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AGS

Converting Voltages

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Hi there folks,

I decided to make a UAV but everything in it has so many different voltages, I have gone with the T-REX 450 as the platform with a Sony 1/3 CCD board.

I need a little bit of advice on converting a 7.4V lipo to supply 12V for a camera and pre amp, and 8V for a TX. I know its a bit last minute but I have only three days to do this by for my project.

I have already put together a step down converter circuit but if I step 7.4V down to 5V for the DC-DC (fixed to supply 12V with 5V input) up to 12V and then back down to 8V will it still work? Will I lose voltage or current? Also I have already purchased two DC-DC converters CLICK HERE the pdf is also supplied, I just dont know if it will work and what do I need to complete it?

Thanks Ayo.

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The DC-DC module you picked is only good for a couple hundred mA's, which is unlikely to be sufficient to drive your camera and Tx. Also, it has a lot of ripple. Plus, I suspect that EMI/RFI noise issues are going to haunt the installation.

Given your short time frame, I'd suggest a 3-cell LiPO and a 8V LDO linear regulator with a big heat sink on it. When you have the time to debug it, go can always go back to designing something with a DC-DC switcher.

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I use a 2s lipo for 12v gear, the part to use is PT5401 I believe. It's good for up to an amp. I power a KX171 and a 300mW 1.3ghz TX with it and have no issues what so ever and the power is fairly clean. I believe you may find reference to it in one of Mr RC-Cams projects on the website.

Part costs about $15 at digikey and you only need one additional electro cap.

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Thanks for the answers guys, I have been given more time, I now have until tuesday for my final deadline for a presentation.

OK, I have made a circuit that consists of an ESKY 7.4V lipo 800mA (2s) being stepped down to 5V using a resistor in order for a DC-DC converter to step it up to 12V.

I have used a DC wire splitter used for CCTV cameras. It converts 1 DC output into 4 DC outputs. I am now getting 12V on all four outputs.

However my new problem is that I am going to power a CCD camera (100mA), transmitter (26mA) and audio pre amp (20.5mA) for the mic using up 3 of the 4 wires. But as soon as I connect the audio pre amp the voltage drops down to 8V on all four wires. I have already measured the current draw and it is only 20.5mA so I really dont know what is causing the voltage drop and how can I resolve it so that I get 12V on all 4 wires and solve the voltage drop? Thanks.

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I have already measured the current draw and it is only 20.5mA so I really dont know what is causing the voltage drop and how can I resolve it so that I get 12V on all 4 wires and solve the voltage drop?

The answer may be here !!!

7.4V lipo 800mA (2s) being stepped down to 5V using a resistor

Terry

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Terry I really think it might be the current drain, I guess voltage doesnt work like I thought it would.

I feel like giving up, people are saying use this and use that but I dont have many resources like some guys in the US. I have been using yahoo answers and the guys over there say:

"powering 4 devices from one battery?" YOU ARE USING BATTERY? my ans is out of topic..yes you can build a voltage regulator using transistors, but the more you add passive devices like resistors, the more you waste across that resistor considering that your supply is only a battery to power-up those 4. Batts are good source of pure DC and for experiment, to achieve an ideal result, but it can not sustain multiple loads drawing current at the same time..Just build a basic transformer supply. Look fo IC's with 4 outs as regulator, you may spend less and save time"

and...

"you need to build yourself a linear regulator!! you lose regulation by dropping voltage across the resistor, as your demand on the supply goes up, you lose regulation.

Copy the linear regulator in the ASTRON 12v 35a power supply (schematics at www.repeater-builder.com). it uses a stable LM723 IC and 2N3055 pass transistors"

The problem is I order my parts from this website (click here), does anyone know what IC's etc I should order?

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I'd just use a 3s lipo. Connect the 12V devices directly to it, and use a linear regulator (7808 series) to power the 8V device.

Why make it overly complicated with switchers when you can do it simple and spare yourself a headache and other possible problems due to the switchers?

By the way, I don't know what your 8V device is, but if it is what I think it is, it could maybe even also take the 12V directly.

If you really want to use a 2s lipo and switching regulator, make one that can take your 2s lipo as direct input to make 12V, don't step down to 5V to step up again...

And then connect the rest the way described above, all 12V gear to the switcher's output and a linear reg for the 8V device if required.

Edited by Kilrah

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I was trying to point out that you said you were dropping 7.4v down to 5v using a resistor, the voltage drop will depend on the current drawn so it will not stay at 5v.

Terry

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I'd just use a 3s lipo. Connect the 12V devices directly to it, and use a linear regulator (7808 series) to power the 8V device.

Why make it overly complicated with switchers when you can do it simple and spare yourself a headache and other possible problems due to the switchers?

By the way, I don't know what your 8V device is, but if it is what I think it is, it could maybe even also take the 12V directly.

If you really want to use a 2s lipo and switching regulator, make one that can take your 2s lipo as direct input to make 12V, don't step down to 5V to step up again...

And then connect the rest the way described above, all 12V gear to the switcher's output and a linear reg for the 8V device if required.

Hi thanks Kilrah the 8V device is a TX which I ripped out of an imported CMOS camera from HK. The current draw I measured is around 26mA at 8.11V using a multimeter and a regulated power supply to power the TX.

Funny though I tested in the workshop and I was getting an image on the TV from the camera when it was plugged into the TX but it was really bad (like a badly tuned camera) it seems the TX could work from 4V to 8V still transmitting an image but the change in voltage slightly altered the frequency I guess.

I have to stress though I am using a T450 V2 and I cant imagine carrying two 11.1V lipos but true you do have a good point, I am just trying to do with what I have since a battery would set me back at least £20. But as I have seen its not as simple as stepping it down to 5V then DC-DC up to 12V, it does work but as soon as you start adding (load) devices e.g camera (100mA), TX (26mA), audio amp (20.5mA) the voltage readings drop, but im suspecting it is the DC-DC converter because it is rated 1W with 84mA-AT-12V. BTW 20.5mA is tiny, I dont expect the battery output to give up and take 6V because of that one device. I need to get 12V regardless of devices attached.

Thanks Ayo.

P.S and eickst I dont think I can get hold of that convertor in the UK, the online store I am using is here. Can anyone recommend anything similar or better because I find choosing the right one confusing. Thanks a bunch guys. :)

Edited by AGS

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the 8V device is a TX which I ripped out of an imported CMOS camera from HK.

That's why I ask. Usually those cams that are rated at 8V have a 5V voltage regulator somewhere (sometimes hidden in the plastic plug) and both the CMOS cam and the transmitter actually run on 5V. If you still have the rest of the ripped camera you should check the voltage inside when the 8V supply is connected.

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