Lighting Up a Model - Circuit Help/Review Needed

[Jerry Brady]

I'm not much of an electronics guy, but I've gotten this far with a project to light up a model that I've been working on for 5 years. The problem I am having is that the flasher circuit I built causes all the other lights in the model to dim a bit when the flasher circuit flashes its lamp. It's really annoying.

I know that there must be some kind of current/voltage drop because of the load and I don't know whether it is something in the circuit or the power supply I'm using just can't keep up.

Here is the circuit:

**broken link removed**

Questions:

Should I swap the lamps for 6.3V/150mA units to draw less current so I can rule out over driving the power supply?

Is there a way to isolate the load from the flasher circuit so it won't affect the other lights?

Any help appreciated.
- Jerry

 

[philba]

maybe I don't understand but from your circuit, you have 6.3V lamps hooked up to a 12 volt rail.

What is your "dc supply" - a wall wart or some regulated supply? more info would be useful.

have you measured the voltage of the circuit (with out the flasher) but with the lamps on? I bet it has dropped quite a bit below 12V since you are pulling about twice the current (1500 mA vs 750). O bet the power supply runs pretty warm.

 

[Jerry Brady]

The power supply is a wall wart rated 12V/1200mA.

I haven't measured the voltage with just the lamps on, but can do that tonight and report back. I couldn't find 12V bulbs locally, so I just went with the 6.3V bulbs. Pardon my ignorance, but do the 6.3V bulbs pull twice as much current because of the 12V rail?

Should I install series resistors for each of the bulbs? Would that help? If so, what value?

As you can see, I know just enough to be dangerous. Thanks for the help so far.

 

[jrz126]

the bulbs will draw more current at a higher voltage. V/I=R 6.3V/250mA=25.1ohms then 12v/25.1ohms=476mA

You could possibly install some capacitors near the bulbs to help with the dimming.

Replacing the flashing bulb with an LED would also fix the problem.

EDIT: oh and resistors would help. use: R=(12V-6.3V)/X where X is the desired current (in Amps)

 

[philba]

I don't know if they come in the right form but automotive bulbs are 12V and pretty rugged.

As jrz said, ohms law means the bulbs will draw about twice the current at twice the voltage. The lights didn't blow probably because the wall wart voltage drooped under the overload.

I would not use resistors to drop the voltage for the lamps since they would need to be 2 watt which are probably more expensive than the bulbs. Just run 2 in series. (12.6V instead of 12V but it's ok).

On the flasher, I would use a voltage regulator because you need to have relatively stable voltage, more on this in a sec. The problem is that the bulb is probably pulling around 400 mA which is well more than the 555 can handle. I wouldn't put more than about a 200mA load on the 555's output.

On the output current of the 555. I would have no more than a 200 mA load. Now, the output voltage is about 2-2.5V lower than Vcc. so I would use a variable regulator like the lm317 to generate a Vcc of 8.5V so that the output voltage of the 555 would be 6-6.5V. Use the 150 mA bulb.

 

[Jerry Brady]

Thanks for the input, I really appreciate it. Good point about the resistors, because once I close the model up, I won't be able to get back inside (easily) should something fail.

Since the LM317 is rated at 1.2A could I simply install the regulator so it would cover the flasher and the 3 bulbs? I could set it at around 6V and see if it helps.

Otherwise, I'm going to verify the current draw and voltage drop for the two LEDs tonight. Just to be sure. I'm also going to replace the flasher bulb with a 150mA/6.3V unit and unless I hear otherwise, wire two of the bulbs in series.

That still leaves one bulb on the rail at 12V.

More later.

 

[ZIGGY_DAN]

You could also place a lareg capacitor across the supply to help cope with heavy currents drawn when the bulb suddenly flashes, or an large inductor in series with the bulb that flashes and a diode reverse biased across it to eliminate electromotiv feedback.

hope this helps?

 

[ZIGGY_DAN]

sorry about spelling!

 

[Nigel Goodwin]

Placing 6.3V bulbs on a 12V supply will blow them instantly, the only reason they haven't done so is that your supply is too weak, so it's not providing anywhere near 12V on load - the others dull as it flashes because it's even less capable of providing the extra current demand.

So for a start get 12V bulbs, and a better supply - you can't start trying to cure a problem when it's only the faults which let it work in the first place.

 

[philba]

I don't think you need to run the non-flashing bulbs off of regulated voltage. You can keep the 6.3V bulbs but run 2 in series to handle 12V. I'd put 2 series sets of 2 in parallel (hide one of the bulbs) which would draw 500 mA. I would only use the LM317 to drive the 555/flashing bulb. I'd use the 150 mA bulb for that. The reasons for this are that a VR like the LM317 dissipates the voltage drop in heat. 12 - 6 is 6 V drop and multiplied by 500 mA is 3 watts. waaaaay too much with out a pretty major heatsink. I'd run the regulator at 8.5V like I said earlier so your 555 would put out 6V to get the best brightness. You would see 150 mA * 3.5 for about 500 mW of heat. you still might need a heatsink. I'd keep an eye on the 555 since it's total dissipation is 600 mW. It will probably be ok but write back if it gets too hot.

If you kept the total current in the 650 mA range, the wall wart should be good enough.

 

[Jerry Brady]

I've got the parts and I'm going to pull the circuit and do some careful testing along the way (voltage and current loads).

I picked up an LM317 and the caps for it as well as four 6.3V 150mA bulbs.

I post updates after I finish. Thanks again for all the help.

- Jerry

 

[Jerry Brady]

Thanks to all who replied, especially philba. This exercise ended up being a solid and practical lesson into Ohm's law.

The resulting tested and fully working circuit:

**broken link removed**

Cheers,
Jerry