (relay or solid state ) to determine which taps to connect.

Can't be any more specific with the minimum information you have supplied

Dan

```--
Dan Hollands
1120 S Creek Dr```
• posted

Use a diode and capacitor to make a simple dc power supply from the ac in put voltage The dc voltage will be approximately 1.4 times the AC voltage

```--
Dan Hollands
1120 S Creek Dr```
• posted

1) what power level are you talking about? 2) the load on the transformer secondary is a heater? 3) why do you need this voltage sensing to be automatic? how come a terminal jumper will not work? a consumer item?

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