Servo strange occurances

[paul borgmeier]

In your first post, you mentioned “The servo runs off a separate power supply, so it can't be a power issue.” However, your schematic contradicts this.

It sounds like the servo might be pulling your power supply down enough to upset (continually reset) the 876A – this could be the cause of the sputter you describe as it "restarts" each time. Can you describe your power supply setup for us?

You might want to try 1) increasing the size of the capacitors on both sides of the regulator and 2) adding some decoupling and bypass capacitors as close as possible to VDD and VSS on the 876A. Let us know ...

Good Luck,

Paul Borgmeier
Salt Lake City, Utah
USA

[kamet]

YES it works! Thanks, paul borgmeier, for pointing the grounding misunderstanding out. I *assumed* that it was separate from the 5v out of the regulator and was being fed directly from the power, so it was considered "separate." Sorry if that confused everyone and made it more difficult to help me.

I rewired a 9V battery to the servo instead so it appears that it WAS a power problem. I would rather use the power supply instead. Paul, you suggested some capacitors to the VSS VDD of the PIC, can you give me some cap numbers I should be using and how to wire them up?

My current power setup is powered from a wall wart at 7v into a 7805 5v regulator with a 100uF on the V-in pin, and a 10uF on the V-out pin. Much like this http://www.tkk.fi/Misc/Electronics/circuits/psu_5v.gif


Thanks everyone that helped.

[paul borgmeier]

For the 876A try a bypass of 0.1uF ceramic (C2) and a decoupler of 10uF electrolytic (C1) as close to as possible to the VDD and VSS pins as shown. Every design is different and it depends on trace/wire lengths, other components, and everything else that is going on. (These are the values I start with and use in almost all of my designs.)

It is hard to say what to try with the power supply without knowing more about your wall wart and your servo. You mentioned 7V but nothing more. You could try adding one or more 100 – 470 uF capacitors in parallel with the capacitor on the “regulator” side of the regulator and even possibly a 100 uF on the other side.

You should note that most wall warts supply the rated voltage (7 V in your case) at the rated amperage (you did not list). Further, they usually supply a higher than rated voltage if loaded less and a lower than rated voltage if loaded more. If your load is less, then you will not have a problem. However, ... most 7805 regulators have a Vdrop of 2.0V, meaning if your wall wart voltage drops below 7 V due to a larger than rated load, the regulator will drop out (i.e., “turn off”). This too could be adding to your troubles. Can you provide more information on your servo and/or on your supply? You also can try the above. Again, let us know.

Good Luck,

Paul Borgmeier
Salt Lake City, Utah
USA

[Bill H]

With 7 volts in, just as Paul said the headroom to the 7805 is limited.
You could replace the 7805 with low dropout regulator such as an LM2937(5v). It's a pin for pin replacement.
Your headroom increases from ov to 1.5v in your example.
Bill H

[mister_e]

as far as i'm aware of.. those wall-wart transformer are really not accurate as we can wish. 7Volt can also be 10-15 volt. BUT measure it with a scope to see what's happen when you load it with your PIC stuff. LOOK what the voltage looks like before going to your voltage regulator? A bigger capacitor at the input Help?

You can be surprise... a LDO is a good option btw.

Or choosing another Wall-wart voltage 9Volt and up