DC ~ Watts/Volts/Amps

  • Thread starter arimike
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  • #1
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Basic question here: I have a simple circuit consisting of a 12 vdc battery, a level switch and a water pump. The switch is rated for 1/4 HP @ 32 vdc, and the pump is 12 vdc rated at 15.5A.

If 1/4 HP = 185 watts, I assume that equals 5.8A @ 32 vdc, and is not capable of handling the motor load of 15.5A. Is this right?

I keep wondering if I am looking at this wrong because I see this exact motor/switch combination in use in various places...with the switch used to make/break the NEGATIVE line. What is the advantage of placing the switching mechanism on the negative side of a DC circuit?

Any insight would very appreciated!
 

Answers and Replies

  • #2
russ_watters
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Your analysis seems correct to me: Typically, switches are rated for a certain amperage. It is odd to see a switch rated at 1/4 hp @ 32 vdc (strange voltage too!). But 185/32=5.8A

Location of the switch in the circuit really doesn't matter much in a dc circuit.
 
  • #3
Is this switch is electromechanical or purely electronic? In purely electronic switches, negative side aka "low-side" switches can be simpler to implement usually. One reason might be that they don't need any voltage to be in the "off-state" but they need a positive control voltage to turn on. That is usually a desirable characteristic. I agree that rating a switch in terms of hp is not intuitive.
 
  • #4
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I am working with a mechanical switch.

Some further thinking about these ratings are confusing me even more. The switch is rated 1/4 HP (185 watts) @ 32vdc which equals 5.8A. Does this mean at 12vdc and 185 watts it is good to 15.4A?

Another way to look at it is the pump is rated 15.5A @12vdc which I believe equals 186 watts. In this analysis it looks as if the switch is ok to use with this pump.

I am just not sure what the important part of the spec is...1/4 hp @ 32vdc. Is it the calculated watts (185) or the amperage (5.8 @ 32vdc) that should be used as a limit??
 
  • #5
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Basic question here: I have a simple circuit consisting of a 12 vdc battery, a level switch and a water pump. The switch is rated for 1/4 HP @ 32 vdc, and the pump is 12 vdc rated at 15.5A.
Yes, that's an interesting rating. It's all very fine if you have a 32 volt supply, but you are not given a rating at 12 VDC, right?

I would go with Russ on the current rating. Don't use it for over a 6 amp inductive load.
 
  • #6
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No 12vdc rating listed in spec sheet. I wish I could get the manufacturer to give me a spec...I guess I dont present enough buying potential to get them to do that though...

Thanks for all the comments and help!
 
  • #7
4,239
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No 12vdc rating listed in spec sheet. I wish I could get the manufacturer to give me a spec...I guess I dont present enough buying potential to get them to do that though...

Thanks for all the comments and help!
The points being made are that common relays are governed nominally by two independent parameters: current rating and maximum voltage across the points. You normally shouldn't exceed either.
 
  • #8
You should draft shortly 12vdc battery rating.
 

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