Re: [DML] Fan Sequencer (delay box)

[Martin Gutkowski <webmaster@xxxx>]

Hi Shain

The reason you need to delay both fans by DIFFERENT amounts is because the switching signal comes along at the precise instant the ac compressor engages. The compressor produces a considerable "kick" to the engine all on its own, so you let the engine recover from that, then switch the first fan on,then the second. You also have to examine how the fans behave as the signal is killed - do they cycle down in sequence as well or do they shut off immediately. Both ways have advantages and disadvantages which I've addressedbased on personal preference - ie it's up to you how to build it.

I don't understand the function of your low side switch, and why therefore the load has to be at least 75 ohms total. All you're doing is creating a potential divider so that the relay switches at a lower voltage and the tranny controls a lower current. Just use a fatter tranny and let the relay have 12V - it's what it's meant to use after all...

My low side switch is one common emitter 2n2222 with the relay coil from supply to collector. My timing comparator switches the base through a resistor with an additional pull-down. (and of course a catch diode across the relay).

There is no UK equivalent of RS's website (rswww.com), and all the part numbers are common across the world (or so I thought....).

With the fan-fail light, I knew how to do it sometime ago, it was just a matter of finding the right part at the right price to allow it to be done simply. Suffice to say, I'm using proper hall-effect sensing - more details to come :-)

You say you're not worried about sparking - I would say it's a case of "just because it works doesn't mean it's done properly" - most of the DeLoreanselectrics fall into this category!!! Switching a relay with a rising (or falling in this case) voltage such that you get a "timed" effect is one of the main reasons the lock module is so notorious. Drive your relays with a good hard sudden switch and they'll last twice as long.

I've just been playing with my system - the 2n2222 quite happilly switches the relay I bought - it has a DC resistance of 80 ohms.

Martin
#1458

shain@xxxx wrote:

> Hey Martin,
>
> Thanks for the input. Unfortunately i'm the only one who had the chance to go over my design until now. In order for the circuit to operate properly i need at least 75 ohms of resistance on the coil, unfortunately the rs model is only 66 ohm, so using the resistor brings it up enough to function. The 3904 (while not as common here as in the Uk) are readly available, and radio shack carries them so why not. The 2n2222 should work fine but i haven't tested them yet. I don't know why your rs part numbers aren't working, i just bought the parts the other day (don't know how rs work in the uk). I'm very eager though to see your fan fail indicator circuit though. As for sparking, i'm not worried about that. I haven't had any sparking yet, and if there is a momentary low of signal, it still takes 2-3 seconds for the capacitor to step back up in order activate the relay. I though thepurpose of the fanzilla was to turn one fan on then have a delay on the second. The main fu!
> nction of this was to have one fan warm up before the other clicks on, creating a gradual instead of an imediate request of power for the car. If there both delayed i can't see how it would help, if one waits say 4 secondsto activate, the the other waits 12 seconds, one is still turning on before the other, please clairfy on this so i can make the nessary changes.
>
> - Shain