Is there any difference between pulling power from the under hood fuse box main post vs. coming off the alternator?

 

Technically you want to source power from the battery as it is "filter" for the power produced by the alternator. I wouldn't grab power for anything with ICs there, but I can't see any harm coming to a simple fan motor.

 

this spal fan I have pulls 21amps, so the startup load is higher. I was going to run a .5 cap in line to aid in the startup. The cap install sheet shows it connecting to the alt, and I just wondered if there was going to be a significant difference from that to the main fuse box post.

 

The cap would be fine hooked up either way. I would install it in the trunk between the battery and main power feed going forward. Then source power for the fans under the hood.

.5 farad is a lot of energy. Use caution when connecting it to your electrical system. Do not hook the cap up without charging it first. I always use a lightbulb in series with the cap and power source to monitor chrarging--when the cap is charging and power is moving, the bulb is lit. When the bulb goes out, the cap is charged and you can then hook it in.

You'll also need to be be careful if your battery goes dead and you need to jump it or charge it--the cap will need to be charged up first.

 

BTW, the fan really pulls 21A constant? I assume that this is a single fan--and you're not going to have a pair of these?

I think the OEM fans pull on the order of 8 or 9 A each.

 

Yes. The spals are tremendous power hogs compared to everything else. It's a single 16" rated at something like 2500cfm. I'm running a 70amp 626 alternator, but the startup load on this fan is still a major burden with the headlights on. I've found others with the same experience who considered the same option- so I figured I'd be the gineau pig.

I've got the resistor that came with it for the charging- adding the bulb is a good idea- I'll do that too.

 

time to whip out your electronics engineering skills and make a soft-start circuit for it. The car wont mind if it takes a few extra seconds to spin up, right?

 

no, not at all. Of course that introduces a entirely new problem - engineering school. I spent my college days in design school. A circuit builder I ain't. I can build a welder cart though.

 

smells like a trade is in order!

(all I need now is to know how to design the circuit...)

 

one instance where a design requires an engineer :gay:

 

Definitely possible, but a circuit that would soft start then "leave" the current path would be a bit complicated. I would do something along the lines of PWM controlled by a 555 or 556 timer. It would basically be a fast acting switching circuit that would switch the fans on for X, then off for X, then on for 2X, then off for X, then on for 3X, then off for X, until the circuit reaches 100% duty cycle. You could set the ramp rate for whatever, say 4 seconds.

 

that works for me. reminiscent of my old R/C car speed control or a class D audio amp?

 

Yes, it's a motor control circuit is all... it just happens to ramp up automatically, instead of comparing or regulating or whatever.

I was a bit technically off in my earlier description, so if you look into it, you'll see that the circuit should run probably around 160 Hz. That should be long enough to not stall out the motor. If you use a 555, for each cycle the minimum duty cycle is 50% and the max is 99%. You can set the time period for the ramp, I think 3 or 4 seconds is appropriate, so over that time period the duty cycle will linearly raise from 50 to 99% duty cycle.

Less than $5 worth of crap at radio shack. Don't know if it'll be necessary with that big *** .5fd cap tho (but certainly safer).

 

so capacitors are unsafe? or they can be unsafe? I figured this would be a reasonably safe item since it seems to be a typical component in higher end stereo installs.

 

Your concept is sound. The cap should be substantially large to conceal the sudden current demand caused by the fan from the rest of the car's electrical system.

Like any other modification, you need to be mindful as they can be dangerous do to their ability to rapidly charge and discharge. Observe proper charging/discharging procedures and all will be fine. Anyone who drives your car needs to be informed that the cap is there, in case they get a dead battery. Do not ever apply power directly to a large discharged cap, ever. At best case, it may instantly weld the electrical lead to its contact and shock the hell out of you. Worst case, it will kill you. And the cap will need to be discharged before removing it or handling it.

Did you already have a cap? If not, I have a few 1 Farad caps laying around. They're the neat-o extra fancy ones with volt meters on top. You are welcome to use one for proof of concept.

 

I bought it on ebay NOS- Monster brand, assuming that meant better quality- $20 delivered.

 

Spal makes a pwm fan controller, which (as mentioned above) is a preferred solution to powering up a high load fan.
ebay auction 1
ebay auction 2

 

Not exactly the same thing, this is a more complex, very neat, elegant little item. This would be a nice fan controller on the a/c side for the guys running the dual spals.

This controller is a comparator; it modulates fan speed with engine temp.
What I was thinking of has a cost of around $5, and is a straight timer circuit.
Either would suit your purpose though.

 

I'd rather have the $5 unit. Does it exist as a complete "box"?

 

It could if you put it all on a bread board and tape into one of those radioshack "project" boxes....