Plz help - trying to make a clean 5 volt source for fpv video receiver...  - Page 1
Forum section: electronics / projects

author: bigjoncoop
author_rank: Regular Contributor
timestamp: June 21, 2019, 06:13:06 am
content: I decided to make my own 5.8 GHz fpv video goggles. The LCD monitor takes 12v and the diversity receiver module needs 5 volts. I plan on using a 3s battery pack. If I just use a buck converter there is a lot of video noise. So I decided to make a 5 volt linear regulator circuit. The problem with that is is that it gets extremely hot! (Dangerously) way too hot to touch and melted into plastic with in 5min...I tried to add a piece of metal of the heatsink with thermal compound but it's still not going to work.The specifications for the receiver are:Input Voltage: 5VCurrent consumption: 5V @ 380-400 mA (1.9-2W)So since I'm using a LM7805 to drop from 12v down to 5v with a .4a load its just to much for the LM7805 to handle. I watched Dave's video on getting rid of power supply Ripple and tried to make a capacitance multiplier. So I made a RC filter with a NPN power transistor on it. 'Schematic Attached'Maybe I used the wrong value components because it did not seem to help at all. I've never built a RC filter before. I used a 1k resistor, 330uf capacitor and a NPN D44H11 transistor and wired it so the output voltage was coming out of the emitter just as Dave shows in the video. I can tell that it was doing something because the voltage wasn't fluctuating like it usually does coming out of the buck converter. I then tried to put a LM7805 after it to see if that would help... I also tried to replace the 330uf cap with a 1000uf cap and still didnt help.The only way I have found to get a clean video signal out of the receiver module is by using just the linear LM7805.Am i not using the correct value components for RC Filter/Capacitance Multiplier? I did find a few forums with people saying they use the D44H11 for the capacitance multiplier.Any help with this would be greatly appreciated. I've been working on this for days now and I cannot figure out how to supply a clean 5v signal with the materials I have. And I double-check that it is the receiver module that is causing the video noise and not the monitor.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 21, 2019, 06:29:34 am
content: 2W 12V->5V using a linear reg in small space, from a battery source, is total nuts.There is absolutely zero reason why a switch mode converter should cause any video noise. Your particular buck is just shit. They are not all that noisy.I'd pick a modern, high-frequency small integrated switcher IC and design around it with a tight layout. Extra filtering might be added, but it's unlikely to be really necessary once the switcher itself is properly done.(Remember that if your switcherinputis shared with anything else (the 12V monitor, assuming you run it directly from your 3s battery?), you may need to filter theinputside, too. Especially relevant in a buck converter, where the input current is discontinuous.

author: bigjoncoop
author_rank: Regular Contributor
timestamp: June 21, 2019, 06:42:40 am
content: thx for your reply..."They are not all that noisy"--- that's the first time I've ever heard that. Since a buck converter is a switching regulator doesn't that automatically imply that's it is noisy? Full of high freq. Noise?? And I also tried a Mini-360 DC-DC Buck converter with same results.Specifications:MPS MP2307DN buck regulator IC4.75 - 23VDC input voltage1.0 - 17VDC selectable output voltage3A surge current, 1.8A continuous rated currentMaximum conversion efficiency of 95% (5Vin, 3.3Vout ~200mA output)340kHz switching frequency30mV no-load output ripple0.5% load regulation2.5% voltage regulationThe switching regulator is causing high freq noise..??I need to make this work by Saturday and would like to use materials o have already. I haves boxes of components.

author: bigjoncoop
author_rank: Regular Contributor
timestamp: June 21, 2019, 06:45:52 am
content: What about if I made a LC filter??? What value coil, and cap???

author: H713
author_rank: Regular Contributor
timestamp: June 21, 2019, 06:53:00 am
content: I do agree, you should be able to get a buck that is much quieter than that. With that said, some circuits are very sensitive to noise, and if you insist on using a linear regulator, there are ways to minimize how much it has to dissipate.You're dropping 7 volts across that regulator- quite a lot. If the load will be consistent, you can put a dropping resistor in series before the regulator. You're still dissipating the same amount of heat, but in this case you are dissipating it in a resistor. You will probably want about a 5W resistor for this. This is a trick I've used when I need to add a regulated DC supply to a piece of tube equipment with a 12.6V filament system. Switchers are out of the question with some tube circuits (very high impedance, very prone to noise).You said this is running off batteries- you are wasting and insane amount of power. This doesn't matter in tube gear (40 watts of filament power is already being thrown away), but your battery life will be horrific.Switching regulators do not have to be noisy- they can be very quiet if designed right. The problem is that a lot of them are designed to the bare minimum cost. I am partial to linear supplies. From my point of view, nothing can replace a big transformer and big caps. But I would not use one where efficiency mattered.

author: H713
author_rank: Regular Contributor
timestamp: June 21, 2019, 06:56:26 am
content: Quote from: bigjoncoop on June 21, 2019, 06:45:52 amThis is probably what I would try- there are plenty of online calculators you can use.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 21, 2019, 08:07:38 am
content: I would try adding LC filters to both input and output sides of the buck converter.If you have to work with whatever components you have around, that makes your component choices easier now, doesn't it?Use larger electrolytic caps as the lossy element, in parallel with small ceramic caps for high frequency suppression. Optimally, the inductors should be somewhat AC lossy as well, but use what you have. Inductance? Something about an order of magnitude smaller than the inductor used in the converter should do. Something very roughly around 10uH?Also consider adding a makeshift metal casing with only small holes for the wires to go through. Twist the + and - wires together. Remember to ground the metal case well.Note that eBay DC/DC modules tend to not only use cheap components; they also use ancient designs, which run at fairly low frequencies, and the layouts tend to vary from slightly sucky to disastrous. Saving on capacitors reduces their lifetime and increases EMI. And sometimes they use unshielded inductors.