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Old 08-16-2007, 10:29 AM   #21
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Quote:
Originally Posted by Arkmage View Post
It could if you put it all on a bread board and tape into one of those radioshack "project" boxes....
Yup, and the box would likely double the cost of the project. :gay:
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Old 08-16-2007, 10:52 AM   #22
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I found a kit for under $25, but it requires another mosfet to raise continuous duty to 20amps... which still isn't enough. And that doesn't include the box... nor the time to make it. I still have this capacitor sitting on my desk though. :gay:
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Old 08-16-2007, 10:57 AM   #23
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Quote:
Originally Posted by m2cupcar View Post
I found a kit for under $25, but it requires another mosfet to raise continuous duty to 20amps... which still isn't enough. And that doesn't include the box... nor the time to make it. I still have this capacitor sitting on my desk though. :gay:

so give ben $15.

a cap that size will make your pubes tingle.
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Old 08-16-2007, 11:53 AM   #24
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Rob, just use the timer circuit to drive the fan relay, then current handling won't be a concern.
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Old 08-16-2007, 12:48 PM   #25
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I'll trade Ben the cap (to add to his collection) for the circuit. I have a box for it. I'll put a logo on it, make install instructions, and he can sell them and retire early.

I need an explanation on the timer circuit - how it works and is applied. Plz.
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Old 08-19-2007, 10:32 AM   #26
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Sorry, Rob I didn't see your last post there until this morning. I was thinking about this circuit on my morning walk around the park with the dog, and had an idea so thought I'd reply.

The LM555 is a timer. It can have a few different modes, but basically it is just a switch. When used in what is known as astable mode, the LM555 will automatically switch its output on "high" and off "low" over a given frequency. Its output is a square wave. The simple explanation of how it works is there is a capacitor, and while it charges the 555's output is high. When the cap gets nearly full and stops charging, the 555 goes low and discharges the cap. When the cap gets nearly empty, the 555 goes high again and recharges the cap. The cycle is continous.

This can be used for voltage control. To keep the concept simple, let's assume our 555 works at a frequency of 1Hz and our duty cycle is 50%, and we are using the timer control a transistor or relay that is switching 12V DC. Over each 1 second period the LM555 is activated, it's output will be high for .5 second and its output will be low for .5 second. So for half the cycle, the transistor/relay is triggered and allows 12V DC to go through the circuit. For the other half of the cycle, the transistor/relay is not triggered and 0V DC goes through the circuit. Therefor the average voltage in the circuit over the time period is 6 volts. If we increased duty cycle to 75%, the average voltage would be 9V, and so on. This is a form of voltage regulation known as PWM "Pulse Wave Modulation." This is how a dimmer switch works.


I've been stewing on this for the past few days about this circuit. I was thinking of the route of a pair of LM556's, a CMOS CD4017, and a SPDT relay (556 is a pair of 555 in a single case). The design concept: Your Link applies ground to trigger the cooling fan's relay coil. Intercept this ground wire with a PWM voltage regulation circuit that would ramp its duty cycle from 50% to 85% over a short period of time, say 3 seconds. The CD4017 would allow us to add resistors into the circuit one at a time, progressively increasing the amount of time it takes to discharge the capacitor mentioned above. The longer it takes to discharge, the longer the timer's output will be high. Which increases our circuit's duty cycle, and thus the average voltage. However the best we can muster is 99% duty cycle. So the second component of the circuit is an additional timer that at the end of the ramp up period removes the voltage regulation component of the circuit and allows a direct connection again between the Link and cooling fan relay coil.

It can work, but it will also take a couple handfuls of resistors and capacitors to get the timer sequences right. Which isn't terribly difficult IF you know a few other variables. Which I don't know. I think it would be a great ordeal of trial and error, that would eventually succeed brilliantly. But I also think it would be a great pain in the ***, and really over complicated and unnecessary. Over kill. It would also be hell on the cooling fan relay.

Easier solution:
Instead of dealing with the ramping rate and all the complications it entails, IMO a better solution involves just a pair of LM555's. The first would run PWM voltage regulation at fixed 60% duty cycle. The second 555 would act as the timer circuit, just as above, to remove the voltage regulation from the circuit after a fixed period. So, the Link determines the cooling fan needs to run and connects ground. The first LM555 controls duty cycle to 60% as the fan starts spinning, and after 2 seconds or so, we have the second LM555 remove the voltage regulation.

So cooling fan off, trigger occurs, starts up at 60% and maintains for 2 seconds, then after 2 seconds goes to 100%. Not a smooth ramping start up, but also hella easy, and should accomplish your goal of not shocking the car's electrical system when the fan kicks on.
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Old 08-19-2007, 11:26 AM   #27
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thanks Ben- the latter sounds adequate for my application. A project for my spare time.
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Old 08-20-2007, 03:57 PM   #28
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I bet there's a way to use one of these RC controls.
Just need to figure out the programming procedure... or borrow an RC plane/controller.
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Old 08-24-2007, 12:03 AM   #29
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Why is the fan draw a problem? Is the voltage drop interfering with something? The alternator should be compensating for the draw. Try using the post at the alternator. You won't have the resistance of the vehicle's wiring at that point. If there's still a problem I can help design a ramp circuit for you. A bypass relay won't be needed. Mosfets can handle the load just fine by themselfs.
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Old 08-24-2007, 09:58 AM   #30
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Yes. The fan is rated at 20+ amps constant and startup is likely more. If my headlights and interior fan are on it will stall the car when it comes on. No headlights and it starts up fine, though there is still a voltage drop. There's about a full second for the car to react to the load.

Is there a difference between the post at the alternator and the post in the engine bay fuse box where the battery ties down? (it makes sense) Not a problem to move it at all.
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Old 09-20-2007, 07:42 PM   #31
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I forgot to get back to this thread. I'm just thinking that, the engine bay fuse box is where all the accessories are drawing power from. The alternator should compensate almost instantly for any loads. So if there is any point in the electrical system were you could draw the most power, straight from the alternator without any resistence, would be the alt post itself. Idle air control is another story. When you say the engine dies, is it dying from the load of the alternator pulling the rpm down? or from your a/f ratios going whacky from the voltage drop? What engine management system are you using? If it's the oem ecu it has inputs that cause the ecu to compensate for things like the blower being on speed 3 or higher, the ac being on, or the power steering demands. It would be easy to wire the fan activating to trigger one of those compensation inputs. The ecu then makes an instant change in iac position and maybe even timing to account for the loads.
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Old 09-20-2007, 07:50 PM   #32
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y8s do you want to design something? I'm always up for "electronically complicating" the Miata. haha The 3525 PWM IC can do soft start. Then check out the FPQ85n06 mosfet. 300a pulsed, 85amp cont. ([email protected]), .01 ohm rds-on. And that's in a to220 case!

Option #2 is a time delay via. 555 one-shot that activates a relay bypassing the (big) series resistor on the fan circuit.
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Old 09-20-2007, 11:00 PM   #33
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I'm no ee. i can only poorly design electronics. i am great w/ concepts tho
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Old 09-21-2007, 12:27 AM   #34
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As in, comming up with them?
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Old 09-21-2007, 12:30 PM   #35
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I think that's what they call a "dreamer"
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Old 09-21-2007, 05:25 PM   #36
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Did you ever solve the problem?
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Old 09-21-2007, 06:18 PM   #37
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No. I looked at my car for the first time in three weeks this morning. Too busy at work, and I need to build the new oil cooler first.
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