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Topic: Simple video interference problem. I'll pay to have it resolved. (Read 1 time) previous topic - next topic

David82

Jan 07, 2013, 04:47 am Last Edit: Jan 07, 2013, 06:17 pm by David82 Reason: 1
Here is the challenge:

When the switch is closed, the video momentarily blacks out. Add components to the diagram such that any capacitor or DC motor can be switched on without causing the video to momentarily black out

Magician



Magician

Than you need LPF connected to A terminal (in series with a load). Assuming than high current involved, 1A motor has about 5A start current, you bind to use LC based filters, with a choke appropriate for 5A or whatever your load.

David82

#4
Jan 07, 2013, 02:22 pm Last Edit: Jan 07, 2013, 02:38 pm by David82 Reason: 1
how do I find out what value inductor (choke) would just allow 1A @12v dc to pass through it?

Magician

Just find out there is other 4 pages thread on the same topic.
http://arduino.cc/forum/index.php/topic,140951.45.html
Reading briefly, I completely agree with:
Quote
« Reply #39 on: January 06, 2013, 08:31:51 AM »    Bigger  Smaller  Reset Quote

Quote from: David82 on January 06, 2013, 08:20:27 AM
Wouldn't an inductor of the right size smooth out current spikes and solve it?

An LC filter between the power supply and the motor, and/or between the power supply and the video camera, would fix it. But the inductor needed would be very large, so I don't think this is a practical solution.
Putting huge capacitor across the motor actually would make situation worse.
If you can't change wiring and put inductor in series with cam (in this case smaller size of choke required) than choke should have inductance big enough to "buy a time" for power supply to react on changes in load. If you don't know settle time of the power supply, you have to experiment. Time constant: t = LxR, where R is a load, L is inductance.  Diameter of chokes wire would defined max current.
Something like this:
http://www.abra-electronics.com/products/159ZA-Choke-DC-Filter-300mH-1A.html
Or you can try secondary windings of transformer, with appropriate gauge.

David82

#6
Jan 07, 2013, 06:37 pm Last Edit: Jan 07, 2013, 06:42 pm by David82 Reason: 1
1. What is the critical spec on DC Choke Filters in my situation? Is the amperage what's important here? Does 50mA vs. 2A matter?

2. What allows it to sustain the current longer? a bigger Henry rating?

3. is it going to be wired as shown in the image below by the green dots or red dots?



4. I CAN put an inductor in series with the camera. Should I do that instead of a choke filter? If so, what would the specs need to be of the inductor? isn't a choke and and inductor the same thing?

Magician

Quote
1. What is the critical spec on DC Choke Filters in my situation? Is the amperage what's important here? Does 50mA vs. 2A matter?
Max current has to be more than max current your load.
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2. What allows it to sustain the current longer? a bigger Henry rating?

Henry rating makes a current to change slower, slow enough to power supply track and adjust. t = R x L, where L is in Henry.
Quote
3. is it going to be wired as shown in the image below by the green dots or red dots?
Red, in series.
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4. I CAN put an inductor in series with the camera. Should I do that instead of a choke filter? If so, what would the specs need to be of the inductor? isn't a choke and and inductor the same thing?
Inductor and choke is the same in this context.

David82

It blanks out for about a second or less. Amperage is unknown so I'll just get the highest I can. The question is, what minimum Henry value should I look for?

Magician

Quote
It blanks out for about a second or less.
This time shouldn't be in equation. More important transition time constant of your power supply. I'd think about 10  milliseconds , than t = L x R has to be mo than that.
Let say you cam consumes 1 A, than equivalent R of the cam R = 12 V / 1A  = 12 OHm.
t >= 10 millisec, so
L >= t / R = 10^-2 / 12 ~= 10^-3, or about milliHenry.

David82

1. Does it matter if the choke gets put on the + rail vs the negative rail?

2. Does matter which way the black and white wires of the choke are oriented?

3. Should the location be as close as possible to the camera, or can it be near the power supply even if the power suplpy is 15ft down a cable?

Magician

1. Depends on a load, simple resistive or motor doesn't matter which rail, even better to put in both. Load with more than two wires (power plus signals lines) doesn't tolerate inductor in ground, so in +rail only.
2. No.
3. Doesn't matter. Nothing should be connected or disconnected in between inductor and cam.

David82

I did some tests. I need help interpreting the results.

measuring equipment: Fluke 287 history graph feature.

Theory: 90W PC power supply isn't good enough because it can't supply enough current when needed
Test: use car battery
Result: same video flickering problem

Theory: a drop in voltage when a load is applied is what causes the camera to flicker
Test: apply 4700uF 35v cap to circuit (which will also cause the video to flicker) and read voltage data from DMM.
Result: voltage only momentarily dropped from 12.1v to 11.8v
Conclusion: voltage drop is not significant enough be the cause of the problem.

what else should I test?


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