Hello All,

I am in need of replacing a potentiometer with voltage ramp circuit, at least that's what I think I need. I have purchased a PWM controller off ebay to control a 12v fan. The issue is that I want to eliminate the POT and replace it with a circuit that ramps up the voltage as soon as the power is applied and be ready for a second ramp should the power be shut off and then on again. The ramp should be about 3-5 seconds.

After searching the forums, I found this circuit: https://forum.allaboutcircuits.com/attachments/rampdriver-gif.87260/

I believe it will do what I need but it looks like the ramp takes 10 seconds. I think to shorten the ramp, R4 would have to reduced by half? And to set up a "restart" there would need to be a pull-down resistor but I think that is what R1 can accomplish?

Sorry for the newbie descriptions. I'm really good at building and putting things together but electronics is a very weak area for me.

I would appreciate any help, Thanks.

 

The ramp has to produce what level of peak voltage ?

Regards, Dana.

 

Hi,
The input voltage will be 12v, so the output should also be 12v.

 

I think, for your purposes, an exponential ramp would be acceptable, so just use a RC circuit at the input.
Try a series resistor of 10kΩ followed by a capacitor to ground of about 270μF.

Below is the LTspice simulation of the circuit from application of power.

The diode is to rapidly discharge the capacitor after power is removed, if you want the circuit to recover faster than about 10 seconds after power loss.

 

Awesome, thanks for that! I downloaded LTspice and was able to build and simulate the circuit I had referenced. I changed the R4 resistor to 33 and was able to simulate a 3 second ramp!

I guess my question now is what would be a better, more reliable circuit? Or, it doesn't matter, simple is better?

I tried to code and on/off switch in the Sim.....it didn't work out so I don't know how the other circuit behaves when power is removed.

 

I don't know how the other circuit behaves when power is removed.

Change your input voltage source from a DC one to a Pulse one with a suitable 'on' time.

 

Ok I'll try that, thanks.

 

my question now is what would be a better, more reliable circuit? Or, it doesn't matter, simple is better?

I take my motto paraphrased from Einstein, "Make it as simple as possible, but no simpler".

 

I tried using a simple Delay Off circuit and removed the coil and could not get pulse to show any delay. It seems a simple switch is overly complicated to make in this program. All it did was pulse on and off no matter how I changed values in both the pulse and circuit.

 

All it did was pulse on and off no matter how I changed values in both the pulse and circuit.

You have nothing to slow down the charging of the capacitor.
You need a resistor in series with the diode in front of the capacitor.

The first circuit should work better for a delay than the second.

The first circuit does not need a diode in parallel with the relay coil since the inductive transient will just keep the transistor on for the short time of the transient.

What's the purpose of the diode in series with the transistor?

 

I think that in the first one you would want the cap to charge as fast as possible so that there is a fast turn on. When power is off, it bleeds out in only one direction bc of the diode resulting in a "delay off". If I remove the coil, I would need to replace it with a low value resistor to slow down the bleed. I could be wrong since I really have limited knowledge of this stuff. I'll have to try it again tonight.

 

! Principial ! linear ramp (with not energy optimized discharge networks) j-Fet - MOS-Fet + Bipolar

with power optimized discharge

 

I think that in the first one you would want the cap to charge as fast as possible so that there is a fast turn on.

Yes.
Scratch what I said.
I was thinking delay for on, not delay for off.

How long a delay do you need?

 

Ultimately I need to combine a delay off and a voltage ramp. Delay off can be 7-10 seconds. Ramp can be 3-5 seconds. I was trying to simulate them separately then combine to see if the results work out.

 

I need to combine a delay off and a voltage ramp.

So what is the exact sequence for the ramp and delay?

 

It would be: power on, ramp up, run for how ever long, power off, delay off.

 

Do you need the relay you showed in post #9, or just the voltage ramp, with delay off?
Could the delay off just be a ramp down?

 

I was aiming for either the circuit in post 1 or the one you suggested in post 4 and then combine one of those with the delay off circuit. A ramp down to off would also work, it would make a nice touch! I was only using the post 9 circuit to try and get it to work in the sim. The relay isn't needed.

 

Here's a modification to the simple RC circuit.
You can play with the values of R1 and R2 to adjust the off and on ramp speeds respectively.

 

Dude, That is awesome! Thank you! I'm going to build it in the sim and tweek it a bit. I'll post results.