time-delayed relay for funbus

I have a selfmade funbus that uses the LM317 voltage controller. My problem is that I'm using it, to power my 120mm fans. I want them to go the slowest possible, which is on ~5.5V, the problem is that they start only at something not less than 7V. What I'd need is a some sort of a timed relay, that within the first, let's say 3-5 sec, runs the fans by the 12V rail to speed them up, and then changes to the regulator's 5.5V adjustable rail... anyone know how to build such a 'relay'...

I'd really apreciate some feedback on this issue

The way I did it was to add a capacitor across the potentiometer, this gives the effect of shorting out the pot until the cap is charged, it then uses the resistance from the pot,

I used a 4700 uF but any large cap will do. just connect it the correct way around, this supplied full 12v to the fan for the first 4 seconds or so from switch on,

the only prob is a restart where the cap is already charged

Kevin

That's f*(&^ brilliant m8. One thing though. How do I connect the capacitor correctly (i assume we're talking about a polarized 4700uF). The pot goes from adjustment on LM317 to the ground, thus i should connect the '-' leg to the pot leg closer to the grounf right?

Sorry m8, I'm not an electronic g33k... yet as i hope

Ok. I've bought a 4700uF electrolithic (polarize) cap, and connected it to both legs of the pot. I tried both ways (i'm aware of the risk), and in both cases the output voltage supplied to the fans by the fanbus was at 1.25 at start and kept slowly growing... what did i do wrong?

Another option is a simple 555 timer hooked up to a relay. Radio Shack has cheep books(if needed) for 555 timers and should also carry any needed parts.

witchhunter said:

Ok. I've bought a 4700uF electrolithic (polarize) cap, and connected it to both legs of the pot. I tried both ways (i'm aware of the risk), and in both cases the output voltage supplied to the fans by the fanbus was at 1.25 at start and kept slowly growing... what did i do wrong?

Click to expand...

It's been a while since I've used the LM317, but I seem to remember that the voltage across two of the pins should only be 1.25V.

I'll think about it today and see if I can come up with anything.

I'd really appreciate that m8