How can a reed switch be used for high current applications? - Page 1
Forum section: electronics / beginners

author: Swapnil
author_rank: Contributor
timestamp: May 14, 2022, 12:32:14 pm
content: I am planning to use a reed switch as a power switch in my underwater robotics project. The switch will be placed between the LiPo battery (16.8V, 20.8Ah) and the load. The load mainly consists of two thrusters (30A each), and other low-power consuming electronics.Keeping the inductive loads in mind, how can the reed switch be used reliably for such high power operations without getting damaged?Is there another substitute that can be used in place of a reed switch for magnetically switching the power from batteries to the load?

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 14, 2022, 12:38:19 pm
content: What would activate the reed switch? I ask because I think you choose the reed switch because you need to be able to activate it. In that case I would investigate another type of switch for that. I found reed switch is very unreliable and I don't know of any that can handle 30A.

author: Gyro
author_rank: Super Contributor
timestamp: May 14, 2022, 12:43:46 pm
content: No, I don't know of any reed switches capable of carrying or switching 30A. Your best bet is probably to use the reed switch to drive a relay, maybe via a transistor buffer to reduce the reed switch current even further. For that sort of voltage and current range an automotive relay is probably the best bet.You may want to use two series reed switches as a failsafe.EDIT: I'm assuming that you're using the reed switch as a waterproof switching method with an external magnet.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 14, 2022, 01:56:26 pm
content: Reed relays are designed for small signal switching- originally for telephone systems.I'd use the reed relay to switch a pair of 60A automotive relays. They are available woldwide from any auto repair shop and they're low cost.What ever you do, dont take the AC contact rating to be remotley the same as the DC rating of any relay. A 30A AC relay will be destroyed by 30A DC.See the difference?Unless you spend a small fortune, a solid state switch will have higher power loss and voltage drop than a relay at these currents.

author: Brumby
author_rank: Unknown
timestamp: May 14, 2022, 03:27:40 pm
content: Quote from: Gyro on May 14, 2022, 12:43:46 pmI will go a step further and askWHYhave you specified a reed switch? A little explanation of what you're trying to achieve could help us identify some more appropriate suggestions.Also, I will add my voice to the others when they say reed switches are not known for high current applications - which this is (in general electronics terms).

author: SmallCog
author_rank: Frequent Contributor
timestamp: May 16, 2022, 02:42:08 am
content: I expect it is being used for water proofing reasons.In submersible sensing, or heavily ruggedised stuff that isn't intended to be submerged but would probably survive it, it's not uncommon for the on/off switch to be operated by a magnet.Are you intending to leave the magnet attached whilst it's running?The commercial devices I've seen like this tend to have a momentary action, the magnet turns the device on but doesn't need to stay there whilst the device is running. Not sure if the magnet is moving a physical switch, or it's a momentary magnetic sensor and is then latched as onInterested to see what you're trying to do

author: Swapnil
author_rank: Contributor
timestamp: May 16, 2022, 05:04:24 am
content: Quote from: BeBuLamar on May 14, 2022, 12:38:19 pmI chose a reed switch as a waterproof switching method with an external magnet. But after going through its specifications, I found that the switch is unreliable and won't be able to handle 30A as its switching current is in some milliamps. So, is there any additional circuitry that I need to add to be able to use the reed switch for such a high-current application?

author: Berni
author_rank: Super Contributor
timestamp: May 16, 2022, 05:11:17 am
content: Yep reed switches are nicely waterproof.Like others have said just use the reed switch to power a relay, then have the relay switch those 30A. The relay coil will be using a fair bit of idle power, but i assume so will the rest of the submarine. If you want something more power efficient you will have to use a power MOSFET activated by the reed switch.

author: james_s
author_rank: Super Contributor
timestamp: May 16, 2022, 05:12:57 am
content: This is easy, use the reed switch to control a transistor, and use the transistor to control the coil of a high current relay.

author: Swapnil
author_rank: Contributor
timestamp: May 16, 2022, 06:22:37 am
content: Quote from: Gyro on May 14, 2022, 12:43:46 pmHey, thank you for the input. Yes, the reed switch's switching current is in a few milliamps that's the reason why I was worried about its reliability for high current applications where inductive loads are involved. Is using a reed switch to drive an automotive relay via a transistor buffer surely won't affect the switching current of the reed switch so that it could be used reliably for high current applications? Can I use a solid state relay instead of the automotive relay?

