# Reducing 110VAC to power a very small coil on a relay

#### mjscott

Joined May 21, 2014
24
Hello,
I'm trying to design a very simple circuit where I can switch a 110VAC 10A load with a reed switch (max switching current 0.5A). I will need a relay to switch the 110V, but what I'm trying to determine is if I can avoid using a transformer to power the relay coil. I'd like to use something small and simple (resistor) to lower the already present 110V to a voltage small enough to power the coil and not fry the reed switch. The load of the coil will also be very small, so I'm guessing there wouldn't be much heat dissipation from the resistor.
This is a rookie guess at a solution! If I'm on the right track, I don't know how to calculate the necessary resistor size for whatever my coil specs are (I haven't bought a relay yet - still trying to discover if I'm on the right track).

With thanks
Mike

#### Ya’akov

Joined Jan 27, 2019
8,510
Your description is confusing. Are you saying you want to switch a 10A with a .5A reed switch?

#### mjscott

Joined May 21, 2014
24
Yes, that is exactly what I am saying.

#### Ya’akov

Joined Jan 27, 2019
8,510
I can’t follow the meaning. You are saying the maximum capability of the relay is .5A but you want to switch 10A through it?

Something is missing.

#### ErnieM

Joined Apr 24, 2011
8,362
If your reed relay coil needs 0.5 amps and you use a resistor in series you need to dispose of some 55 watts (110V * 0.5A) of power. That's kinda warm.

#### MisterBill2

Joined Jan 23, 2018
16,550
It seems that the TS has a very small reed relay and wants to use the 0.5 amp rated contacts to switch a n 1100 watt, ten amp load. The solution is not at all in using that tiny relay to directly control the load. The solution is to use that tiny relay to control the gate of an adequately rated triac device mounted on a suitable heat sink, and with proper noise filtering added. So there is a bit of additional hardware required.
OR, the TS can get one of those very small 5 volt output wall wart supplies and get a CRYDOM, or equal, solid state relay with adequate ratings and a 5 volt control input. Use the small reed relay to control the solid state relay, it will work very well.

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#### BobTPH

Joined Jun 5, 2013
8,101
Where are you getting the 110V signal to switch the reed relay? Why can it not switch a 110V relay directly? Is it really that limited in power that you need two relays?

I cannot see how your scenario would ever make sense. The same signal should easily be able to switc a sold state 10A relay.

Bob

#### MisterBill2

Joined Jan 23, 2018
16,550
Hello,
I'm trying to design a very simple circuit where I can switch a 110VAC 10A load with a reed switch (max switching current 0.5A). I will need a relay to switch the 110V, but what I'm trying to determine is if I can avoid using a transformer to power the relay coil. I'd like to use something small and simple (resistor) to lower the already present 110V to a voltage small enough to power the coil and not fry the reed switch. The load of the coil will also be very small, so I'm guessing there wouldn't be much heat dissipation from the resistor.
This is a rookie guess at a solution! If I'm on the right track, I don't know how to calculate the necessary resistor size for whatever my coil specs are (I haven't bought a relay yet - still trying to discover if I'm on the right track).

With thanks
Mike
You do not give us the reed relay coil resistance or the coil current, or even the required coil voltage. So with none of the required information I can not even make a bad guess. I did post approaches that will work, but you do not even know how to power the reed relay.

IF you use the reed relay to switch the ten amp 1100 watt load, be prepared to put out the fire that you will get when the reed relay engages.

#### sghioto

Joined Dec 31, 2017
5,095
Read post #1 again. The TS wants to activate a low voltage relay to switch 110vac to a 10 amp load. This relay coil will be activated by a reed switch rated at 0.5A
Wants to use the 110vac to power said relay without using a step down transformer.

#### DickCappels

Joined Aug 21, 2008
10,104
The interposed triac sounds like the to go, band that is the thread-starter’s decision. The load (Inductive/capacitive/resistive) needs to be defined.

#### MisterBill2

Joined Jan 23, 2018
16,550
The goal was clarified in posts # and #3. I provided an answer that will work in post #6
The only information about the reed switch, now I se it is not the relay, is the contact rating. I provided the answers in post #6: Use either a triac circuit OR a suitable solid state relay. The brand mentioned is available with a number of different operating voltages. I also suggested a good alternative to a transformer, using a switcher type wall wart supply. A direct mains connection will be unsafe for quite a few folks to work with.
AND this whole site has rules about unsafe circuits.

#### Ya’akov

Joined Jan 27, 2019
8,510
Looking back, I can see what I missed in the TS' wording. For some reason he wants to use mains to have a reed switch (door switch?) to operate a relay with a 10A, 125V load on it. OK.

The answer to his question, if not his problem, is to use a suitable, isolated SMPS like these. They are very small (1"x.5"x.5") and appear to be properly built if not beautiful.

I know he wants something like a single resistor (without a transformer) but that it unworkable. Other, non-isolated solutions are dangerous, so we are left with this. It is pretty small, and it should operate the relay coil without damaging the reed switch.

Just be sure there is a flyback diode on the relay coil if you want you reed switch to have a long, happy life.

#### MisterBill2

Joined Jan 23, 2018
16,550
Looking back for a third time, it is not a reed relay, only a reed switch. So my suggestion for using a solid state relay of suitable rating is still workable. And not even a small supply. Use three AA cells to provide 4.5 volts, cose enough to the required 5 volt input for the CRYDOM SSR module, rated 15 amps and 50 volts. The reed wil only be switching about 25 milliamps, far below the rated max, and a dry load so no protection required. Just not very cheap.

