# How much power does my relay need to turn on.

#### Somarl

##### New Member
Hi all! I am currently building a simple setup off a 12v system (its for the car) that will turn some lights on when drawers are open. For this i have decided to use magnetic switches but the current is far too low to make the leds anything but very very dim. Now thats where a relay should come in. I have purchased a simple 4 pin 12v 20amp relay which should work for the 4 or so leds that will be running from this system. However the magnetic reed switch does not seem to be giving the relay enough power to turn on. I thought the whole point of a relay was for very little current to turn on the main power. If i got a magnetic switch that allowed more power through it then I wouldnt need a relay.

I am unable to test for certain things as I do not know how to use a multimeter properly (I know only very very basics of this stuff) but I have tested the orientation of the relay and the magnetic switch (N/O vs N/C) and they are correct (otherwise the opposite of what I wanted would happen but it would at least still happen). I have tested the magnetic switch separate and it works great, and I have also tested the relay on a separate normal on off switch and it too works great. So I know its a) wired correctly and b) nothing is broken.

How can I test how many amps switch the relay on? How can I test how many amps the magnetic reed switch is allowing through? And how do i find a reed switch that will allow enough through to power the relay?

Thanks.
Steve.

#### Visitor

##### Well-Known Member
You need a relay module like this. It has a transistor, so the small amount of power a reed switch can handle will be enough to operate the relay. Be sure to find a 12 volt version.

#### MacIntoshCZ

##### Active Member
Measure the resistance of windings. Then do math.
P = U*U/R
P = 12*12/Resistance

#### Somarl

##### New Member
Thank you for the replies. I will attempt to source a 12v version of the above board then. Do i search for "relay with transistor" for the right one?
As for measuring resistance, I would not know how to do that. When I said basics I meant very very basics. I have too little understanding of circuits to do the maths behind them just yet. So while some of the formulas are out there I don't know how to use devices that allow me to get the numbers to fill in the blanks I'm afraid.
But thank you all.

#### Pommie

##### Well-Known Member
OK, a modern LED needs about 10mA to be bright, why can't your "magnetic switch" supply that?

Mike.
BTW, what is a "magnetic switch"?

#### Somarl

##### New Member
I may have found one but I dont know if it is correct.

If this is what I am looking for then would it be adequate for multiple LEDs from different reed switches. I dont mind all LEDs coming on when only one drawer is open this does not bother me. But each drawer needs a reed switch to detect opening movement. Will all of these go together into this and work or does something else need to be added to ensure that any one of the switches will power all leds? Surely any one reed switch would provide power to trip this relay and then power everything up.

Does anyone know where I can find wiring diagrams for the sort of thing I am looking for, everything I search for seems to be bringing up diagrams with Arduinos or something unrelated.

Kind regards.

#### Somarl

##### New Member
OK, a modern LED needs about 10mA to be bright, why can't your "magnetic switch" supply that?

Mike.
BTW, what is a "magnetic switch"?
These are simple 12v LEDs and from the bench power supply it seems to only draw 1amp max and it is bright enough I guess, there are only small strips. What I meant by magnetic switch (you will have to forgive my namings as I am unfamiliar with correct terms) is one of those switches usually two wires into a magnet, like a reed and it has a separate magnet as a separate component that when it is near to the other part, the circuit it open, when it is pulled away the circuit is closed and anything linked comes on. They are found on some cupboard doors that lights come on when you open the door. However they carry very low amps and are usually unable to power an LED or multiple LEDs without something else like switching a better power source.

#### Somarl

##### New Member
On ebay, you can search for "relay module 12v".

Here's one listing that has them, a US seller.
Thank you, I will find a UK version of something like that. I am thinking I need a multiple channel one (one for every magnetic switch and one for every LED) and this may be the best solution perhaps?

#### sagor1

##### Active Member
A lot of ebay and amazon sellers list those relays as "Arduino Relay Module" or just "Arduino Relay". Search for those terms, you will find lots of them. Make sure they are 12V, and based on your description, you have to find one that has "active high" properties. Most of the Arduino relays are active "low", not high. There are several that have both high or low activation.
High activation means you supply +5V or +12V (depends on relay design) to the control pin to activate the relay. That may be an issue with 12V, you have to check. The active "low" relays (most common) are activated when you supply a ground to the control pin.

#### Somarl

##### New Member
A lot of ebay and amazon sellers list those relays as "Arduino Relay Module" or just "Arduino Relay". Search for those terms, you will find lots of them. Make sure they are 12V, and based on your description, you have to find one that has "active high" properties. Most of the Arduino relays are active "low", not high. There are several that have both high or low activation.
High activation means you supply +5V or +12V (depends on relay design) to the control pin to activate the relay. That may be an issue with 12V, you have to check. The active "low" relays (most common) are activated when you supply a ground to the control pin.
Ahh, ok thank you. I think then I have found something more suitable. I would still be unsure how to wire it correctly or even if it would power on with the magnetic switches very low power passthrough but still I think this is the one (it does seem to have the active high, or at least be capable of that).

