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kima

help with dc dc converter and rf frequencey filte

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Hello every one,

I would like someone to give us a hand with my small problem as i do not have much knowlege in this area. I have a rc helicopter which i am useing for aeriel photography. The camera mount i have built myself and i am also useing a ccd wireless camera. The ccd camera, Rx and servos for pan and tilt run on 4.8 volts.

Now this is where the problem is at the moment the rx and servos run off their own supply and the Tx and ccd camera also has its own battery, but if i conect them to the same supply there is some RF frequencey interferance and was wanting to know how to correct this. I am also wanting to use a video overlay and GPS but the video overlay runs on 12volts so what i am wanting to do is to run every thing off the same supply, being 12 volts and then use a dcdc converter to drop the voltage down to 4.8v for the camera and servos as i think it would be better to drop the voltage than to try and convert it higher. Now would i need a filter for each peice of equiptment i am running off the main supply, and would just one dc dc converter be ok. All your sugestions and help are welcome and if there is any thing else i need to add to the circuit please can you advise me.

Cheers Kimberley Attwell

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Hi Kimberley,

Here's my thought. If I were doing this, I would probably select a battery voltage that was a happy medium *between* my two requirements, perhaps from a pair of lithium-polymer cells which give out a nominal 8v.

I would use a 12v dc-dc switcher to get the voltage up for my 12v components and a 5v ldo linear regulator for my 5v stuff.

Because of heat dissipation concerns I would very likely gange two 5v regulators together to spread the load and to add a small amount of redundancy.

In this manner you would be powering your servos from a linear regulated supply instead of a more noisy dc-dc supply, and you would be saving some weight in your batteries.

Kind Regards,

Bill

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Hi Bill,

Thank you for your advice, I am going to go about this as you suggested, thanks alot.

There is just one thing which i am worried about with all my appliences connected to the same supply will i not have to worry about RF frequences interfering with the servos or the servos interfering with the ccd camera, or shall i just use a couple of filters?

Cheers kimberley Attwell

Edited by kima

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It is so installation dependant that it is hard to say what interference you'll get.

All I can say is build it and try it! Fight the interference gremlins after they rear their ugly heads.

Regards,

Bill

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I am using a TI dc-dc converter with a 2s2p poly cell.

The ESC gets 7.4 but the cam and tx are on 12v. I got/get massive interference. I wrapped the dc-dc in electrical tape, then in alum tape, but still had mega problems.. What has anyone done to quiet down thier TI dc-dc converters??

Also, does anyone get interference problems when using the 500mw 2.4ghz and higher powered transmitters? If so, What have you done to slove them? I get a little bit off my 500mw unit, but I relocated my planes rx antenna out to the wing tip, and that seemd to help... I just don't want to get out to far, and still have video, but loose control.. Any small prebuilt bandpass or reject filters available out there?

BTW, I am using older futaba radio gear....

Thanks

Edited by lvspark

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I got/get massive interference.

Is this interference related to the video or the R/C gear?

If it is video related, is it aggravated by ESC/motor speed?

What has anyone done to quiet down thier TI dc-dc converters??

- Short leads on EVERYTHING.

- Do not daisy chain the grounds. Instead, use a star ground layout. That is a single point ground with lots of conductive area. The Tx's sardine can case is one such solution.

- Use very low ESR caps, per the data sheet. Use VERY short leads (SMT parts are nice).

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lvspark,

Can you shoot a picture of your installation and show us?

Thanks,

Bill

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The interference I was refeing to was RF. The planes controls were erratic at a close distance, and a total loss of signal at 75'.

This was on my slow stick.

I cannot show a picture, I have removed it.

It was the standard TI module with the 100MF cap soldered on the leads as required per the data sheet. I did not put on the (2) optional, small caps. I was pulling power from my reciever on one of the channel connectors with 2" leads for power in and ground and then went from the module to my cam/tx set with 6" leads. I noticed some R/C reciever problems on the bench, so I wrapped the dc module with aluminum tape in a lame attempt to block RF.

For today, I used a 7.4 1500ma poly pack for the motor/rx and (2) 7.4 650ma poly packs in series with a 7812 (12v reg)for the cam and tx. It works real solid this way, but having a single battery on this little plane sure lightens the load and makes the pre-flight easier with only one battery to charge. Although it does lift the multiple packs just fine, and with the seperate power supplies, it is safer, so I might just do it this way in the future....??? I remember mention of seperate power supplies from someone who found out the hard way. ( maybe mr rc-cam himself?)

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The 100uF cap is not effective at reducing the EMI/RFI issues (it is for ripple reduction). The EMI/RFI problems are partially handled by the external small value ceramic caps (must have low ESR ratings).

As mentioned, ALL component leads must be VERY short. A single point star ground is recommended. Power leads must be as short as possible too.

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So if I understand correctly, your ESC regulated down to 5v for the receiver, and then you were tapping that 5v to the dc-dc converter. If this is the case, then it is not an efficient method of powering the dc-dc converter. You'd be better drawing power directly from the battery (I think you said 7.2 volts). Your current draw will go down by a factor of 1.4 and you won't be putting unnecessary stress on your BEC.

Regards,

Bill

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Good catch, Yb2normal. If true, then a typical ESC Vreg would not be able to handle the added load (most ESC's are limited to for about 5V @ 1A). I am surprised it did not go into full current foldback and shut down the servos.

Try again, but this time power the PT5041 directly off the 7.4V LiPo pack (be sure to install all the caps shown in the data sheet). This method will work well on e-models that draw low motor current. Otherwise the video will be noisy from the excessive trash on the battery voltage.

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I'll give it a shot with the external caps and direct to the 7.4v source... I hooked up the power to the unused rx channel because it seemed so easy at the time (not thinking about current). makes perfect sense.

Thanks for the help guys.

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Hi There,

I Ended up useing two voltage regulators and a 12v battery pack. I have one volatge regulator fro the servos and one for theTx and ccd camera and the 12v battery supplys all including the video overlay which is 12v and it works great with on inter ferance at all.

Have fun and thank you every one for all the advice.

Cheers kimberley Attwell

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Kind of slow but hope I picked this up. You will be using one battery to power the motor and the cam insted of the usual motor per item. Could you show a picture or diagram, Im more of a visual learner when it comes to the electronics, Thx

Edited by frebaln

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