limiting inrush current at the relay

Hi all.

Im needing a special relay to drive a radiator fan from a Ford Taurus. At startup the inrush current can be as much as 130A, steady state is about 35-37A.


At my relay on the output I need a current limiter. Am considering a NTC thermistor ( inrush current limiter ). this one here.




Would this work? Placed on the output of the relay with a diode?


Inrush current limiter

No

1 fan or 2?

1 fan

Does it have separate low and high speed windings?

Yes I am planning on using the high speed for engine cooling and the low speed for A/C operation.

Is it 130 amps when starting the low speed? There is a way to start it in low speed and then seconds later turn to high. This should make it much easier on your system. Any idea of the current draw at low speed startup?

Low speed is about 80 something. Still too much as the vehicle I plan to put it on only has a 100A alternator.

I wouldn't worry about the inrush current as relays and fuses etc will generally ride thru that. As long as the relay is rated for the normal full load current plus a bit extra (typically 20-30%).

Contact arcing etc (being an inductive load) is a different issue, but I'd suggest suck it & see.

PS - those NTC thermistors are not suitable, they are for different applications - not current "limiting".

There's something wrong here, the Taurus should only draw about 85 amps to start the engine in warm weather, so 130 amp fan draw, sorry but feel free to prove me wrong.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

howie ll wrote:

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There's something wrong here, the Taurus should only draw about 85 amps to start the engine in warm weather, so 130 amp fan draw, sorry but feel free to prove me wrong.

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It's the current spike when set to run from a dead stop that is over 100 amps but only for a few miliseconds.



Taurus Fan Inrush current spike video

OK, then an 80 amp relay from a reputable manufacturer will handle that quite easily.
They will handle twice their rated power for a few miliseconds.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

howie ll wrote:

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OK, then an 80 amp relay from a reputable manufacturer will handle that quite easily.
They will handle twice their rated power for a few miliseconds.

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Cool. I assume that even if my alternator is 100A that that few ms of the fan drawing over 100 should be fine? Think the vehicle would stumble?

Another way to avoid the current rush would be to add a circuit that ramps the voltage to the fan over about 5 secs or so. I ;ve been scouring the web for something like that but can't find anything.

{Deleted & re-posted...]

Just a mere current limiter would do, but WHY are you even bothering to do that? What problems are you suffering?


The alternator is irrelevant. Lights may dim etc, but "stumble" as in misfire etc - very unlikely - vehicles with much larger fans do fine... (EMSs have caps...)

Yes I think we're disobeying the KISS principal OR if it ain't broke don't fix it rule.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Or overthinking - without sufficient experience/knowledge.

Transient and inrush over-currents are not an issue for relays or fuses (and cables) etc - just look at their surge and withstand or blow-time curves etc.
If they were, all relays etc would be rated for the peak inrush - ie, maybe 200A for a 55W halogen bulb and probably much higher for your 30A fan (check its inrush using a CRO).

Usually it's the resulting voltage spikes that are the issue... eg, see the current "returned" to your battery when you switch the fan off - and that voltage spike.


Usually the only consideration is to use relays that are rated for inductive loads (contact endurance & separation), but that should not be an issue for automotive 12V relays.

jaxzin wrote:

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Another way to avoid the current rush would be to add a circuit that ramps the voltage to the fan over about 5 secs or so. I ;ve been scouring the web for something like that but can't find anything.

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A circuit of this type to handle that much current will be costly to build. You need to try it before you go through any trouble. I really see no problems.

Two points;
a) I remember reading an Omron data sheet mentioning that their relays are rated at twice the switching current for a few milliseconds each cycle.
b) With regard to Mr. I's last post that could explain why my Mits had a black box on one of the electric fans.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I doubt that anyone would use a ramp-up circuit for a fan due to the risk of motor burn out.
Current limiting is a different issue and that's merely 2 resistors & 2 transistors for low current circuits...

BUT YET AGAIN - WHY!!!?

You're right guys. I am ditching the Taurus fan and going with a Ford Contour fan setup. That one only draws about 60 amps inrush and settles at 30Amps when running. Will simplify things by a ton. Thanks for the advice.

I would still like to know what it simplifies.
Were you burning relays or suffering other problems?