#### Pommie

##### Well-Known Member
LEDs that draw 1A with 12V must be around 2W LEDs + a current limiting resistor. If you get LED's without a current limiting resistor then a reed switch should easily be able to turn them on. An LED with 50mA continuous forward current will be very bright. Put 3 in series with the right resistor will give you a very bright source. And, still only 50mA from the power supply.

Mike.

#### Somarl

##### New Member
LEDs that draw 1A with 12V must be around 2W LEDs + a current limiting resistor. If you get LED's without a current limiting resistor then a reed switch should easily be able to turn them on.
Mike.
They do come on with the reed/magnetic switch but they are so very dim, when I hook them up direct to the power supply (a fused distribution box hooked up to the 12v bench power supply), then they go as bright as they are needed. So thats why I was thinking the magnetic switch isnt letting enough power through but thought it would still trip a relay into coming on which I was wrong about. It seems I do not have the right relay so I will attempt to use the one I listed above. I just wonder if anyone has any wiring diagrams for something like that hooked up to a few LED's.

Im just guessing here but I think the 12v should come straight from the distribution box to the "DC+" part of that relay, the negative to the "DC-". Then I am thinking the magnetic switches all go to the different inputs labeled in1, in2 etc on that same bit of the board (so lets say 5 switches for 5 LED's in 5 drawers as an example). Then at the back of that unit for each one of the relays (up to five) I would send out a positive and negative to the appropriate LED and just make sure I use either NO or NC (I think i would need them on NO to work properly). I may draw up a dreadful paint based drawing to illustrate what I think I mean.

#### atferrari

##### Well-Known Member
To the OP:

Could you show a schematic or a block diagram of what you plan to build?

The title of your post seems basically not related with what you intend to discuss.

Somewhere in the OP you seem to describe the use of a reed? relay to activate another more powerful relay for the LEDs. Is that your idea?

Once you have it expressed by a reasonable block diagram you should go into details. Not even knowing how to use a DMM to measure DC current denotes that you should go for the basics first. How powerful those LEDs should actually be?

Ignoring this post is an option.

#### Somarl

##### New Member
To the OP:

Could you show a schematic or a block diagram of what you plan to build?
Im not all that great with drawings. It is basically a set of storage drawers in the back of a vehicle. When opening these drawers I would like to turn an LED (located inside the drawer). Similar to how some display cabinets have the "light on open" option. I have seen this done with magnetic switches and also push style switches, I wanted to go down the magnetic switch route.

The title of your post seems basically not related with what you intend to discuss.

Somewhere in the OP you seem to describe the use of a reed? relay to activate another more powerful relay for the LEDs. Is that your idea?
What I had come up with so far was a NO magnetic reed switch on the inside of a drawer that when the magnet would separate then the LED would come on like an automatic switch. Now when I tried this the LED barely lit up, it was very dim compared to lighting it directly from a power supply or through a simple switch (like a carling switch). With this I then presumed that I needed a relay, something that takes a very small current to switch that would allow "full power" to the LED's. I.E use the reed switch to send current to the relay and the relay powers the LED's.

My mistake was that I believe the relay I bought seems to want more power to "switch it on". My original question I suppose was how much power do I need to turn my relay on, how would I find out how much mine needs so I could find a magnetic reed switch that would provide this power. Now it seems there are better options (better relays) to allow me to do what I originally intended. So now my question has turned to "how do I wire up the new relay that I found?".

I hope this makes things clearer, I apologise if it does not but I fear a drawing of my standards would probably confuse the matter further.

#### Pommie

##### Well-Known Member
I'm going to leave you to it, you seem to have enough help. Good luck with your project.

Mike.

#### alec_t

##### Well-Known Member
What I had come up with so far was a NO magnetic reed switch on the inside of a drawer that when the magnet would separate then the LED would come on like an automatic switch. Now when I tried this the LED barely lit up
Strange. Can you post a link to your reed switch or its specification? 1A for the LED seems excessive. A link to the LED spec would also be useful.

#### Somarl

##### New Member
Strange. Can you post a link to your reed switch or its specification?
Yes, it is this one.

#### Visitor

##### Well-Known Member
Yes, it is this one.

There are two troubling features for this switch:

1. It's rated for AC current. The DC rating may be much less.

2. This line of the description: "This micro switch has a built in 20k resistance which blocks the voltage and dims the supply. " I'm not sure what this means.

But for the moment, let's assume you can find a suitable NC (Normally Closed) switch, which means the switch is open when the magnet is near (meaning the door is closed).