Voltage drop problem

[Pete Prodoehl]

I'm working on some electronics to control a rotating light which runs
on 12 volts. There's a microcontroller that controls a relay that can
take 12 volts input which it drops down to 5 volts via the built-in
voltage regulator. Basically 'm trying to power the rotating light and
the microcontroller from the same power supply.

Everything works fine until I connect the rotating light. It seems to
draw enough that the 12 volt / 5 amp power supply I'm using drops way
down and causes the microcontroller to reset.

Aside from powering the microcontroller separately (which does work
fine, but I'd like to avoid a second power supply) what might a solution be?

Would a capacitor across the pos/neg for the rotating light solve the
problem? Or is there another solution?


Pete

[Brent Bublitz]

I would throw a good size cap across the connection to the microcontroller. You need the voltage smoothed out there more so than the motor. But that's just my unsophisticated brain thinking of things. 

Pete



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[Elroy Pearson]

Pete,

I would start with the capacitor. You could use a motor driver driven with a PWM (pulse width modulated) signal for motor speed and direction control as well as to give the turning motor a start speed and reduce the current in-rush. I'm not sure whether your relay would react fast enough to drive it with a PWM signal.

Elroy

On Tue, May 24, 2016 at 2:58 PM, Pete Prodoehl <ras...@gmail.com> wrote:

Pete

[Pete Prodoehl]

Oh yeah, across the microcontroller makes more sense. One problem, since the 12 volt power is going to both the microcontroller and the light, wouldn't I really be putting the capacitor across both things? Would that still do what it's supposed to?


Pete

[Have Blue]

Well, it wouldn't hurt, anyway.  How low of a voltage will the microcontroller take?  Can you add a 78L05 voltage regulator (and then add a smoothing cap to the 5v output of that) so that the microcontroller power supply is better isolated?

[Tom Gralewicz]

I would start by looking up the specs on the bulb and rotating light, or better yet, just connect them through an amp meter to a battery and see how much they draw.

I'm going to guess its more than 5A on startup.

Most of those use a standard 1156 automotive bulb, these are rated at 27W at 12V, that's  over 2A.

Both the motor and bulb have really big initial currents, take a look at this for incandescent bulbs starting up:

http://www.lamptech.co.uk/Documents/IN%20Starting.htm

Yes, a 2A bulb can draw over 20A when it starts.

Add in the motor - especially when it starts - and you are likely to see way over 5A.

A really big capacitor across the power line After the micro-controller might get you past the start up.  A few thousand uf at least.

The other solution is to support the micro-controller when the light turns on.  Add a diode before the micro-controller and a good sized cap after the diode, 500uf or more.

This gives the micro power even in the main buss drops.

You might want to see what your micro-controller brown out voltage is and if its adjustable.

Other notes:

The sudden surge might be overtaxing your power supply, try a different brand.

Add in in-rush limiter to the light to keep the initial current lower.

Replace the incandescent bulb with an LED model.

Get rid of the motor and incandescent bulb, just use some bright LEDs and let the micro-controller make it look like its spinning.

Tom Gralewicz

Chronic Maker

(414) 380-1716

milwaukeemakerspace.org

[Pete Prodoehl]

Wow, thanks, Tom... lots of good info!

I'm after the simplest solution, so I may try the LED bulb first and see if that solves things.


Pete

[the_digital_dentist]

Is the motor running from the same power supply?  You might have noise from the motor riding on the supply line to the uC and that can cause all sorts of weird behavior.  If it's a DC motor put a flyback diode and a capacitor across it to limit the noise level.