author: Ian.M
author_rank: Super Contributor
timestamp: May 16, 2022, 06:23:43 am
content: I suspect you were intending to use the reed switch to implement a safety lockout, to disconnect the motors when it is nessercery to handle the robot, without having to open the presumably hermetically sealed housing.However I have serious concerns about using a reed switch for this purpose as the simple solution of using it to switch voltage to the gate of a big MOSFET is inadvisable as any current leakage (e.g. due to surface condensation) to the gate terminal could cause the MOSFET to turn on, ignoring the reed switch. Using multiple reed switches in parallel to short gate to source to shut the MOSFET off when the lockout magnet is present would be more reliable, but I'd suggest duplicating the circuit (using the same lockout magnet position) with the MOSFETs in series, and even so, I'd want to open an access cover and physically disconnect power before working on propellers, any parts near them, or any other motorized mechanism.

author: pqass
author_rank: Super Contributor
timestamp: May 16, 2022, 06:46:13 am
content: Maybe two reed switches; one for on, another for off.See simulation:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKDAzxABZsUQUUXCoSoChhcFGh9UGFAgxdCzfAi7E8SqZCRh48bXyLawSGInAsASt179BtvsxlRuoqh6kIWAFxAjOLjhhKiDPCBhCbGIEPAwFDDA0PARiCRhFMCiUYj0uAUpICD0qABM6ADMAQwBXABsfFgBzEO48WhEKJNdIFgAnfwQJMMGJFDxOUTRegHdWwSpCIf4gqH6QPEIhXCFN7c1XVha9kGwDk5GPFmwMKhOFja3TyCEItYBnUbdF5a52w5APj6NToLDmQ1C-yWw3+s0c-F+djEaxuHVCWVafyob16pW4aX4E3h404fHK1XqjTmPCMRJpCPSLE+9JGLNhkiBILB+IkZyEENOB16AwFJNafNcXAw3PpaiEGEInDlKNuXxGAW+kiuAGMeYTAgSJR5YHBiihoHgVGApRwpTwNNoijKCWLRUTeqj5ss3Zwcdz7qtoSsrgAPfxxbiEPg4IRKJBCBwAeQAYsmWGGlBA4-jAoruCsQImAHLp7g4cCbEA4bAV4j5sCcXAAQQAOu9tQB7AB2Piq2p8Hb6bYAFEkAGoAGjbRCsbc7AEs6gBKUu4JVEvC5fzYGuvTgABw7MzoQ-e7yqADd512mnOqvuRyUAKpLttVLulNt9OjvefvAensOUpWK+7wALZVAA1j+LYtl2VRtuOc4douyFgfuVQ+POABGdR0G2Mzzj4AAWb7TpAY5tu8HY1H02r4eeH6wV2+59B2V7lO8VHwfuDDdm+-bzvx7zYTUPhtnUR4nlxZ4+HQVSlAAngw-6YQxC51MhXbvDU6FYd20DrCywT0mKUxwNyQYPFZ7rcgqSpcPKeaXHZznBEGLmemyoR2Mqfphmu-iEOQwWcMFoQVoWRYAPQpsmX50HQn5noRPjasRMHnnUkkzG2AA8ehga24FVCGhlAA

author: Swapnil
author_rank: Contributor
timestamp: May 16, 2022, 07:05:42 am
content: Quote from: Brumby on May 14, 2022, 03:27:40 pmYour assumption is correct regarding my use of the reed switch in the project. The reed switch will be used as a power switch to magnetically switch the power on from the batteries to the load as the vehicle will be fully submerged underwater and there are no chances to open the sealed housing. I am planning to use the reed switch in the latching configuration, i.e, before putting my vehicle inside the water, I will magnetically switch on the power from the battery using the reed switch and put the vehicle in the water. And after retrieving the vehicle back from the water, I will again use the magnet to turn off the power from the battery. My only concern is regarding the switching current of the reed switch which is in a few milliamps as drawing over 60A through a reed switch is perfect to destroy the switch during the first closure. How can this problem be overcome?

