# Help with 5v power supply

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#### ElectronsFlow

##### Member
Hey Spec, Could I bug you with a problem I'm working on? I've got a very low current 5vdc power source, It's approx 20~30ma only!!! Now here's the catch, I need to switch a relay from that, I can use any subminiature DPDT form factor relay (though preferably not over 12vdc on the coil) I figured the only chance I've got to run the relay from such a low current power source is with an optocoupler/BCXXX NPN transistor combo? I've used them in the past but I'll be the first to admit that after browsing all the types of optocouplers I feel rather overwhelmed as to my best option. An optocoupler through mount PCB style would be ideal, but it's not essential. Any suggestions? TIA

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#### ChrisP58

##### Well-Known Member
Hey Spec, Could I bug you with a problem I'm working on? (unrelated to this project) I've got a very low current 5vdc power source, It's approx 20~30ma only!!! Now here's the catch, I need to switch a relay from that, I can use any subminiature DPDT form factor relay (though preferably not over 12vdc on the coil) I figured the only chance I've got to run the relay from such a low current power source is with an optocoupler/BCXXX NPN transistor combo? I've used them in the past but I'll be the first to admit that after browsing all the types of optocouplers I feel rather overwhelmed as to my best option. An optocoupler through mount PCB style would be ideal, but it's not essential. Any suggestions? TIA
Is the 5V, 20-30mA power source the only supply available for the relay coil? If so, then the only real choice is a 5v coil relay that needs less than 20mA to energize. All of these fit that requirement.
https://www.digikey.com/product-sea...=0&page=1&quantity=0&ptm=0&fid=0&pageSize=500

#### ElectronsFlow

##### Member
Sorry I should have given more info (came home pretty burnt out yesterday) The 5v power supply is from an alarm output, basically when the 5v is grounded and therefore pulled down to 0 volts that is 'normal' If the loop is broken (e.g by a door switch) the alarm goes off. When I tried connecting a relay coil across the 5v power supply the alarm wouldnt go off due to the loop technically not being broken. I chucked a garden variety led on the loop and it seemed to work so I figured my best bet was an optocoupler? the opto needs to draw as little current as possible to leave a floating voltage behind for the alarm to detect. if the whole 20ma was used for instance that could lead to the alarm not reliably detecting a change of state

Is the 5V, 20-30mA power source the only supply available

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#### ElectronsFlow

##### Member
I've got a couple 6N138 photodarlington opto's, I know they are used to amplify very weak signal. But my knowlage isn't great enough to know if they will do the trick or perhaps the best way to utilize them for my application?

#### ChrisP58

##### Well-Known Member
Unless you've got some other source of power for the relay coil, even an opto may not help.

Taking a step or two back to get the bigger picture, what are you trying to do?

-Chris

#### ElectronsFlow

##### Member
Hey Chris, Many thanks for your time

Taking a step or two back to get the bigger picture, what are you trying to do?
It's complicated to say the least, but I'll try my best to simplify. There is an existing alarm system I'm trying to piggyback off of (this saves the client many \$) My own alarm system has several more advanced features I've developed. The idea would be to combine both the old with the new alarm system. Basically my own alarm has a bank of relays (can be either 5 or 12v) and yes they do need to be relays and not semiconductors for more complex reasons I won't go into) Now normally this wouldn't be a problem, I'd wire in any pre existing alarm inputs to one of my relays for my alarm to interface with. However due to the aforementioned uniqueness of this 5v very low current pre existing alarm my usual approach does not work. I might be on the wrong track? but I figured the most convenient solution might be an optocoupler? But if you have any suggestions I'm open

#### ChrisP58

##### Well-Known Member
OK. So your new system has it's own power source and you're just trying to get a status signal from the old one.

That can be done with an optocoupler.

If the current limit on the existing alarm sense loop is in the 0.5 to 20mA range, I would suggest inserting the led of the opto into that loop, rather than in parallel across the loop head terminals. If you do put it in parallel, you'll need a current limiting resistor.

#### JonSea

##### Well-Known Member
Why not use the 5v / 0v signal to control a transistor to switch the relay? If the two systems can share a common ground, this will be the easiest way to do it.

#### ElectronsFlow

##### Member
If the two systems can share a common ground
That's the problem, they don't share the same power source.

If you do put it in parallel, you'll need a current limiting resistor.
It's going to be parallel so the resistor will be required.

Is there any particular opto I should go for? I don't know if I should choose a photo transistor or darlington type?

##### Well-Known Member
Why not use the 5v / 0v signal to control a transistor to switch the relay? If the two systems can share a common ground, this will be the easiest way to do it.
That would be my thinking. Yes, you could use an opto-coupler but a simple 2N2222 transistor will drive a pretty beefy relay with a 12 volt coil. You do not mention what the relay needs to handle but there are no shortage of relays which could be easily driven.

Ron

#### ElectronsFlow

##### Member
That would be my thinking. Yes, you could use an opto-coupler but a simple 2N2222 transistor will drive a pretty beefy relay with a 12 volt coil. You do not mention what the relay needs to handle but there are no shortage of relays which could be easily driven.
Hi Ron, The problem is the 2 alarms don't share the same power source. So a simple transistor won't work sadly

##### Well-Known Member
Yeah, I caught that later. Oh well back to the opto coupler if the twopower supplies can't share the common ground.

Ron

#### JonSea

##### Well-Known Member
I suspect you can tie the low (common) sides of each system together without any problems.

#### ElectronsFlow

##### Member
I'm going to try a PC817 with a NPN transistor. Who knows how well it will work on such a low amount of current. I can always pull out the hammer if I get into trouble

#### crutschow

##### Well-Known Member
I'm going to try a PC817 with a NPN transistor. Who knows how well it will work on such a low amount of current.
A transistor can switch a collector current 10 to 20 times the base current.

#### ElectronsFlow

##### Member
I've put together a circuit with an optocoupler that can draw very little current, in the order of 1.5mA. Yet to test it on the alarm (only bench tests thus far) but in theory it should work