Author Topic: Needing a clean 5v reg  (Read 1481 times)

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Offline David97

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Needing a clean 5v reg
« on: September 13, 2016, 06:17:02 am »
Hi everyone. Working on building a small video receiver to use with FPV models.

Ive been playing with a rx5808 module. First I was using a arduino board to get 5v power from for testing. But this was not giving me enough current. The regulator was getting hot.

I measured the current to be around 200mA. I was dropping the voltage from a 12.6V battery.
That's (12.6-5)*0.2=1.5Watt, that's probably why it was getting hot.

So I took a cheap Chinese, no name, buck converter from my box of parts, and hooked that into it. Problem was that even with a 470uf cap, was producing noise (I think, I dont have a scope). Anyway the video sucked compared to my other receiver (store bought), which I'm certain uses the same module.

So cant definativley prove that that was the problem without a scope, but not going to want to run the cheap anonymous buck regulator from china anyway. I know having low ripple is important with this project anyway.

So weather or not that was the problem, I need a new 5v regulator.
Needing something with low ripple, has a input range of 6V-12.6V, and can provide the necessary current.

Switching would be nice, but not necessary, since I know switching regs tend to produce more noise.

Thanks for your're help! Sorry for the noobie nature of this question
« Last Edit: September 13, 2016, 06:18:53 am by David97 »
 

Offline David97

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Re: Needing a clean 5v reg
« Reply #1 on: September 13, 2016, 06:28:59 am »
Discussions on this thread about a similar project have led me to believe that this would be a good option

http://www.rcgroups.com/forums/showthread.php?t=2477752&page=11


http://nz.rs-online.com/web/p/switching-regulators/6664379/
 

Offline homebrew

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Re: Needing a clean 5v reg
« Reply #2 on: September 13, 2016, 05:01:48 pm »
Hm, why not using a good old 7805 linear regulator in a TO220 case?
200mA doesn't seem to be a big deal - just select a proper heat sink.

Add two 100nF bypass caps (input and output) and a 470uF bulk capacitor at the input and another 100uF on the output and you should be fine. A 1N400X diode as reverse bias protection and another one to protect from reverse polarity.

Adding a fuse would make it even more safe.

However, this won't work anymore with 6V. You probably need something around 8V to get this working ...
 

Offline blueskull

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Re: Needing a clean 5v reg
« Reply #3 on: September 13, 2016, 05:39:36 pm »
I would generally recommend a buck converter followed by a CLC pi filter, then a high PSRR, low noise LDO. I use this topology in high end portable audio circuits where absolutely low noise and low power is needed.
It's unlikely that a video receiver will require such high performance, but it is always good to know you can cascade regulators to get goods from both of them.
 

Offline JPortici

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Re: Needing a clean 5v reg
« Reply #4 on: September 13, 2016, 06:34:23 pm »
+1
i will be using the same configuration in a future project. i am basically a noob in smps but it wasn't hard to achieve good-ish performance, maybe with poor efficiency but a rather clean signal

blueskull might correct me but i wouldn't go crazy high in switching frequency as LDO PSSR usually become rather poor over 10-20 kHz
« Last Edit: September 13, 2016, 06:44:07 pm by JPortici »
 

Offline blueskull

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Re: Needing a clean 5v reg
« Reply #5 on: September 13, 2016, 07:17:39 pm »
+1
i will be using the same configuration in a future project. i am basically a noob in smps but it wasn't hard to achieve good-ish performance, maybe with poor efficiency but a rather clean signal

blueskull might correct me but i wouldn't go crazy high in switching frequency as LDO PSSR usually become rather poor over 10-20 kHz

You need very high switching frequency to shift noise band to high enough that inductor and beads can handle them easily.
<50k is usually handled by LDO, >100k can be easily handled by small inductors, >100M harmonics can be easily handled by feed caps or beads.
What you need to worry about is gap between LDO PSRR cutoff frequency and inductor cutoff frequency, as well as between inductor's SRF and bead's cutoff frequency.
 


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