Author Topic: Using RF connectors as power connector - experince?  (Read 1076 times)

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Offline martin1454Topic starter

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Using RF connectors as power connector - experince?
« on: January 16, 2023, 01:27:05 pm »
Hello everyone!

I'm in the middle of a project where I need an compact 'cheap' IP67 rated connector to mount in the back of an small chassis.
The project is just running a small micro controller with a few sensors, and the plan is to use 12V with a buck converter, so I will only draw up to around 100mA.

Looking at "compact" waterproof connectors that can be panel mounted, I realize most of them a quite expensive - A norocomp M5 connector in IP67 easily runs around $7-11 for the panel mounted connector and another $15-20 for the cable with connector. It makes it even more complicated since I need to extend this connector, so cutting up a cable also looks quite bad.

But I was looking around, and found that some RF connectors are available for a decent price, IP67 rating and compact size (SMA or SMB for instance)

What are your thoughts on using an RF connector for running 12V 100mA? 
 

Offline martin1454Topic starter

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Re: Using RF connectors as power connector - experince?
« Reply #1 on: January 16, 2023, 03:43:37 pm »
BNC is a bit too large, I'm in the kind of "small is better" situation. My dream scenario would be something like a JST PH2 connector that would be water proof and chassis mounted receptacle. But since it is not possible to get in IP67, then a small RF connector might be easier to get.
 

Offline martin1454Topic starter

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Re: Using RF connectors as power connector - experince?
« Reply #2 on: January 16, 2023, 04:21:37 pm »
What I think you are forgetting here is voltage drop of the wire required over the distance that you want.

Seeing as you haven't said the distance required then I am erring on the side of caution and recommending to you a larger connector. Running 12-24v DC over F on coax is tried and tested for a long time now.

JST PH only accepts a small gauge of wire so therefore your distance is limited before voltage drop sets in. Its not a connector that I would be using for a long run at 12 volts and the wire it uses is not rated for outdoor use in the UV bathed sunlight. Either up your voltage beyond 12v and use a stepdown DC-DC converter inside of your project box or use a larger connector. You might get away with using SMA/SMB with a quality coax that is rated for outdoor use for 12v at 100mA but it all depends on your distance from the power source. The connector determines your wire gauge that you can use.

It would be very useful if you could tell us how many meters your sensor microcontroller is away from its power source. Also don't forget to factor in the power draw of your sensors, some gas sensors can draw a large amount of power.

Upping the voltage will let you use a smaller connector. 24v DC would be decent. Then you can step it down to whatever voltage your microcontroller needs inside of the project box.

Remember that 12v at 100mA at the power source will not be 12v anymore at the end if its 20 meters of wire. So you need to compensate for that by either upping your voltage or using a thicker wire.

25% voltage drop for 20 meters of RG316 using an SMA/SMB connector:

Ah, yes sorry for not clarifying - The 12V is fed into a buck convter taking 12V to 3.3V, so it doesn't really matter if it is 7V-10V-12V etc....
 

Offline martin1454Topic starter

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Re: Using RF connectors as power connector - experince?
« Reply #3 on: January 16, 2023, 04:29:05 pm »
I don't have the schematic with me right now, but it is a TI buck converter that goes up to 28V, so close ;)
 

Online langwadt

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Re: Using RF connectors as power connector - experince?
« Reply #4 on: January 16, 2023, 04:39:51 pm »
100mA and 12V should not be an issue at all, the datasheets I've seen for SMA list something like few mOhm and power handling any where from 50 to several hundred watt into 50R
 

Offline tszaboo

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Re: Using RF connectors as power connector - experince?
« Reply #5 on: January 16, 2023, 04:41:28 pm »
Most active GPS antennas will send power down the coax cable. But it's nowhere near 100mA, it's more like 10mA. And there could be quite a voltage drop on the cable it's high DC resistance as others said. It will probably work, you might need to up the voltage or work a bit on the power consumption. If you terminate it wit 50Ohm, just make sure it's AC terminated and not  DC.
 

Offline martin1454Topic starter

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Re: Using RF connectors as power connector - experince?
« Reply #6 on: January 16, 2023, 04:55:23 pm »
I don't think we are going to get much smaller than a waterproof SMA connector:
https://www.gradconn.com/Products/RFConnector/Waterproof-SMA
https://www.gradconn.com/News/Looking-for-a-Waterproof-SMA-Jack-Connector
You would also need to limit the amount of power that your power source delivers to the coax as it leaves the psu. You could easily fuse a piece of thin coax if you don't current limit it to around 100-200mA.

Definitely! luckily, I'm a bit more flexible at the power souce than in this end! Thank you for the suggestions from Gradconn - They look promising.
 
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