Voltage supply specification and power supplies 1

[jumacao]

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 ?

 

[davidbeach]

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.

 

[waross]

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

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

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.


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

 

[7anoter4]

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.

 

[jraef]

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.

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