when amperage draw is greater than supply...

Sean's picture

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Joined: Feb 18, 2021

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when amperage draw is greater than supply...
February 18, 2021 - 2:31am

Generally speaking, what happens when draw exceeds capacity?

In this case, i would like to connect a 12v 18w water pump to a pc motherboard fan header which (presumably) is rated for 1amp (an industry standard from what i understand)..

At 12v and 18w, the draw from the pump is 1.5amp (watts=volts * amps), which exceds the asumed capacity of the header by 50%.

I understand that the pump cant draw more than what is offered by the header, but what, if any, risk is there of drawing at 100% on the motherboard. Conversly, what is the affect of under supplying the pump?

The fan header is able to run in either pwm or dc mode for controlling fan speed (or, pump speed in my case).  Obviously, i will be setting it to DC mode, which reduces the voltage as a means of controling the pump anyways. (there's a clue...)

Still, i wont be able to get it to 100% capacity at a minimum.

As a potential solution, is there some sort of "variable" voltage relay i can use?

Many thanks for your kind consideration. As a curciutry-newb, any sound information is golden.

best regards,

Sean

 

 

Harry's picture

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Joined: Feb 15, 2021

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"As a potential solution, is there some sort of "variable" voltage relay i can use?"

No, you can not increse the power from the supply. Plus you are looking for more current not more voltage.  You may try using the power supply as is by putting a "fast-blow" one ampere fuse in the circuit.

Sean's picture

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Joined: Feb 18, 2021

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A fuse is a good idea.. thanks for that. Might not be a bad idea to place one on each of the fans as well. since they have the potential to generate current.

Sourav Gupta's picture

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Joined: Feb 12, 2018

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From the Motherboard side -

1. The PCB traces are designed to tolarete upto the rated current it should provide. Drawing more than that can be troblesome as the heat in the traces will increse potentially.

2. The connector has a rated ampere. More than that could potentially damage the connector socket (Heat).

3. The motor is designed to give adequate generated tourque for a potential current flow. The motor may generete high surge current draw when connected to a load. Situation is, the motor having proper load not getting proper current and not providing proper tourque can be troublesome for the motor as at this situation the motor may or maynot generete heat depending on the mechanism. Heating a motor armeture is a dengarous thing. The life will be 40% less when a motor heats up. However, this point can be discarded if the motor is current controlled but at the same time check the driving load, tourque and current relationship.

4. You can not change the current ratio given from the PSU. Try a higher voltage Higher current psu and then use a Buck converter to provide the adequate current. Use proper polyfuse. You do not need to change the fuse every single times.