# How can a reed switch be used for high current applications?

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?

• Why are you using the reed switch directly? What's stopping you using a reed switch to switch a FET or similar? Or even a microcontroller then you can do various other things with the input Commented May 9, 2022 at 12:03
• Drawing over 60 A through a reed switch is perfect to destroy the switch during the first closure. You should read some datasheets of reed switches very carefully.
– Uwe
Commented May 9, 2022 at 14:44
• I think this is what is called an "xy question". It's better to start by asking the larger question, "What, exactly are you trying to do? Why do you think a reed switch is a good choice here?" Have you done some homework and tried to find a reed relay which can handle 60 amps? (Hint on the last one - you won't find any.) Commented May 9, 2022 at 18:17

Don't use the reed switch directly. Use the reed switch to switch the coil of a larger relay. Alternatively, use the reed switch to switch the gate of a FET large enough to handle to load current.

• Hey, thanks for the input. Can you show a reference circuit for implementing the switching of the gate of FET using the reed switch so that I get a good insight into the solution that you just proposed? Commented May 14, 2022 at 7:57
• @Swapnil Meshram I find it slightly alarming that you're asking for a reference circuit for this while you're undertaking what seems to be a somewhat challenging electronic/electrical engineering/mechanical engineering project. Are you sure you're not out of your depth (pun acknowledged)?
– Frog
Commented May 14, 2022 at 9:24
• @Frog This reference circuit might help a lot of folks out there who will be going for such kinds of challenging projects. I am not asking for a circuit with exact values of components but just a reference circuit to start with. Commented May 14, 2022 at 10:14
• Nice answer. Correct, short and to the point. Follows the KISS principal. Commented May 14, 2022 at 13:25

For long-term reliability, a 60 A load requires relay contacts rated for at least 100 A. The coil current for such a contactor almost certainly is greater than the reed switch rating. There are at least two ways to solve this. One is to have the reed switch control a small relay with contacts rated to handle the coil current of a larger relay or contactor. Cumbersome and inefficient, but a method that has been around for over 100 years.

If you are nervous about switching that type of load with a solid state device, an intermediate step is to have the reed switch control a small power transistor, and have the transistor control a contactor.

Finally, use one or more power MOSFETs to switch the load directly. Both size and weight are much less than either relay approach. Over the last 50 years we've learned a lot about safe operating area (SOA) and other device and circuit parameters, to the point that a FET switch is considered more reliable than a contactor in many applications.

AND - you can reduce the turn-off inductive kick significantly by controlling the gate voltage. If the voltage to the load is ramped down over 1 to 5 milliseconds, the di/dt is 100x to 1000x less than the microseconds ramp caused by a relay contact. Some switching power supply control chips have this feature built in to reduce EMI. The tradeoff is a thermal spike as the FET(s) spend more time in the linear active region, but this is an easy calculation.

Note that large power MOSFETs have a significant gate capacitance. Surge currents can be several amps, which also can shorten the life of a reed switch. You probably would need a gate driver, but that is the perfect place to implement a voltage ramp.

You will want a protection circuit (or BMS, stands for Battery Management System) for the LiPo pack, to implement overtemperature and overcurrent cutoff.

The protection circuit will have a MOSFET that is used to cut off the power in case of a problem. Most protection circuit boards have a connector or pins where you can connect external power switch that will also turn off the power. You can connect the reed switch here.

By using the same MOSFET, you avoid extra resistance and losses. It does mean that if the single MOSFET fails in short-circuit, you have no way to turn off the current without physically disconnecting wires. It will depend on your risk analysis whether you want redundant protection systems.