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Discussion Starter #1
Correct me if im wrong
I have been reading about how some of you have problems with high current draws when switching on electric fans,this might help.

What if you were to put 2 12v light bulbs inline with the fans.When you turn the fans on the lights will take up the high current then when the fans get spinning the lights will still stay on but the fans would work as usual also.

When the fans arn't spinning at startup its like having a short in the system,but having the lights in there will take up the initial "shock".They will glow bright at startup then dim once the fans are going.

Its kinda like a "cusion" for the relays and fans.

If someone has already thought of this,sorry for the "New Idea"
 

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Ya lost me bud


You mean to put in an inductor to limit in rush current?

This "hit" isn't so bad for me. MAybe a slight flicker of the lights, but that is only because the voltage goes from 14V down to 12V suddenly, then rounds out at 13V. All happens pretty quickly though.

adding more power draining "stuff" into the equation would only cause more current draw in the long run.

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71' 3880# with me. Big Block 402, Merlin oval heads, 10.2:1CR, TH400, 3.73 posi,
1/8th: 8.0 @ 88mph
BEST 60': 1.85 w/street tires.
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Discussion Starter #3
Well,the current draw will still be there but the bulbs will take most of it instead of the relay contacts.When you flip the switch the current will spike so the bulbs will glow bright disipating the increased current as heat and light instead of the current melting the contacts in the relay.Then once the fans are moving the current will decrease so the bulbs will just be on.

I'll try it on a blower motor tomorrow,im not completly sure if it'll work because if you use a fan that doesn't use brushess it won't work as good.Some fans use an oscilator to switch the polarity of the magnetic field inside the motor instead of brushes and a comutator.
 

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SS has got a point, but I'm not sure how it will help. Isn't the bulb more resistive instead of being an inductor. The motor being an inductive load? This is a DC circuit. Don't mind me, just goofing.
 

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Discussion Starter #5
OK,the increased current is because the fans havn't started turning yet so they act like a short so the current is high.Thats where the bulbs come in to take up the high current at the time that the fans are not turning.The bulbs will GLOW BRIGHT because of the high current,then they will then dim as the fans start turning and the current decreases.

Better?


[This message has been edited by supersport396_2000 (edited 04-22-2002).]
 

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edit

[This message has been edited by Coppertop (edited 04-22-2002).]
 

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By goerge I've got it.
the CAPS usage helped.


I getcha now. "divert" the sudden rush of current from the contacts of the relay. But, the "system" still sees this load and your car will react the same.

Pros: Your relay will last longer

Cons: You just installed a light bulb in series with a fan and people will laugh at you.
Not to mention the labor time and parts involved will buy you another relay.

Check out that Bosch relay I use. I think it is a 45 Amp piece. the data sheet listed it's potentual for switching high loads, and I'm sure they tested it for wear/tear. Mine still lasts after a year of use.


something cool I turned up looking for said data sheet. http://www.eatel.net/~amptech/elecdisc/coolfans.htm

Even has a neat "How relays work 101"

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71' 3880# with me. Big Block 402, Merlin oval heads, 10.2:1CR, TH400, 3.73 posi,
1/8th: 8.0 @ 88mph
BEST 60': 1.85 w/street tires.
BEST 1/4: 12.5 with 1.89 sixty foot (street tires)
BEST MPH: 109mph

Picture of me roasting the tires and other guy stuff
Video of me staging (smoke of course)

[This message has been edited by BB_Mike (edited 04-22-2002).]
 

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The light bulbs would be adding a constant resistive load, and with them in series with the fan, would lower the current in the circuit regardless of the state of the fan( start-up inrush or just flat out running). Since they are a constant resistance, there will always be a voltage drop across them, which means the fan will never see the full potential of the battery. If two light bulbs, in parallel, draw 12 Amps when connected directly across the battery, they would have the equivelent resistance of 1 ohm. If the fan draws 24 amps once it's running, it has a resistance of 1/2 ohm. I don't know the real currents/resistances, but that it probably close enough to make my point, which is that once the fan is past it's inrush ( although in reality, it probably would never turn on to begin with), once everything has leveled out, you would have 8 volts across your lights, and 4 volts across your fan.