author: Swapnil
author_rank: Contributor
timestamp: May 16, 2022, 08:11:29 am
content: Quote from: SmallCog on May 16, 2022, 02:42:08 amYes, it is being used for waterproofing reasons and I don't intend to leave the magnet attached while the vehicle is running. I am planning to use the reed switch in the latching configuration, i.e, before putting my vehicle inside the water, I will magnetically switch on the power from the battery using the reed switch and put the vehicle in the water. And after retrieving the vehicle back from the water, I will again use the magnet to turn off the power from the battery.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: May 16, 2022, 09:54:57 am
content: Your submersible will need a high power circuit breaker (maybe a solid state relay) that is operated by a control subsystem. A subsystem with it's own independent power supply.With any safety critical system, big or small, good engineering is considering how systems and their control subsystems will fail. As a mechanical device, what if the reed switch is subjected to vibration or mechanical shock? You can 'function' a reed switch by flicking it with your finger. What if the reed switch is affected by another magnetic field, either top side or in the water? What if the reed switch bounces when being operated? What if the operator loses the master magnet? Consider, does the power circuit failsafe to the on or off state if something else goes wrong?You need to be 100% certain if the battery system is ON or is OFF. In this context, a reed switch is a significant point-of-failure which introduces a random factor that risks your ROV.An alternative might be an immersion detector. Maybe an 'Arduino' rain water detector or a sexy optical refraction detector?

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 16, 2022, 11:35:18 am
content: What kind of power do you have in this device? 12VDC?

author: Swapnil
author_rank: Contributor
timestamp: May 16, 2022, 11:40:55 am
content: Quote from: BeBuLamar on May 16, 2022, 11:35:18 am16.8VDC 20.8Ahr

author: vu2nan
author_rank: Contributor
timestamp: May 16, 2022, 12:39:47 pm
content: A 1 A reed switch would be more than adequate to drive two 18V DC, 60 A relays.Nandu.

author: CaptDon
author_rank: Super Contributor
timestamp: May 16, 2022, 01:54:19 pm
content: There are mechanical relays for 12vdc systems that are designed for continuous duty for R.V.'s and such. They look like the old Ford fender mounted starter relays but are for continuous ON duty. A starter relay has a high current coil and will burn up if powered continuously. Using the reed switch to operate a much smaller relay to drive the big relay is a good option. If you have a normally open reed switch to close the small relay and also a normally closed reed switch wired to the small relay as a self holding circuit you can definitely create your idea of 'Temporarily place the magnet at point A and the unit energizes and stays energized UNTIL you place the same magnet temporarily at point B and the unit de-energizes. Basically two reed switches placed maybe two inches apart. You only need to wave your magnet across either reed switch. Be sure to use anti-kickback diodes across your relay coils as snubbers!!

author: pqass
author_rank: Super Contributor
timestamp: May 16, 2022, 02:29:53 pm
content: Here's another version but with single ON/OFF reed switch. It's an adaptation of Dave's soft latch power switch (EEVBlog video #262).Also, I corrected my last (two reed switch) version to split the contactor coil resistor into two; one to adapt common 12V automotive contactors to a higher source (20V) and the addition of a parallel cap+resistor snap-on action but allows for lower steady-state coil current.My preference is for a two reed switch solution because it is a simpler circuit and is unambigous in intent (definitely ON or OFF). A single ON/OFF reed switch may have timing issues depending on how long you keep the switch pressed. If too long, the load will be switched on again!Simulation here:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEDYCYDMqDsrUE4AOLSAgsJBSZZKhAUwFowwAoMLAkAqvANhFQkQfEPwgxUIACxp0eQegSop3BLzCNmUWEjDx42yJNT8YkcochJILAEpce-biCamo0qoPdUYCFgBcBNRB0Xk5BTlDOKggmPGg8RLwsQiksdLC8SzhBSF4VdLh2MHkLPSoAEzoAMwBDAFcAG38WAHMBISkVDs4VHygWACcQyCkQsJGx5klPXJYAdx7wMEl0UeWZodFg9n5eYNRgmLbt-kO9g5xvFnRiU4373fBrgGdJ8d7uqPcIf0H6ugLBzgVCcZwiGy3TxCdAEMYRaTdcQDCrgLrgXhUZxgTECEBVOpNFqLbG47GggaLMDoxTGGFKAbDBHnJa09y4G53NZjb4s77ImwAYzR8J2NIZPlgRS40AQUjAcmS+2UeBUjGiUtYVPROOhnF1AyhSxW4SEBoFQKkImm7xtNgAHiE5E6sNJ0BB0FIqPCxgB5AByAHpfQAxEMsR0rCAISJkUQQKaRSAAQQAOi9BQB7AB2-lqgv8mcG6YAFCsAGoAGnTJls6azAEtGgBKCPSCjo3h3GNZRMgeoABwHdGLL0GdBeDZehdHJdwtmb6YAtrUANYT9O1dMV+uZpu7pcD2r+BsAI0adHT8wb-gAFpua5By+mXpn6oNBZfU9mB5n5iOX1qAA3Bts1aetagHUtygAVUXSds3TI9BlqRoL0aK8b3vF5Gj-ACxwnKcZ1Led4IHQZMxAqoXlTb8XmzSCGBzTcCwbZiXlPep-HTXD-0GGiXmnOhagqABPBhp2PS8Mz3DCZOzF56kPE8c2gTkqExXpMGkEQpG0i1HSkJBeHddt0FET1pHATgENaC90wDYMw3Tcc6AqF9r38QVsNQ3j0wAHj0Jc0xeFd7TUpkzVxZkjgEVAgWpUUziEFkbECK1+D0yQui0mZnjiBJEjgAheESDIVFMKVICKghCBKPSsEydBzPKfEagaZoThynTMu6BAVmuRZuvlXrIgmIVRE6bTNJ67wqtGYh9CW-QxjMOAarq1VbiaywtUm3pdO6DLKURA7RtOxlZu+Y67WkRabCG60Bv2MQBpsYZuuupFGp+fRLW+pwYh+h7gSeUkfBYAcPmkHFoZuwa4cOs6Tuu2HhthmwobCXLRFIWaLUWbG3UkGaspO0mRCJ47ITuY7rt0-TrkM10uz2PSQEWqywE4E8l1A8CXjLSAAGlqxeNJRfTNI4IgxCOOkv4G0FVdRJfUDP3Tdc6AHfmHKDUMQyQ8dBLc2jsxXVXp3fOXfz49NT3zVd03Y2pWlqUC1KAA