#### AnalogKid

Joined Aug 1, 2013
10,785
I'm trying to design a very simple circuit
As you can see from the thread so far, we have no clear idea of what you are asking. Please post a simple wiring diagram of what you want to achieve.

Post a photo / part number / vendor site for the "reed switch", because this could be several different things and it affects the responses. For example, is the "reed switch" a magnetic reed switch like the kind used in security systems on doors and windows? If yes, that is a problem. That type of switch usually is not rated to handle 110 V (AC or DC). Also, it cannot handle the contact arc created by switching an inductive load like a relay coil.

ak

Joined Jan 15, 2015
7,256
Hello,
I'm trying to design a very simple circuit where I can switch a 110VAC 10A load with a reed switch (max switching current 0.5A). I will need a relay to switch the 110V, but what I'm trying to determine is if I can avoid using a transformer to power the relay coil. I'd like to use something small and simple (resistor) to lower the already present 110V to a voltage small enough to power the coil and not fry the reed switch. The load of the coil will also be very small, so I'm guessing there wouldn't be much heat dissipation from the resistor.
This is a rookie guess at a solution! If I'm on the right track, I don't know how to calculate the necessary resistor size for whatever my coil specs are (I haven't bought a relay yet - still trying to discover if I'm on the right track).

With thanks
Mike
This is a typical Reed Switch Note the data sheet specifications as to current handling capability.
This is a typical Reed Relay The reed relay includes a coil and is named a reed relay.

They are not the same. Since you mention Reed Switch those wanting to help will need a data sheet similar to what I linked to.

Also since you mention a 110 VAC 10 Amp load you will need switching at least 20% greater than your max load. Using a simple reed switch magnetically turned On/Off I would apply a low voltage like maybe 12 VDC to your reed switch. I would use an AC SSR with a 3 to 32 volt DC control voltage. Let the reed relay drive the SSR. I can only guess you want to maybe use a low voltage DC to switch a 110 Volt 10 Amp AC load.

Consider also if you were to use a reed relay that if you apply AC to a Reed Relay coil it will be switching at the AC frequency applied.

Typical inexpensive AC SSR using a low DC control voltage. I can't speak for the quality of these imported SSRs, personally I used brands like Omron and other more expensive versions. In closing this will not be as simple as using a few resistors.

Ron

#### seanstevens

Joined Sep 22, 2009
231
I may be wrong, but I am guessing the reed switch will drive the relay which can switch the load. If that is the case. I would suggest you do some searches for "capacitor drop power supply". BUT be very very careful, these circuits are live non-isolated mains supply. Meaning they can be very dangerous if they are not designed and deployed properly. In short, used incorrectly, that can explode, catch fire, or electrocute you - BE AWARe!!

#### ThePanMan

Joined Mar 13, 2020
707
I, too, am not seeing a clear picture. You have a reed switch OR you have a reed style relay. If the reed style relay is designed for 110 VAC then you don't need to drop any voltage or current. It's designed to handle 110 VAC. If the reed style relay is designed to operate on 24 VAC then you need a 5:1 drop in voltage. That is the mains power is five times the relay coil rating. 110VAC ÷ 5 = 22VAC. That's enough to operate the 24V coil.

IF you have a reed switch, one that takes a magnet to close it - or a magnetic field - then it doesn't have a voltage dependency. HOWEVER, its contacts are rated for a specific limited voltage, AC or DC specific, and a specific amount of current it can conduct. The leaves inside a reed switch are quite small and definitely not going to be able to handle 10 amps of anything. If your goal is to use that to activate a larger relay, one that IS designed for the voltage and current AND the control relay (the big one) is rated to operate on a specific voltage then your reed switch has to be rated to match or exceed the relay coil rating.

As for what the big relay will handle, that, too, will have to be considered. If the relay contacts are rated for 110VAC at 10 amps then you're on the raged edge of being over the limit the contacts can handle.

Before anyone can give you a good answer we need a good description of everything it is you're trying to accomplish. Short of that - pictures will help a lot. Part numbers, if you have them, will also help. But from what I've read so far - I don't see a clear answer for you. And voltage dropping caps on mains is not a good way to approach the problem. It's dangerous, which is why it's somewhat regarded as a forbidden topic on AAC.

#### eetech00

Joined Jun 8, 2013
3,648
Hello,
I'm trying to design a very simple circuit where I can switch a 110VAC 10A load with a reed switch (max switching current 0.5A). I will need a relay to switch the 110V, but what I'm trying to determine is if I can avoid using a transformer to power the relay coil. I'd like to use something small and simple (resistor) to lower the already present 110V to a voltage small enough to power the coil and not fry the reed switch. The load of the coil will also be very small, so I'm guessing there wouldn't be much heat dissipation from the resistor.
This is a rookie guess at a solution! If I'm on the right track, I don't know how to calculate the necessary resistor size for whatever my coil specs are (I haven't bought a relay yet - still trying to discover if I'm on the right track).

With thanks
Mike
A recommendation cannot be provided to you without further technical information because it could lead to an UNSAFE DESIGN. Please provide part numbers and/or datasheets for the reed switch and a simple wiring diagram showing your desired design.

#### ericgibbs

Joined Jan 29, 2010
18,222
Hi,
Clip from AAC T&c;s

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#### ThePanMan

Joined Mar 13, 2020
707
Does this mean Mains topics are no longer off limits or out of bounds?