What your really trying to do is incorperate some kind of "soft-start" circuit, in which you could have some kind of current limiting device, which is physically taken out of the circuit once the fan is up to speed. Leaving anything in circuit will just slow down the fan.
 

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Discussion Starter #10
Gene,that sounds good to me.It was just a thought.I was trying to think of that "soft start" but i couldn't remember it.I see what your saying but i have only tried this on small radio shack motors and lights.When i put power to the ckt the light glows bright because the motor isn't yet spinning to use the power but once the motor gets going the light dims because the motor is using most of the power.

Its been awhile since i tried to figure out R,V,I using ohms law or anything so i'll have to freshen up
I thought about some kind of multiple FET setup like a stepper motor type of thing.Do you think that would work?,or just use the old 2 relay in parallel setup for the fans.

Hey what about using a capacitor in series with the bulb then putting that ckt in parallel with the fan wire.When you turn the power on the bulb and cap will be in series then as the cap charges it will take it-self out of the ckt.When the cap reaches full charge it doesn't conduct,right?
so it will be out of the ckt.Changing the capacitance of the caps would changed the time that it stayed in the ckt.Oh and put a resistor across the cap leads to discharge the cap everytime the ckt is turned off.

[This message has been edited by supersport396_2000 (edited 04-22-2002).]
 

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The best way I can come up with to "cushion the load" would be to run a 1 farad Capacitor. Similar to what car stereo guys run on big amps.

Might help.

Later
Steve

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66 327 Malibu
80 Malibu Wagon(summer 2002)
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Not overly. 3"x6" Ok then use a 1/2, or 1/4.
I think 1 farad is usually recomended for 100-150amp amplifiers.

Just a thought. Would it work?

Later
Steve

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66 327 Malibu
80 Malibu Wagon(summer 2002)
"Rice Eating Pony Killer"
 

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Discussion Starter #15
Audio capacitors must be complety different in the sizes.A standard 1 farad capacitor,i thought was about 6' tall.I still think they are.I searched for the 1 farad audio caps and i see that they arn't that big.I've seen 500uF caps larger than those so something is fishy.

This is what i thought of when you said 1F.This one is only 500uF 1500V http://www.vortexia.com/coil/forsale.htm http://www.vortexia.com/coil/images/4.jpg


[This message has been edited by supersport396_2000 (edited 04-22-2002).]
 

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From the start-up aspect of the fan starting, say, when the relay closes, a capacitor across the fan will just draw more current through the relay, as the cap will just be another current draw as it goes from 0 volts to 12 volts. Putting the relay between the cap and fan would do nothing except perhaps supply an intitial current supply if the wiring from the battery to the cap can't handle the initial current demand.

As far as putting the cap in series with the light bulbs, you would have to take into consideration the ESR ( equivelent series resistance) of the capacitor to see how it would interact with the overall circuit current at any given time as it reaches a steady state.

If all you are concerned about is trying to make the relay more reliable, just replace it with a SSR (solid state relay) like this($150).

[This message has been edited by Gene McGill (edited 04-22-2002).]
 

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I am not sure what the purpose of all this is. A good automotive 70amp relay ($7) will take alot of inductive load and not mind at all. Starting caps would be useless unless you are trying to get the motor to speed faster.

PUtting a light bulb in series will limit the current and therefor the work the fan can accomplish.

If anything...I would want to limit the spike that is created when the contacts open and break a load of high current with a capacitor or MOV.

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Steve Jack - ConceptOne Pulleys and Brackets / Engineering & Marketing Technologies
 

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Two things come to mind on this. One is that the fans probably draw about 20 amps when at full speed. That is a lot more than the bulb would draw. The second thing is that the filament in the bulb has a positive temperature coefficient for resistance. This would allow an inrush surge followed by an increase in resistance and then a drop in current.
 

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I may be way out in left field on this one, but if you run the hot wire right to the fans, and use the relay to open/close the ground, it should be a lot easier on the relay. Same reason you hook up the positive cables first when boosting a car, if you hook up the ground first, it arc's when you hook up the positive, but if you hook up the positive first, it does not arc when you hook up the ground.

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Chris Dagenais
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