EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: analogo on May 31, 2020, 04:27:18 pm
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I would like to limit the inrush current of a water pump. It is working fine up to now, but I can hear how violent the initial impulse is.
Reading around I have found that the most basic way to implement a soft start is a NTC thermistor. But I haven't found any application note that deals with simple cases like mine.
Here is the my trivial circuit: https://crcit.net/c/38a809a8be3c4096a4255481784daee1 (https://crcit.net/c/38a809a8be3c4096a4255481784daee1) (The capacitor is there to block EMF.)
(https://api.circuit-diagram.org/document/store/render/38a809a8be3c4096a4255481784daee1.svg?r=1)
Would such a trivial circuit be OK?
How can I determine the right Ohm value for the thermistor?
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It must be an extremely small pump with a 4.7 k NTC. As such, start current should not be an issue.
???
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Benta, that was just a diagram, I have no idea which value the NTC should have. :)
The (el cheapo) motor is consumes ~ 1A (24V) in steady state, and about 1.5A when it starts up.
Is there a rule of thumb for which Ohm value the NTC should have for such a motor?
In addition to the inrush current, I would like to use the soft start to avoid the initial transient wave of water that violently shakes the pipes.
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Not sure how to use a thermistor for this but you could connect switch through RC delay circuit to transistor to slowly turn it on.
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When using a NTC thermistor to limit starting current, you must remember that it may not cool off sufficiently between power-ons, if you need to start and stop the load repeatedly.
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Benta, that was just a diagram, I have no idea which value the NTC should have. :)
The (el cheapo) motor is consumes ~ 1A (24V) in steady state, and about 1.5A when it starts up.
Is there a rule of thumb for which Ohm value the NTC should have for such a motor?
In addition to the inrush current, I would like to use the soft start to avoid the initial transient wave of water that violently shakes the pipes.
So now the motor is not 12 V, but 24 V. It consumes 1 A steady state. Why didn't you post this information in your original question? Talk about wasting people's time.
1: get the facts together
2: present them in an orderly way
3: ask your questions.
How hard can it be? Jeez...
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Part of your preparation should be to obtain a scope plot of the inrush current response. An NTC is rated to absorb a certain level of energy during the initial transient event, and a datasheet would provide an example of that energy capability in the form of charging up a capacitor (as that is the most common usage).
Another part of the prep is to determine what minimum current is needed to start the motor, as it may be that even using a constant current power supply, the start-up current required must be larger than the steady state current, so you won't be able to insert more than a certain maximum series resistance.