help with a 12 volt dimmer

[swtmike]

I'm the village idiot in the electronics world but have ran out of ideas and am in need of some new ones.

I need to put a dimmer switch on a 12 volt spot light that plugs into a vehicle cig lighter plug. The light is 3 million candle power and I think draws around 8 amps (has a 10 amp fuse in the plug).

I figured a rheostat or potentiometer would work, but it is virtually impossible to find one large enough to handle the watts while keeping the ohms low (if that even matters), and the ones I've found have been pretty expensive. Have looked at several schematics on the internet which show a potentiometer and/or a rheostat to hook into a triac. However, I can't find who carries a triacs and don't know if that would even work since that was on an electronic motor.

It's funny how something sounds so easy to do but it ends up being extremely time consuming and complicated to produce! If you have any ideas or suggestions, I would really appreciate them.

Thanks
Mike

 

[IanP]

If you want to experiment a little bit, here is a circuit that will do the job:

https://freecircuitdiagram.com/2009/02/01/pwm-motor-control-circuit/

Although it’s been designed to drive dc motors, driving lights is not much different ..
All what you need is the 555-timer, power MOSFET and some associated components – all of them you should be able to get in your local store ..

Rgds,
IanP

 

[betwixt]

PWM is definitely the way to go - forget Triacs, they only work in dimmer circuits if the power source is AC and in any case, they would drop to much of the 12V and permanently dim the lamp.

Brian.

 

[swtmike]

I think you took my electronic village idiot with a grain of salt. I'm about at a preschool level compared to y'all, but I'm willing to give making a circuit board a whirl.

I've ventured to all the electronic places in town and all keep pointing me in different directions about who knows how to do it and who carries the parts, guess they don't know enough about it either. I went to radio shack and was able to get all the parts except the diodes and the transister, which I've found on the internet. I have a couple of questions, so please bare with me.

1. I just got all the parts which the skematic had, but am wondering if I needed to get larger parts for the watts that light is running on, or if you think the parts listed are more than fine for the application?

2. I'm at a loss on how to connect the POT. It has three prongs and I'm assuming the middle is the in and either one of the sides is the out, but how do you connect it? Do you solder a wire to the board and out to the POT, then a wire from the POT back to the circuit connected to the swiggly line (for lack of a better term)? And is that same line supposed to connect to both diodes?

3. The spot light has 2 wires going to it, where in the skematic does the pos enter, and where does it come out, on line 1 (on the right)? Where does the ground connect to?

4. The gap between 1 and 3 (on the right), that is where the transister goes, correct?

5. Is the negative going to continue out of this if I lay it in-line before the on/off switch built into the light, or will this assume the on/off switch? If that is the case then it might be best to mount it inside the housing of the light.

Sorry, I think I'm making it more complicated than it really is, but that's what happens when you don't know a whole lot about it. If anyone would rather discuss this over the phone, shoot me a pm with your number and I'll give you a call at your convenience (if it's within the US).

This is what I'm working on.
http://www.brinkmann.net/shop/Detai...ies=FLA-2003-3&seriesname=Max Million&id=1227


Thanks in advance for any further help on this matter.
Mike

 

[IanP]

A number of your concerns are connected with the power MOSFET ..
If you can not find IRFZ-type try some other power MOSFETs that are available ..
Here is an example:
http://www.irf.com/product-info/datasheets/data/irf540n.pdf
From the pin allocation .. see attached picture:
1=Gate[G]
2=Drain[D]
3=Source
In general, the MOSFET should have: I[D]>15A, V[DS]>50V and R[DSon],0.05Ohm ..
Also, it will require a small heatsink ..

.. The spotlight is connected between +12V of the car battery and the [D]rain of the power MOSFET ..

.. on potentiometer:
http://www.allaboutcircuits.com/vol_6/chpt_3/6.html

.. For the spotlight it really doesn’t matter which way pins are connected ..
One pin is connected to the +12V, the other to the [D]rain, 0V, or battery[-] is only connected to the circuit, including of the power MOSFET ..

Rgds,
IanP

 

[swtmike]

Man I'm sorry, I'm still at a loss. I know you're having to explain this to me like you would have to explain it to a new born and apologize for that, and greatly appreciate your help.

In my basic knowledge of 12 volt (which I thought was pretty good until I encountered the engineering side of it!), I'd have a 12v+ in and out, and either bypass the 12v- leaving it straight to the light, or have the 12v- end at the circuit, and it's still unclear where they tie in. What does the dot under the 12V (in bold) in the upper center of the skematic represent? I'm assuming, I would solder the 12v+ in to the peg on the mosfet (S) and 12v+ out from the peg of the mosfet (D) or from the top of the mosfet(D). Oh man, I'm really confused now. Just tell me where 12v+ in and out get soldered to and 12v- in and out get soldered to. Also, how does the grd coming out of the 555ic tie into the 12v-?

When I look at one of these circuit boards, I see a bunch of holes that need to be tied together. This is where I'm getting confused on the POT (as well as +/- in and out). I don't know where along the line on the skematic I need to tie the three terminals on the POT into, surely there not tied to one another. I can see doing two, one from/to each diode, but don't know what the third would go to. If there were separate lines on the skematic, like on the link provided, it wouldn't be as difficult.

And where does the heat sink tie into the skematic? Or is this just used during the soldering.

I'm going to give my circuit a whirl when I hear back. I am also going to make a pre-assumption that it's going to flare up in glory, should be a site to see! If my attempt fails, I would be happy to compensate someone to construct one and ship it to me.

Ian, thanks for all your help. I'm sorry you've expended so much time on it already, but it's greatly.

Mike

 

[IanP]

The attached (modified) picture should explain where the battery terminals should be connected ..
Keep in mind the line thickness indicates “thicker” wires ..

It also should give you an idea how/where to wire the potentiometer ..

On the drawing of the MOSFET I added [D] at both sides .. it just means that the tab of the MOSFET is connected (internally) to the middle pin ..

The role of the heatsink is to maintain the temperature of the MOSFET at reasonable level, as the switching process is not ideal and while switching the MOSFET will convert a portion of power into heat ..
But keep in mind two things: 1st, it doesn’t need to be the exact shape (as shown on the picture), in fact it can be a piece of aluminium sheet or anything made of metal with a tapped hole, so you can attach the MOSFET to it, .. and second, the MOSFET tab is not isolated, so once you attach the MOSFET to the heatsink, it will have the same potential as the [D] pin of the MOSFET and therefore shouldn’t “touch” any other part of the circuit ..

Rgds,
IanP

 

[swtmike]

Well I'll be, I was wondering why all those broken line triangles pointing down just ended. That explained everything! Huh, funny how simply it comes together. Ian, thanks for your patience in explaining this to me. I really appreciate your time and help.

I've got to wait for those parts to come in and then I'll let you know how it worked out.

Thanks,
Mike

 

[IanP]

G'dAy mAte, yOU'rE wElcOmE ..!!
IanP