Is there such a thing as a simple diode to restrict voltage flow?

Thread Starter

scott375

Joined Sep 2, 2022
3
Hi... I'm working on a pretty simple collection of off the shelf networking parts and without going into too much detail, what I'm looking for is a component (diode I'm pretty sure) to insert at a certain place to ensure the 24vdc doesn't flow in the wrong direction. I've been doing a fair amount of reading on the subject and I often find myself too deep into the conceptual weeds. I'm no electrical engineering wizard but I know enough to be dangerous. :)

As you can see from the napkin sketch, I don't want my 24vdc input power to flow back out of my LAN port through my POE injector (which has no diodes) because if someone plugs in a device it could be damaged by the unexpected voltage. That said, I DO want power to be fed into the LAN port via POE (Power Over Ethernet) so that it can make its way over to the CP (CradlePoint) device.

Is there enough information here for someone to point me to the type and size of diode that would best prevent backflow of the 24vdc as described?

Thanks!napkin sketch.jpg
 

Tonyr1084

Joined Sep 24, 2015
7,093
Welcome @scott375 to AAC.
The diode will prevent current from flowing back into the lan from the 12.6 V step-down.

1662126692218.png

Just for edification, voltage doesn't flow, current does. And the diode blocks current in one direction but allows it to flow in the other. It flows in the direction of the arrow. Voltage is akin to "Electric Pressure", meaning it has a potential. 12 volts has a certain level of pressure whereas 240 volts has a much greater potential. Imagine hooking up a garden hose to an air line with 250 pounds of pressure with the end of the hose closed off. The hose will burst. Like the hose, anything not able to withstand the voltage will be damaged.
 

Thread Starter

scott375

Joined Sep 2, 2022
3
Thanks for the quick response Tony, and also the correction of my terminology - much appreciated!

You said the diode with prevent the current from flowing back from the 12.6v step down but it wouldn't be coming from the step down - it would be coming from the 24 DC In port. Not sure if that matters to your answer but though I'd mention it in case it did.

So I do need a diode then but the question is how to find the right one. Finding a single component on Digikey is like trying to find the right bolt on McMaster-Carr. I've done some googling and have determined that I need to know what the peak inverse voltage is and what the max forward current is.

Peak voltage anywhere in this system is going to be 24V and both the CradlePoint and the UPS say they shouldn't draw more than 2A of current. Given that - how do I find what I need from all the different varieties available on x number of websites? I'm also a little leery of just soldering in a component like this to to wires. Seems like there should be some support to keep things from moving/breaking over time.

Thanks again.
 

Tonyr1084

Joined Sep 24, 2015
7,093
1N4002 would be sufficient.

Here's a cut-n-paste of a partial data sheet for the 1N4000 series diodes.
1662129060656.png
The 4001 should handle the job but since it's fairly close to the blocked voltage I would recommend the 4002. The cost is the same but the protection is more than double. As you can see the 03 through the 07 have reverse voltage protections from 140 volts (reverse) all the way up to 700 volts (reverse). Me? If I had any one of those laying around I'd use them. Cost is virtually the same across the board, and if I had an 07 I'd go ahead and use it. If I had an 04 and an 07 I'd use the smaller just because at some point I might need a higher voltage protection at some time in the future. But if all I had was an 01, I'd try it, probably with success. But if you're buying, get the 1N4002. Won't cost you anything more.
 

DickCappels

Joined Aug 21, 2008
9,317
Normally I would go with a 1N400X device but @scott375 said that the load will draw less than 2 amps. If it is really less than 1A then a 1N400X device would be fine. For higher currents a 1N540X device (3.0 amps) might be a better choice. The 1N540X devices are more expensive but it he only needs one a few extra cents probably won't break the bank.

1662130429250.png
 

Tonyr1084

Joined Sep 24, 2015
7,093
Normally I would go with a 1N400X device but @scott375 said that the load will draw less than 2 amps. If it is really less than 1A then a 1N400X device would be fine. For higher currents a 1N540X device (3.0 amps) might be a better choice. The 1N540X devices are more expensive but it he only needs one a few extra cents probably won't break the bank.

View attachment 275426
Agree. But it appears the desire is to block unwanted current from back feeding into the LAN connection. Does a LAN power source run at 3 amps? (not a hijacking).

Often I see newly indoctrinated (to electronics) people look at the power supply, which may be rated UP TO 2 amps and assume the circuit draws that much current when often it draws a lot less.

Nevertheless, the 1N5400 series is another good option. Likely a better option. D. C. often has excellent advice. And when it's not "excellent advice" it's "good advice".
 

crutschow

Joined Mar 14, 2008
30,788
You might consider a 3A Schottky diode since they have a lower forward drop when conducting, which also means less dissipation and heat.
 

Thread Starter

scott375

Joined Sep 2, 2022
3
I have two possible power inputs to this system and they (should) never be in place simultaneously:

1) POE into the LAN port. This won't ever exceed 1A so a 2A limit on a diode in this system should be fine.
2) 24vdc from a standard "plug into the wall" power supply. It's designed to power the CradlePoint and max amperage on that power supply is 1.5A.

I walked down to the BioMed area here and found a handful of 1N4005-TP diodes. Seems like overkill since they're rated at 600V/1A but they'd probably still work, right? Would it be better to pick up a 1N540X (is the 'X' a variable here?) or 1N4002?
 

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panic mode

Joined Oct 10, 2011
2,301
Schottky diodes have smaller forward voltage drop which is useful at low supply (5V etc.). But they also have lover breakdown voltage so check the datasheet.
1N400x is generic description for 1N4001, 1N4002, 1N4003 etc. (x is place holder for last digit 1-7)
1N540x is generic description for 1N5401, 1N5402, 1N4503 etc.

You can always use one that is rated for higher voltage in place in place of lower rated 'sibling'. People often got for higher one since the price is often same or very similar so there is no need to stock different parts.
 

Tonyr1084

Joined Sep 24, 2015
7,093
Schottky diodes have smaller forward voltage drop which is useful at low supply (5V etc.).
I don't think this point has been mentioned before. Forward voltage (Vf) is the voltage that is lost when current passes through the diode. Suppose you have a system that runs on 5V and it's critical that you stay real close to that voltage. A common diode such as the 1n400x and 1n540x series is typically 0.6Vf At 5V that can be a significant drop in voltage. However, at 24V if you're using the 400 or 540 series diodes, your likely going to see at least 23.4V. If that voltage is not super critical then standard diodes should work. However, if your system needs a minimum of (just making this up) 23.8 volts then a standard diode might be a problem. Most 24V power supplies I've seen have a pretty big tolerance and typically lean toward overkill. I wouldn't be surprised if a 24V PS runs close to 28V with no load. When loaded it will come down, depending on how big the load is. Do you have a 24V supply now? If so - what is its rated output amperage? It's my guess you won't have an issue. HOWEVER: Do you want to build something based on "My Guess"? This is why the numbers are important.
 
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