Dimmer/Potentiometer Question

[Hercpilot]

I admit to being electrically handicapped! Hence this question. Can I use a simple potentiometer to control instrument lighting or do I need a dimmer?

[raiki]

A potentiometer will work however it will be quite hot, depending on current. Also, again depending on current, it will need to be quite large.

 

A PWM type dimmer is a much better solution.

[Waiter]

Heres a nice little circuit and description:

 

http://www.electronics-lab.com/projects/power/026/

 

Waiter

[argoldman]

Heres a nice little circuit and description:

 

http://www.electronics-lab.com/projects/power/026/

 

Waiter

Wait, Waiter---(couldn't resist)

 

What are the possibilities of noise coming from this circuit? does the unit have to be shielded (or would that help)?

[Waiter]

I think the unit in the diagram runs at about 80 hz. It wouldn't hurt to shield the unit in a box

 

Waiter

[Hercpilot]

Thanks again for the input fellas! As usual, you guys deliver!

[Hercpilot]

Waiter,

Would this kit work?

 

http://www.electronickits.com/kit/complete/motor/ck1400.htm

[raiki]

Yes indeed that will work, however DC motors are generally switched very rapidy. This high frequency switching may cause noise in your COM. Being that it is only 410Hz I think it will be fine. I am used to motor speed controllers in the kHz range.

 

It's basically the same as Waiters suggested circuit (A couple of 555 timers firing a driver). Fixed Freq (astable 555), varied Duty Cycle (monostable 555).

 

Try it, and if it is noisy swap out a few components to bring the Freq down. It's pretty simple.

 

Another addition I would make is a switch to go full brightness (even a pot with an internal switch). That way you get 100% DC as opposed to the circuits max of 99%. Now you could have the switch power the MOSFET permanently or you could have the switch carry the lamp current, effectively 'shorting out' the dimmer. That approach would extend the life of the MOSFET, as it wouldn't be running at 100% load when not required to dim.

[Hercpilot]

Thanks Adrian!

[raiki]

Here is a screen capture of that circuit in a simulator (with my added switch). Gets about 7A max on the dimmer into 100 watts of globes, and 8A with the switch closed. Controls down to about 3A so you have a brightness range of about 40% to 90% with the dimmer. You'll get 100% with the switch. With some mods you should be able to get <40% if you so require.

 

[raiki]

Controls down to about 3A.

Whoops made a mistake. Create the connection from R7 to Q2 emitter and it controls down to 850mA. Much better. Top end no change.

[Hercpilot]

Adrian,

Since I am no good at reading circuits, which switch did you add? Do you think the board that comes with the kit can be modified to accept your switch design?

 

Justin

[raiki]

My added switch is labelled J1 in the simulator. It connects the lamps to ground in place of the MOSFET. This could be added to the circuit quite easily (even externally).