# reed switch

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#### SteveC69

##### New Member
I'm trying to use a reed switch with a 12 volt 2.6 amp lithium battery. The only problem is the reed switch won't handle the amperage. Is there an easy way to step it down with a relay or something? This is for a scuba light, so size is critical also.

Thanks,
Steve

#### ericgibbs

##### Well-Known Member
hi Steve,
If you limit the current in some way, the light will be dimmer.
What is the wattage of the scuba lamp.?
E

#### MrAl

##### Well-Known Member
Hi,

I am not sure i would trust a reed switch for mission critical applications like diving or caving. I've seen too many fail in the past dispite reading fairly good reliability. There's also the temperature change which may effect operation.

Maybe a hall effect device would be better? I assume you want to use a reed switch because it can be activated magnetically.

#### Tony Stewart

##### Well-Known Member
Automotive markets have accelerated the demand for robust smart electronic power switches making Relays obsolete except for the most difficult loads with motors or lamps which have 8-10x surge currents.

LED Scuba Lamps come in a wide power range, and are well suited to high (+) or low side (-) switches. These come in over four thousands different varieties now with smart protection built in and options for dimming and sensing.

Here's search engine listing from Digikey.

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#### Nigel Goodwin

##### Super Moderator
Hi,

I am not sure i would trust a reed switch for mission critical applications like diving or caving. I've seen too many fail in the past dispite reading fairly good reliability. There's also the temperature change which may effect operation.

Maybe a hall effect device would be better? I assume you want to use a reed switch because it can be activated magnetically.
Reed switches are EXTREMELY reliable, I'd certainly much sooner use one of those than a hall effect device

Presumably he needs some kind of FET switch, and latching system - I presume it's for toggling a SCUBA light ON and OFF?.

#### Tony Stewart

##### Well-Known Member
Reeds & power relay contacts might be rated for 1 million switches mechanically, but this reduces logarithmically with rise rise in current max current ratings are often 0.25, 0.5 & 1A might only be rated for 10k cycles at max current, or unspecified.

I guess the built-in switch is not accessible or nonexistent.

Of course exceed ratings might reduce life expectancy to 100 operations or 1 hour at 10x the rated current

One rated for 2A are priced in the $50 range. So get a highside smart switch.They are smaller than miniature mechanical switches. Last edited: #### SteveC69 ##### New Member Thanks for all the replies. The bulb I'm using is a 7 watt MR16 led. I don't want to reduce the voltage to the bulb. only to the switch. I'm assuming there is some simple way to do this. The reed switch that I tested is rated for about .5 amps, and it seemed to fail instantly. It activated one time and then stayed on continuously. What is a hall effect device? #### kinarfi ##### Well-Known Member #### Externet ##### Active Member I have a few beefy 1/4" x 2" reed switches like the one used in my scuba 'scooter' but do not know the specifications, nor remember where bought. Digi-key shows ----> http://www.digikey.com/product-search/en/switches/magnetic-reed-switches/1114199?k=&pkeyword=&pv587=222&FV=2dc0dc5,fff40011,fff80057&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25 Select in-stock and the highest power they stock to try your luck. Smaller paralleled reed switches may work. A mosfet driven by a smaller reed would need some heat sinking if you chose that good way. A hall effect device senses magnetic field and can trigger a mosfet. ----> https://duckduckgo.com/?q=hall+effect+device&t=canonical&iax=1&ia=images&iai=http:%2/www.alselectro.com/images/AH34_hall_effect_sensor_thumb_zelal3t8.jpg They look like transistors and there is at least one in every 'muffin' fan waiting for a second salvaged life. To me, the hardest part to implement a reed switch for scuba use is not the reed switch; it is making/finding a magnet that slides captive in a plastic channel. Last edited: #### MrAl ##### Well-Known Member Most Helpful Member Thanks for all the replies. The bulb I'm using is a 7 watt MR16 led. I don't want to reduce the voltage to the bulb. only to the switch. I'm assuming there is some simple way to do this. The reed switch that I tested is rated for about .5 amps, and it seemed to fail instantly. It activated one time and then stayed on continuously. What is a hall effect device? Hi, That's what happened to some of mine too. Over the years i realized that the data for some of these switches does not match the real life experience at all. Maybe if the current is kept very low as Tony said it would work longer. A hall effect device is a semiconductor that reacts to a magnetic field. The one i was talking about would be a latching type, so when the magnet comes near it turns on. The current for these things is usually low, so you use a transistor with it. The voltage usually has to be around 5v or more though, so you would have to provide more details on your particular light. Hall effect devices also require some power to operate. Because of the seriousness of the application it might also be a good idea to have a secondary method to turn the light on just in case the first method doesnt work, and of course carry a second light just in case. Last edited: #### MrAl ##### Well-Known Member Most Helpful Member Reed switches are EXTREMELY reliable, I'd certainly much sooner use one of those than a hall effect device Presumably he needs some kind of FET switch, and latching system - I presume it's for toggling a SCUBA light ON and OFF?. Hello there, Well i am basing my opinion on experience over about 20 years time, not some paper that the manufacturer wrote up. In that time i have seen maybe 10 reed switches fail for apparently no reason. The typical setting is they get stuck closed. I have been able to 'flick' them with my forefinger to get them to let loose and then they might work again for a while. I have never had a hall effect device go bad except for one where i applied too much pressure to the plastic case and then it did not work right after that, although it would still sense and provide an output. A couple specific cases i remember well for reed switches: I even got one bad that was never used. I ordered 6 units and 1 was stuck closed already. I used one for a refrigerator sensor to sense the open door. The first one i used died in about a month. Current was about 10ma. the LED was a 5mm white Nichia type, with 3 AAA batteries to run when the door was open. The second one however lasted for more than 2 years now and is still going. Some applications MUST use reed switches because they have to have low 'on' resistance or can not use any power when the device is 'off'. Offhand i can think of one which would be range switching. It would be much harder to use a HED for that. So in short, i can only suspect that the quality of the reed switches come into question. Some are probably better than others. I've even seen plastic types and i stay away from those. Also as Tony said, the reliability is strongly related to the current. PS. I have some really tiny ones, about 1/4 inch long (not including the leads)! I havent used them for anything yet though so i have no data to contribute yet. #### Tony Stewart ##### Well-Known Member Most Helpful Member Most MR16's are 12Vdc and MR16 LED's usually have DC-DC regulators or very small series R's and a diode bridge. Since the current at 7W/12V =580mA plus at least 1uF capacitance for the constant current DC-DC regulator, this is what I think happened. The 1uF cap will have an internal ESR of about 200 mOhm for general purpose and thus the surge current in the contacts. will be 12V/0.2Ω=60A or one hundred and twenty (120x) times the rated current and thus life span was reduced to one operation welded shut. A solution. Use Kinarfi's schematic on right side only, for an Nch MOSFET driver and tell us what kind of switch you plan to use so we. can check if it is rated for low current. (Carbon or Gold plated contacts, not silver). Then use this NMOS FET with thin AWG 24-28 magnet wire soldering to leads. http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00001945.pdf stock at Digi-Key$1

