×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Voltage supply specification and power supplies

Voltage supply specification and power supplies

Voltage supply specification and power supplies

(OP)
Hi:

We have a a power source of 120 VAC. System requires 115 VAC.
The system have a power supply that is suppose to supplies 24 VDC to a weighing cell.  Now it is supplying 30 VDC.  The power supplies input is 121 VAC (specification is 115 VAC).

Why we have 30 VDC at the weighing cell?  Is that variation OK?  Do we  have a power supply issues or everything is OK ?

  

RE: Voltage supply specification and power supplies

It is because somebody designed the dc power supply with no concept of how the real world operates.   Assuming the US, 115 is a nominal utilization voltage designed to accommodate +/- 10% variation.  120-123 is generally what the utility tries to deliver.

RE: Voltage supply specification and power supplies

If you think you are seeing 30 Volts DC you may have metering issues also.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Voltage supply specification and power supplies

The supply may be an unregulated type, in which case the off-load voltage will be considerably higher than when it is connected to the instrument.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Voltage supply specification and power supplies

All the above explanations are possible. But there is another possible explanation.
If it is a simply double-wave rectifier the system is supplied by a 115/26.5 Vrms [k=4.34] transformer.
Then the output [d.c.] will be Vdc=2*sqrt(2)*Vac/pi()  Vdc=24 V.
If Vprime=121 V Vsec=26.5/115*121=27.9 V[rms] and Vdc=25.1 V[d.c.]
In order to get 30 Vdc you need Vsec=33.3 V[rms].
That means the ratio Vprime/Vsec=121/33.3=3.63 [less than 4.34]
Since number of the turns of the secondary winding could not increase no. of primary turns would decrease.
That means there is a short-circuit in the primary winding.
 

RE: Voltage supply specification and power supplies

Or it could just be that you have a 24VDC power supply that has an adjustable output, and someone has turned it up to Maximum; I see that a lot. Most modern industrial grade Switch Mode Power Supplies offer that feature now, but many users are unaware of it and if they notice the adjustment screw, some think that by turning the screw to Max. they will get "more power" from the SMPS.

If your power supply does not have that feature, maybe you should invest in one. A lot of times people think that any old cheap power supply will do, until they run into this kind of situation.

But check with the instrument supplier first to make sure they don't have a problem with using an SMPS.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources