what are you running on your fans as far as size values

 

The diodes used on the two Bosch relays for the dual electric fans...I used Radio Shack #1N4001 diodes. They are .59 cents for two of them in a pack. They solder between the positive & negative spades on the relay, with the band on the diode end always going towards the positive side spade. Is this what your after? Gar

 

yes thanks

 

dumb question but what purpose does this serve?

 

No question is a dumb question, I was wondering the same thing . I have dual fans controlled by 2 relays and I didn`t need to put a diode in. I think you may need one if you are going to have the A/C trigger the fans too???

 

Might have something to do with helping to get the ripple out of the "DC" since the "DC" was formed by an alternator which produces AC and it is not a perfect flat voltage.

 

I am an ee engineer and I don't see the value of the diodes either. I always wire them straight to the relay. I am curious what the diodes function is. The battery does enough to smooth the dc ripple from the alternator that it should not have an effect on fan operation.

 

hmm.
i only know of two common uses for diodes in fan systems.


1.. the fan can act like a generator and actually create voltage as its blown by the air going down the road... a diode can stop this "reverse voltage".
2. AC operation... on the control side of the relay, sometimes you need a diode so that when you kick the AC compressor on, it can ground the fan relay, but when you ground the fan relay, the compressor doesnt kick on.

2002_Z28_Six_Speed:
the alternator has a built in rectifier, and some buffer circuitry, so the end result that comes out of the case IS DC current.. the remaining ripples are extremely mild, but taken care of by the battery... the battery is like a big capacitor in this case... if a device requires a pure, regulated DC voltage, like the PCM does, it has(or needs to have) its own cleanup caps and voltage regulator on the device... the rest of the car also puts some noise in the system..lol

 

Diodes are used to suppress the reverse inductive voltage kick when coils are first energized or de-energized.

 

i thought most automotive relays have that diode built into them..?

 

we have been going over this at my work my tech says I don't need this cause the batt, is a buffer and why would back flow be a problem any way ???

 

Interesting info. I have never used them and never had a problem with a relay unless the current flow went too high and welded the switch to pin 87. If it was such an issue on the control side nitrous systems would come standard with them. I never thought about the fan spinning in the wind, but esentially that should have no effect. The car would have to be moving pretty good to spin the fan, and at that point the fan is not engaged. The postive lead going to the fan from the relay is not engaged so feedback should not occur.

 

well, the problem they're talking about is very common with electronic devices... the relay closing is moving thru a coil, and it does create a slight electrical spike.. no problem for most devices, but when you're dealing with smaller electronics, you need that diode... for example, your PCM has thoes diodes built into it..


as far as the fan thing, that depends on how yours is wired... i normally see that "current backflow" relay in cars where there is something monitoring if the fan is on....


the AC compressor -> fan diode is very common on GM cars.. i know my 91 had it from the factory.

 

I can see the usefullness of it in small senstive devices, but I was just curious for the use in this aspect. I know what your talking about with the ac system, but I wonder why it is being used.

 

Depends on what backflow you are talking about. Backflow from the fan should not be a problem since it would dead end at terminal 87 on the relay if it was not engaged and do no harm.

But if you have backflow from the fan driver in the computer going to the A/C system then depending on how the A/C system is wired you may turn on the A/C compressor because of the backflow.

 

Placing a diode in reverse polarity across a DC coil is common practice for control designers. A diode blocks DC when in reverse polarity but only blocks half of the sine wave in a AC (alternating current) system. With AC relays a Varistor is used which is a variable resistor across the AC coil for the same purpose. A diode used on a dc relay coil allows the coil energy (amps in a inductor hense a coil) collape throught the diode disappating its energy through itself trapping the spike from going down the wire to other electronics to do it harm. Diodes are not perfect as they do have a resistance value that will disapate the current through itself as heat.

As far as the fan motor free wheeling due to air blowing through them in the off condition shouldn't cause a problem as they should be electrically open from its relay control.

I wired my dual fans with low speed, high speed, and force them on high whenever the AC compressor is on. No matter what the PCM is commanding the relays are wired to not allow low speed or high speed to be commanded on at the same time. I didn't use any diodes on my relay coils although I would normally design them in if I were designing a control solution for machinery.

 

A 1N5404 across the fan motor (set-up as a suppressor circuit) is used as a 'freewheeling' diode to negate the effects of inductive feedback to the electrical system.

MrDude_1 has it right:

A Mark VIII fan, which I normally use for all cooling efforts, has a surge current of around 100A.... and a simple fan control circuit that I use in Mark VIII installs is as follows:

 

I beg to differ. The voltage isn't perfectly constant. Maybe a automatic DMM would suggest so but try logging the vehicles voltage with Autotap or using the manual instead of auto range on your multimeter and you will see that the voltage level is anything but smooth.

 

Precisely...this is the reason. Gar

 

Inductor (coils) store current (amps) where capacitors store voltage. A diode across a coil will allow the coil stored inductive current when de-energized to loop through the diode and itself where the resistance of the combined pair will disipate the charge until it is gone, there by elliminating the harmfull current spike in the system.