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Luke

Internal vs. External Video Tx Mounting?

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I've got a question, regarding video Tx mounting...

I'm using a RangeVideo 500 mW Tx, and normally (because it's a temporary installation) I just rubberband it to the side of a foam slow-flyer... Works great, no dropouts, clean picture, etc. The antenna pointed downwards, running down the side of the nose.

Recently, I decided to "clean up" the installation... I mounted the antenna through the bottom of the fuselage, added a canopy... The antenna protruded to just past the little articulating hinge. (i.e. the hinge was outside the aircraft.) The video quality was HORRIBLE... Dropouts at 20 ft., diagonal noise... After about 10 minutes, I lost color, and had to land...

I then (just on a whim) popped the Tx out, put it back on the outside of the nose, and gave it a toss... Perfect video again, great color...

Now.... Was the Tx overheating inside the fuselage? I handled it immediately after the flight, and it didn't seem very warm... I can't imagine I was having an antenna problem, since the antenna was (for all intents and purposes) completely exposed in both installations. Is it usual to have the transmitter mounted in free air?

Any input is definitely welcome!

~Luke

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I handled it immediately after the flight, and it didn't seem very warm

If it was a typical heat problem, you would feel it. Too hot for you is too hot for the Tx. However, the short range and missing color does indicate that something was not happy at all. How about some bench testing with a hair dryer or heat gun?

Also, how about posting clear photos of the model and the two Tx mounting methods. Maybe a fresh set of eyes will ID the problem.

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Well, unfortunately, I recently dorked the plane, in an unrelated incident... :P I'll sketch out what I did, though... Better than nothing.

I'll try bench-testing... But if "hot" means "ouch! HOT!"... It's never done that.

Really quite strange, since outside the fuselage, I had zero dropouts at over half a mile. (And that's literally 5 minutes after I couldn't even get 20 ft., without developing interference.)

~Luke

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I'll sketch out what I did, though... Better than nothing.

No need to create a sketch. I suspect that only clear photos would be useful (with microwave RF, everything matters).

Sorry to hear about the crash!

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Well, here are dimensionally-accurate CAD renders, all the same. (5 minutes wasted, oh well.) The only thing missing is the purple rubberband, wrapped around the external mount.... :D

Two things occured to me, though: First, the Tx was unsupported, except by the antenna mount, and second, the linear regulator would have been REALLY hot inside the closed canopy. Even exposed, it runs quite hot, but has never seemed to cause any problems. (1.5A rating; the camera and Tx draw about 0.45A.)

~Luke

post-4393-1214593522_thumb.jpg

post-4393-1214593530_thumb.jpg

Edited by Luke

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It's not clear to me which regulator you are talking about. There is one inside the Tx and sometimes one on the outside that is added by the retailer or user. When one is on the outside, it indicates that the Tx is a 5V device and someone has added an external Vreg to support 12V operation (usually a 3S lipo). If this is the case, then airflow is absolutely needed to handle the 3 watts of heat. The photo would have have helped us see these little details. :)

In any case, on the next model add some cooling holes to provide airflow to the Tx and any adjunct Vregs. Or simply go back to mounting it on the outside of the model to help keep things cool.

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the Tx was unsupported, except by the antenna mount

This can be a reason too. Some transmitters don't like vibration, and holding by one point only surely doesn't help against that. Mounting the SMA in a hole always caused me problems. You're better off making a bigger hole and not fixing the connector, but holding the TX in the same way you do when it's outside.

But the regulator overheat is higher in the list. You didn't mention the batery voltage and TX operating voltage, by the way.

Edited by Kilrah

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Regulator: I've got an L7805 linear regulator on a wiring harness; it powers both the camera and the Tx, off a separate battery. I've got a 0.1uF capacitor across the output. See attached. (Finally got a picture!) I was wrong about the max. current, it's rated at 1A.

Mounting: I'll look into the Tx mounting, try securing it better, and run some tests.

Voltages: 2s LiPoly for the supply, 5V output.

I think I have a little aluminum heatsink in my collection of parts, I'll try attaching it to the regulator, and see if it helps. Of course, I could also just switch to a better (more efficient) regulator......... :P

~Luke

post-4393-1214610973_thumb.jpg

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The photo and additional information helps identify the problem.

(1) The 7805 Vreg is not the correct choice for a 2S LiPO application. This Vreg requires up to 7.5VDC to ensure reliable operation, which means it could be in trouble soon after a fully charged LiPO has been in use for a short time. Once its input voltage goes below the rated dropout voltage (~7.5VDC), the 7805's behavior is unpredictable. They usually become dumb resistors and operate with a reduce output voltage. Some turn completely off. Some do a mixture of both of these things.

