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oxxyfx

Horizontal lines caused by servo

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Hello,

This topic shows my plane tracking receiver: http://www.rc-cam.com/forum/index.php?showtopic=2058, please take a look at the pictures.

The original setup used a Futaba 3003 servo, and all was good except the servo didn't have enough torque and it was kind of wobleing...

I changed the servo to a Hitech 625MG and it was rock steady in moves, however the LM2940 - 5V regulator (1A) didn't seem to have enough power to power all the components, and when the servo moved it blacked out the video. I then changed the LM2940 with an LT323AT - which is a 3A regulator. The input has a 0.47uF SMT ceramic condenser, the output a 22uF condenser. Now with this setup the video does not black out, but there are horizontal interference lines across the screen when the servo moves. I tried to filter this out by adding a 470uF condenser at the bottom of the servo connector - and it worked - it remover about 95% of the interference lines, leaving only a few. I tried to add one more 470uF in parrallel, but that didn't improve anything any more. I also tried running the servo cable through a ferrite ring - no effect.

Does anyone have any other suggestions on how to filter out the remaining lines? The circuit has a Max4217 video buffer to create 2 video outputs out of the one coming out from the airwave 625 receiver.

Thanks,

Ox.

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Rather than trying to make your common supply work with everything, how about powering the servo from a separate Vreg? Along with the mfg's recommended stability caps, I would add a choke on its input too. Perhap use the LT323AT for the servo, and install another LM2940 for the electronics.

Otherwise, to salvage you existing method, try adding a 33uH choke to the power supply V+ that operates the electronic circuitry to isolate it from the servo's commutation ripple.

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Thank you. Terry also sugegsted the seperate vreg, however I am not sure I can find space on the baseboard to fit it in.

If I put two vregs in, do I still should use the choke?

Thanks, Ox.

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The choke would be a nice touch, but I doubt it is needed in the two Vreg version. But keep it in mind.

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Ok, thanks, one more question.

If I change the design and instead of the servo I use a stepper motor - bipolar driven by an L293D - would you also run that from a separate vreg? Will the stepper motor cause same or similar interference if it is run on the same vreg as the rest of the electronics?

Thanks, Ox.

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Whenever you place complex loads like motors on a shared power buss, you should always be prepared for trouble. Not that it is absolutely necessary, but a no-brainer solution is to use separate supplies. A common ground can also be used if it is a star arrangement.

If it was me, I would stick with a good hobby servo rather than a stepper motor. In this application it seems to me that it would be much simpler to deal with.

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Whenever you place complex loads like motors on a shared power buss, you should always be prepared for trouble. Not that it is absolutely necessary, but a no-brainer solution is to use separate supplies. A common ground can also be used if it is a star arrangement.

If it was me, I would stick with a good hobby servo rather than a stepper motor. In this application it seems to me that it would be much simpler to deal with.

Exactly. I would strongly recommend to apply the seperate vreg solution. This will definitely get you out of trouble, even when your power source gets weak or messed up with other stuff. You can also add another servo, if needed. Just power it from the servo supply. Doing this with only one vreg, would most likely show up with distortion again because the passive filtering might not keep up with the higher current.

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Ok, this is getting better by the minute. So we go with two separate vregs fed by the same battery. I read the explanation about the star arrangement and it just got me even more confused....

So same battery, 2 separate vregs, but the ground cannot be common unless it is in a star arrangement? Is this right? This would mean that the servo does not have common ground with the rest of the circuit. Do I understand this correctly?

Thanks, Ox.

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This would mean that the servo does not have common ground with the rest of the circuit.

The servo's supply will have a common gnd with the rest of the circuit.

In a star ground, your two major ground paths return to a common low impedance location in the wired circuit. In other words, do not daisy chain loop the grounds.

There's an endless number of online resouces that will further explain star grounding. Google will show you the way ...

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I always explain the ground problem with resistors,

lets just pretend that each wire you use have 1 ohm pr cm,

ok then you can much more easy see what lines can be joined

and why others cant, with out making signals into critical video signals.

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I built the circuit today with 2 vregs as suggested. I have to thank you guys, the video is chrystal clear, I haven't noticed any glitches during the servo movement.

Thank you guys.

Ox.

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