220 uF on output of 7806

This is not directly Arduino related, but I'm working on a model railroad application where cars on the railroad are powered by 18V AC.

Some lighted rolling stock I'm tinkering with buzzes/hums when it is powered from the track. The lighting circuit is a bridge rectifier feeding a L7806CV, which powers two small incandescent lamps.

The circuit uses a 10 uF capacitor on the input of the 7806, and a 220 uF on the output. That seems strange to me, since all the 780XX designs I see have much smaller capacitors on the outputs.

Could that choice of capacitors be the problem? Any drawback in using a small capacitor on the output?

Circuits in this field often have big capacitors to deal with intermittent contact between rolling stock wheels and the track, but the big cap is always on the input. I'm wondering if the folks in China switched the specification of the input and output capacitors.

Those capacitor choices are not unusual and will have no adverse effect on the circuit's operation.

From ~25volt (rectified) to 6volt is quite a drop.
No smoothing cap on the input of the regulator is less heat.
Hum, but that doesn't matter for lightbulbs.
If it bothers you, try a 470uF/35volt on the input of the regulator.
Regulator will get hotter though.

Why are you powering incandescent lamps with DC, they will quite happily operate from AC. If the lamps are rated at 6 volts then wire 3 or 4 of them in series so that they'll run directly from the track 18 volt supply. If you only want 2 lights then either hide the other 1 or 2 in a small tin enclosure or hide them where they won't be seen. Alternatively you could use a resistor to drop the excess voltage (a nominal 6 volts for 2 series connected lamps) but this'll take a little experimentation to determine the correct value For example, if the lamps draw 30mA then from the formula R=V/I a suitable value of R would be 6x1000/30 = 200 ohms (220 ohms if using E12 values) Wattage rating would be VxI = 6 0.03 = 0.18 watts so a 1/2 watt resistor would be suitable.

The reason for DC and regulation on the lamps is to provide constant brightness no matter what the rail voltage.

The 220uF cap is to help when track conditions cause brief interruptions to supply, ie dirt, point isolators etc.

Putting an extra cap on the input to the regulator will probably make the regulator run hot as the average voltage will increase, so the drop across the regulator will increase, but its worth a try.

The circuit is not exactly standard by the sound of it, they use the 7806 as a clamp to the pulsed DC on its input, then use the 220uF to smooth the resulting 6V pulsed output.

How old is the construction, the caps will be undergoing considerable pulse current, so they may be drifting in spec.

Tom.... :slight_smile:

Rather than incandescents you might want to look at small LEDs, though they may be a bit “white” for your application. They will be a lot brighter than the incandescents but that can be cured by reducing their current flow. Colour filtering with coloured paper (from sweet wrappers ?) might produce an acceptable colour. The advantage of LEDs is that current demand is very much less.

Are you using any kind of digital control to drive the locomotives - if so that might be the source of your buzzing within the ‘cars’ or “coaches” in UK parlance

This is a recent model from a manufacturer, not my design. The lamps are 6V incandescents drawing about 115 mA total - no idea why they picked those.

The cause of the buzzing was the small size of the capacitor on the input - when I added 47 uF in parallel, the buzzing went away.

However, the regulator was already dissipating about 2W, and as Tom mentioned adding more filtering to the input increased the drop across the regulator further. I wasn't sure if I wanted to add more heat.

So I tore it out and put in a $1.00 eBay switching module instead. Problem solved.