Automatic Potentiometer?

Hi guys,

I'm trying to get something that upon powering up, it basically slowly goes from 0-12V DC. When I say slow, it doesn't need to be that slow, maybe over 2-3 seconds? I know I can use a potentiometer to dial it up manually, but is there a way to do it automatically? I found Digital Potentiometer, is that what I need and how do I use one of those?

 

What are you trying to do?

 

I make 1/4 CCFL light sabers for collectors. The CCFL uses 12V AC power which is done by using a small inverter with a 12V DC power input. When I use my variable PSU on my soldering station to power it up, and I slowly dial up the power from 0>12V I can see it slowly powering up the CCFL tubes and it actually lights up like it does in the movie from the hilt to the tip. So I'd like to be able to be able to turn the power on for 12V DC power, send power to this adapter/device/potentiometer (that I'm trying to see if it's possible to make) so it gradually sends power to the 12V DC >AC inverter to simulate the manual dial up. Does that make any sense?

 

Mmmm, yes. So how linear does it have to be? First thoughts - capacitor and a large resistor.

Or there's always something like this you could try - https://lmgtfy.app/?q=how+do+I+use+a+digital+potentiometer

 

Realistically not very linear. From 0 to about 5V there's nothing on the CCFL tubes anyway. I believe it's towards 10V+ before it starts lighting up at the base and at full 12V the whole tube is lit. I have tried googling how to use a digital potentiometer. Fact is I don't even know if that's what I need. If there's another way of doing it or if people can think of other ways. How does a capacitor and a large resistor work in this case? I've got very little electronic background so I'm rely on what little I know and build from there.

 

I wonder if some sort of DC PWM circuit would work? https://www.electronics-tutorials.ws/blog/pulse-width-modulation.html

 

Isn't that still a manual adjustment? Like I'd need to dial it up/down myself? Unless I'm reading it wrong?

 

I would use capacitor , 2 resistors and power transistor
The capacitor and high value resistor on the base of the transistor to negative being fed by a resistor from switched positive, the transistor would be in the negative of the device path so it switches on the device slowly. By using a variable resistor in series with the one from the positive you can vary the speed.

 

something like this?

 

Ok I understand what each item is and have some knowledge in what they do. But not how to wire it up from what you've describe unfortunately.

Yes I think something like that would work. It doesn't need a power switch to reset so if I just omit the switch, when it powers on it'll do the ramping up then just stay at the peak power after?

Also I need to run about 1A 12V DC through it. I read a few things and even the video only shows it using fairly low power. I'm wondering if there could be a heat issue with higher power.

 

This is what I'm trying to do but without me dialing the knob up/down.

 

So paulvk's way seems probably the least component way to go about this

This circuit is also called a capacitance multiplier. You only need the 3 components that make it work, the resistor, capacitor and pass transistor. in this case you can safely ignore the supply cap and downstream cap etc.

In relation to heat generation, the heat will be generated by the pass transistor particularly during the ramp. Once it is up to voltage the pass transistor will be held fully on at about 0.6V voltage drop (assuming you've omitted the resistor to ground from the capacitate multiplier version), so at 1A it will dissipate 0.6W. Not high but also not low, You will want something approximately TO-220 sized. The transistor would warm up but a small finger sized heat sink will also be enough to increase the cooling and lower the point temperature. A BD139 would do it if you added a heat sink tab.

There are ways of lowering that voltage drop and continuous power loss. This is a bit more complicated, and you'd probably best start from a power supply module and mod it so it starts up slowly instead.

Also regarding PWM dimming - not the way to go. The reason is because the partial fill effect in the CCFL is actually a result of the tubes not being excited enough, but with PWM dimming each impulse fully excites the tube and relies on the timing to reduce the brightness. This is why with CCFL's you can only dim down to about 1/3rd before this and other things happens in current controlled mode, but then PWM dimming can stretch this all the way to over 1/100th but still cause the tube to be evenly illuminated.

 

Thank you! I will look into those parts tomorrow.

 

Sorry, just a slight hiccup there with the power math, you'll actually end up with probably way too large of a cap to get the timing you're after in conjunction with a low pass transistor voltage drop.

Basically you might be able to alleviate this with a Darlington pass transistor (probably about 2.5V loss in practice) but I think a slightly different setup might work better - I'll sketch it up and see how it goes

Here we go:
https://www.falstad.com/circuit/cir...2EIoUggQ6mh0hOgNHoAHtStgQEF1NDSlRsIa1Eplix6EA

Adds a transistor and resistor, steady state losses are about 1 watt. The PNP transistor also needs to be reasonably large like a 0.5A one as it initially sucks down a bit of current.

 

My question would be how do you get the 12VDC when you're swinging this thing around.l? Is it a boost converter from a lithium ion pack? Because there's a fair chance you could modify the boost to perform a soft start fairly easily. This can often be done with a capacitor and diode in the feedback path even if the chip it uses lacks a dedicated soft start function.

 

It's for 1/4 scale statues. Usually powered by a 12V DC adapter.

 

Ah. So plugged in?

 

Yeh. A 2A 12V DC adapter.

 

Ok I'm not good at reading this stuff but going by what the mouse over says I need the following:
Transistor : BD139?
Transistor : BD139?
Resistor : 47 Ohm
Resistor : 1000 Ohm
Capacitor : 10 microfarad

Is that right?

 

Link with some notes. Not sure where you're buying your stuff, but the transistors I've picked are pretty generic and are available at jaycar:
https://www.falstad.com/circuit/cir...CTVWwIc9tEgmNJ8CcKGI2GIQXWSCHKKcCBC06q3ZY9CAA

w.r.t resistors, just normal 1/4w ones are fine in this case, and an electrolytic would be ok for the capacitor. Application wise we can change the 1k/10u for 10k/1u and use a film capacitor for life = practically forever. It might be an idea if you need to put this together really tightly, but if your going to have this in a jiffy box/metal jiffy box in line to the model then it shouldn't be too bad.

Also the tab of the transistor needs to be electrically isolated if you are mounting it to something metallic for cooling.