author: SeanB
author_rank: Super Contributor
timestamp: May 16, 2022, 02:41:50 pm
content: You want to have a latching relay, with 80A rated contacts. Commonly available as they are used for prepayment meters, and very rugged contacts as well. 2 12V coils, which are easy to drive with a pair of reed relays, probably enclosed in a plastic tube stub, so that you can slip a ferrite magnet over the stub to turn on, and then over the other to turn off. a one transistor amplifier to switch the coil will probably be good, but you do get larger reed relays capable of switching 60VDC and 500mA, so use those. Pot the reeds in a compliant compound, I would recommend a plastic sleeve over the relay, then use mirror mounting silicone to fill the inside of the plastic stub, and put the reed relay into the silicone till it cures. Mirror mounting silicone, as it is neutral cure, readily available, and the same price as regular tubes as well, plus it is a lot stiffer out of the tube, and shapes easily.Just a relay example, which will work as most AC relays are also rated for 30VDC use as well, plus you get double contacts here.https://www.ivy-metering.com/80a-relay/180.html

author: Swapnil
author_rank: Contributor
timestamp: May 18, 2022, 04:34:11 pm
content: Quote from: pqass on May 16, 2022, 06:46:13 amHey, thank you for the circuit it was helpful. Can I use Solid State Relay instead of the 30A contactor?And if I am using a 100A rated solid state relay, what will be the changes in the circuit?

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2022, 05:13:06 pm
content: Quote from: Ian.M on May 16, 2022, 06:23:43 amIf it's safety critical, then a contactor with the appropriate performance level and approvals is required.Otherwise, a MOSFET will do. Add a couple of resistors so it won't turn on through leakage and a device which is more than capable of handling several times the current.

author: pqass
author_rank: Super Contributor
timestamp: May 18, 2022, 07:00:18 pm
content: Quote from: Swapnil on May 18, 2022, 04:34:11 pmAssuming you're going to use an SSR like this one:https://www.digikey.ca/en/products/detail/crouzet/84137140N/13175953You won't need any additional resistance to match the higher source voltage since most SSRs can be supplied with a range from 3V to 32VDC.Also, you won't need the 100uF cap+parallel resistor that lowers the steady-state coil current since we're driving an LED instead.And, you can remove the usual flyback diode since there is no coil; just an LED.If you still want to use a two switch latching solution (ie. momentary ON or OFF) then see attachment and simulation below.If you want to have a full safety critical implementation where the magnet stays over the switch for the duration of the on-time (use it like a key), then you can remove the 2K2 base resistor and OFF switch entirely. In fact, if you can tolerate up to 25mA through your reed switch then you can switch the SSR LED directly; no transistor or ON switch resistor either! Just: Vcc----SSR LED----reed switch----GNDhttp://www.falstad.com/circuit/circuitjs.html?ctz=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