Using a 120 VAC to 5VDC PS to sense 120 VAC - how to handle 277VAC?

[robd2002]

Hi,

I needed to make a device to remotely detect whether or not a 120VAC circuit was live. We make a micro-based device that measures 0 to 5 VDC and converts it into a 10 bit number. Being in a rush and not wanting to screw around the best idea I could come up with was an open frame power supply (85 to 264VAC input, 5VDC output) wired to the switched 120VAC. Close the switch, the PS puts out 5V, my A/D says 1023 or thereabouts, and all is great.

Now I'm told I need to sense 277 VAC (one phase of a 3 phase supply to a 480 VAC pump). I have found a 120 - 370 VAC to 5 VDC supply (

http://www.powerconversion.com/assets/nlp65_ds_1216436331.pdf)

but it's $80 and I need 4 of them. And I'm concerned about inserting the device into one of the 3 phases to the motor.

1. Will the pump still pump OK with the power supply inserted into one of its phases?

2. Could I use a voltage divider to cut the 277 VAC in half and use the first supply?

3. Is there...a better way?

Thank you for any help.

Rob

 

[IRstuff]

Is it just me; isn't using an open-frame power converter to sense whether the input is live a trifle bit brute-force?

Using the same basic idea, couldn't you just do a full-wave, or half-wave rectification, resistor-divide and measure the resultant voltage? Not only would you know that the input is live, you'd actually be able to figure out the numerical value of the voltage.

TTFN

FAQ731-376

 

[rbulsara]

480V phase monitors are apparently available for $62.

http://www.dofflorida.com/assets/applets/2008_winter_catalog_feb_2009_REV_1_030609_web.pdf

See page 21 of the catalog.

Rafiq Bulsara

http://www.srengineersct.com

 

[VEBill]

For a brute force approach, it would have been slightly less inelegant to use a relay (and a single digital input) rather than a power supply (and an A/D input).

Ideally, if building anything on your own, one would include an optocoupler stage in the circuit to provide some inherent protection for the poor micro.

If you can't find any off-the-shelf voltage measuring interfaces, then there must be off-the-shelf 3-phase current measuring gadgets. If you have current, then the voltage is probably there.

 

[LionelHutz]

You can get control relays with a 277V coil. Just connect and use the contact as an indicator.

The controller for the motor may have a 480V:120V control transformer. Just connect a 120V coil relay onto the output of this transformer.

Or, just buy a voltage relay like the one already posted.

 

[OperaHouse]

For a motor, what might be more useful is a self powered current sense relay. This would tell you the motor is actually running. This requires a single loop of one of the motor leads through the sense transformer. These relays provide a dry contact for your micro to look at and no power other than some voltage for your micro to sense. The same device would work on any voltage. They generally require 10A to switch on, easily provided by starting current. An extra loop works if the current is really low.

Just sensing voltage to a motor isn't evidence that it is running. If you don't sense current I would use a phase relay.

 

[roydm]

Since your input is 10 bit analog why not use a current transducer, that way you will also be able to monitor the motor current. Greystone comes to mind.
Roy

 

[MikeHalloran]

Lamps and phototransistors?



Mike Halloran
Pembroke Pines, FL, USA