A preferred Vreg for your 2S LiPO application is a LM2940 (or other suitable LDO type). The LM2940 looks like a 7805, but is designed to operate on much lower voltages. It's stability cap values are little different, so consult the data sheet. Be sure to use both caps shown on the data sheet (don't cheat and omit either one).

(2) There's no heatsink on the Vreg. With a fully charged 2S LiPO, it will be burdened with about 1.5 watts of heat, which is waaaay to much without a good sized heatsink. Lots of airflow is what saved you originally, but it was probably borderline at best.

FWIW, the max 1-amp rating for these linear regulators is essentially achieved with an infinite heatsink. The actual current it can safely provide, without going into thermal shutdown, is based on the heat dissipation that the Vreg must accommodate. That is determined by the input voltage, output current, and thermal mass of the heatsink.

In your case, without a heatsink, the current you can safely draw is a fraction of what your system needs. The solution is to mount it to a suitable hunk of aluminum and then monitor the temperature to ensure it is safe for a fully charged battery in a hot outdoor environment. Your finger can be used as a test instrument.

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Gotcha. Yep, I noticed the dropout behavior at around 6.75-7V, but just "used" it to keep from overdischarging the LiPoly... (The CMOS camera would develop dark lines in the feed about 5-10 minutes before the regulator would drop out... Really professional, no? :P)

I assume I should also heatsink the LDO? Also (just being lazy) you wouldn't happen to have some capacitor part numbers I could 'borrow'?

Thanks for the info!

~Luke

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By the way, are switching regulators any good in this application? I imagine they could produce some pretty nasty interference..?

~Luke

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I've used switching regulators with mixed results.

Sometimes I've gotten interference and sometimes I haven't lol. You could always encase the regulator and keep it as far away as possible.

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I assume I should also heatsink the LDO? Also (just being lazy) you wouldn't happen to have some capacitor part numbers I could 'borrow'?

All linear vregs will need a suitable heatsink. To work effectively they need sufficient airflow too. For ceramic caps I normally use the Kemet C332C family (available from mouser and digikey).

By the way, are switching regulators any good in this application? I imagine they could produce some pretty nasty interference..?

Switching regulators eliminate the heat issue. But they often bring EMI/RFI problems that can glitch traditional R/C Rx's. Their ripple currents can cause lines in the video too. Even those that are advertised as providing clean power are prone to do this, so don't take the mfg's word for it.

My opinion is that with a 2S LiPO, and 5VDC A/V equipment, the best solution is a linear Vreg. With 3S LiPOs a switching Vreg is preferred (otherwise the heatsink would be quite large). But, switchers require extraordinary steps to ensure that the EMI/RFI issues do not bite you. This includes additional filtering (chokes and caps) and careful testing.

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Ok... Maybe I'm just doing this all wrong, but...

I can't find any KEMET C322C ceramics in the 22uF range..? Digikey and Mouser both list the largest value as 0.47uF.

The ESR range that the datasheet for the LM2940 gives is driving me crazy. Can I just use "any" suitable capacitor, and drop a small resistor in series...?

(Sorry for being such a newbie...)

~Luke

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I can't find any KEMET C322C ceramics in the 22uF range..?

The Kemet C322 is a family of ceramic caps. For larger values, just use a quality electrolytic or tantalum cap. Keep the leads short (place the caps next to the Vreg IC).

Can I just use "any" suitable capacitor, and drop a small resistor in series...?

The ideal caps for this app would have low ESR values, at least low enough to satisfy the data sheet recommendations. BTW, placing a resistor in series increases the ESR, so don't do that.

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I've used switching regulators with mixed results.

Sometimes I've gotten interference and sometimes I haven't lol. You could always encase the regulator and keep it as far away as possible.

Me too but I realized if I use good quality caps and chokes, plus populate the components as close as possible to the v-reg, this helps stabilize the noise.

Edited by JMS

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The Kemet C322 is a family of ceramic caps. For larger values, just use a quality electrolytic or tantalum cap. Keep the leads short (place the caps next to the Vreg IC).

The ideal caps for this app would have low ESR values, at least low enough to satisfy the data sheet recommendations. BTW, placing a resistor in series increases the ESR, so don't do that.

Ok, I was looking though the datasheet, and it indicated a range of acceptable ESR values, ranging from 0.05 Ohm to 1 Ohm. I was shooting for the middle of the range, so about 0.5 Ohm. (Many of the caps I looked at had too low an ESR, according to the datasheet.)

In any case, I did end up ordering some electrolytic caps, with ESR values at the low end of the acceptable range, so I guess I made the right call there.

Thanks again for all the assistance!

~Luke

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Oh, also tried a Dimension Engineering (I think) switching regulator which I had for another project... Unfortunately, it introduced a lot of noise.

~Luke

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