Since it is rated for 5A you only need to use 3 of the 8 pins.
RdsOn=0.05Ω, so better than a relay

Any questions?

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#### kinarfi

##### Well-Known Member
I drive an off road vehicle (see avatar) a lot and I used to gas it via 5 gal jerry cans. Being crippled made it a chore, so I took an in tank fuel pump and made a setup to fill my Trooper, only problem was / is it takes longer and I had to sit and watch it so it didn't over fill, so I took the gas tank vent hose that goes to the carbon canister and inserted a 1/2 " x 4" brass tube with a float & magnet inside it, the hose attachment at the top come out sideways and the top of the tube is sealed with a piece of copper sheet soldered to it, and the HED is glued to that. Works great!!
The problems I have found with reed switches it that they seem to magnetize and stay closed or the contacts weld or they eventually break from the flexing action. If you use a Hall Effect Device, you have to do something to make sure the flux passes the the HED in the correct direction, that's why I sealed the brass tube on the top end with a copper sheet, which made a good place to mount the HED and the magnet is on the top of the float and I had to test it to make sure I had the correct side facing up. Here's the circuit I used, but I used a pump instead of a lamp.

#### JonSea

##### Well-Known Member
Rather than tell you what you want to do won't work, or what you should do instead, how about we explain how to do what you want to do?

Like others have stated, a reed switch can't handle large currents; they are meant to switch currents in the few mA range. I suspect you want to use a reed switch because it's waterproof, an important consideration in a dive light.

What a reed switch can do is switch a low current to operate a relay, particularly if it has a transistor to drive it. A few mA can control several amps to the bulb this way. I borrowed this diagram from Sparkfun and made a slight addition showing the reed switch.

If the dive light has room, single relay boards including the driver transistor are available on ebay for a few bucks or a smaller version could be made without much effort.

#### alec_t

##### Well-Known Member
In that time i have seen maybe 10 reed switches fail for apparently no reason. The typical setting is they get stuck closed.
I've also had a reed switch stick closed, even though it was switching only ~1mA through a resistive load. I attributed the failure to permanent magnetisation of the reeds, since the switch was sensing rotations of a wheel-mounted magnet.

#### MrAl

##### Well-Known Member
I've also had a reed switch stick closed, even though it was switching only ~1mA through a resistive load. I attributed the failure to permanent magnetisation of the reeds, since the switch was sensing rotations of a wheel-mounted magnet.
Hi again alec,

Yeah i was thinking something like that too but did not investigate any further.

I've never seen such as wide difference between published data and real life experience as with reed switches. I can only guess that this happens because of management and/or due to some older designs and *maybe* if we are lucky the newer models will behave more like they should. In other words, if we read a modern data sheet we get the impression that they are reliable, but that could be because the modern ones are but the older designs are not. That's just a guess though, and it's too bad that we cant always trust management to provide accurate data sometimes. Promoting the product is part of the company's job.

I have also seen one that if i looked inside the clear glass package i could see the two prongs come together when the magnet was brought near, yet there was no electrical contact made based on a Ohm meter reading.

Reed switches are used in some good meters to switch ranges, so they cant always go bad i guess, so i have no exact way to explain the difference between published data and real life experience with these things, only guesses.

Another guess is the sealed glass package starts to leak and degrades the metals inside there. This can happen with IC pin seals too, so maybe it has something to do with the way the reed switch is soldered: too much heat damages the seal. Again, just a guess though, and there could be multiple reasons why we see this problem crop up.

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