Mechanical Relay Limitations

Hey Guys,

I am trying to put together a basic 2 channel relay similar to this design Relay Project. But the relay I am working on will be normally open and powered by 5V. A GPIO from an Uno will control the signal wire.

The problem is there are some circumstances when I need to power the relay for extended periods of time (one month continuously) although there will only be 2A max going through the relay when it is closed and it's rated for 10A, I am concerned that the mechanical relay could burn-up in this situation. I may need to get a latching relay instead. I wanted to know if anyone had any recommendations or knowledge on mechanical relay limitations, especially how long they can stay in their "non-normal" state?

Thanks

Anything oven a hour I would use a latching relay. Both for power consumption, and so the driving electronics are not under load.

feynman137: Hey Guys,

I am trying to put together a basic 2 channel relay similar to this design Relay Project. But the relay I am working on will be normally open and powered by 5V. A GPIO from an Uno will control the signal wire.

The problem is there are some circumstances when I need to power the relay for extended periods of time (one month continuously) although there will only be 2A max going through the relay when it is closed and it's rated for 10A, I am concerned that the mechanical relay could burn-up in this situation. I may need to get a latching relay instead. I wanted to know if anyone had any recommendations or knowledge on mechanical relay limitations, especially how long they can stay in their "non-normal" state?

Thanks

If the relay is passing AC, then there is no problem. If it is passing DC, the metal points will eventually weld themselves together due to the current causing metal migration. Did you consider using solid state relays, either AC or DC? Paul

Consider a DPDT relay and place the contacts in parallel.

If you need 2 amp flow, use 4 amp contacts.

Use a latching relay.

Have you considered power failures in your design.

Consider using a contactor.

Consider a solid state relay.

Consider a MOSFET.

Paul_KD7HB: If the relay is passing AC, then there is no problem. If it is passing DC, the metal points will eventually weld themselves together due to the current causing metal migration. Did you consider using solid state relays, either AC or DC? Paul

The relay is just powering a raspberry pi, which takes 5V DC. I haven't used an SSR before, but would it be able to handle these conditions. Also how would the current consumption compare to the latching relay? Thanks

larryd: Consider a DPDT relay and place the contacts in parallel.

If you need 2 amp flow, use 4 amp contacts.

Use a latching relay.

Have you considered power failures in your design.

Consider using a contactor.

Consider a solid state relay.

Consider a MOSFET.

I think in this project it would be best to fail open on both relays. The DPDT relay is interesting but will it be able to stay active for a month? Also I think its current consumption will be higher than the latching.

I am curious to know if a 5A TIP122 transistor could be used. Would it be ok to have the transistor on continuously for a month with 2 amp going through it?

Steve

Would it be ok to have the transistor on continuously for a month with 2 amp going through it?

You'll probably need a heatsink, and a MOSFET will run cooler. A month isn't much different than an hour. It's the internal temperature than kills it. It doesn't even take an hour for the temperature to stabilize, but it might survive the winter and burn-up in the summer (because the ambient temperature is added to the internally generated heat).

A relay would probably be fine... I've worked in electronics for a LONG time and only remember seeing a dead relay once... It was a horn relay in an old car (under the hood & exposed to a severe environment).

Where I work now, we have a product with 16 relays in it. I do occasionally see a DOA relay, and sometimes one of the units comes back for repair and a relay fails the resistance test (resistance more than 1-Ohm or something when closed). But, an out-of-spec relay has never been the reason it was returned... It's returned for something else and we just happen to catch that during the test.

feynman137: The relay is just powering a raspberry pi, which takes 5V DC.

So what is the point of turning it off?

I would use a latching relay.

Some time ago I had an application in which a small relay was energized continuously for many months. In the event of a power failure, the relay was supposed to de-energize and enable a battery backup system. What actually happened was that the coil became magnetized and/or some welding of the contacts occurred, so the relay remained closed when the power went out. This kind of thing is a known pathology for DC relays.

S.

Unfortunately the link to the relay board does not give relay part numbers or links to data sheets. About all they say is:

In-here we have designed an isolated PCB for 4 relays to operate 4 AC appliances at a time. We have put a three pin screw terminal blocks (NC, Nuteral, NO) for connecting appliances.

So the relay contacts in the link are rated for AC. Some relay contacts rated AC can be used for lower DC currents but in choosing any relay a good data sheet is you best friend. Powering a Raspberry Pi is likely going to be a very low current application. In the interest of simplicity and parts count I would likely opt for using a DC SSR (Solid State Relay) with a low trigger / control voltage (about 3 to 12 VDC). You likely only need one to handle a few amps. I would look to a manufacturer like Crydom, Gordos, Opto 22 or any other reliable name from a reliable source. You buy once and cry once and don't buy junk off the boat if you need reliability.

Just My Take Ron

Paul__B: So what is the point of turning it off?

This process in powered by a well engine, which can run for weeks at a time when irrigating say a rice field. But when the motor is off, there is no alternator to power the 12V battery. I still need to post data once a day though, so I conserve power by turning the Pi on and off with an ESP module that sleeps on a timer. The relay is controlled by the ESP. I do need the relay to fit on the PCB with the rest of the circuit. I am thinking something like the relay in the picture below will work best. It is a flip flop circuit connected to a regular electromechanical relay that basically simulates a real latching relay. But the Songle relay module is cheap and so is the circuitry.

I don't know how to build it quite yet but hopefully it is not too difficult. Let me know what you guys think.

My complaint with the SSR is the price and I don't know how to make it fit on my PCB, maybe I am not looking at the right items. It may be over kill as well though since there is very low frequency switching in my process.

|500x245

srturner: I would use a latching relay.

Some time ago I had an application in which a small relay was energized continuously for many months. In the event of a power failure, the relay was supposed to de-energize and enable a battery backup system. What actually happened was that the coil became magnetized and/or some welding of the contacts occurred, so the relay remained closed when the power went out. This kind of thing is a known pathology for DC relays.

S.

Thanks for sharing the experience, I might have run into something similar.

latching relay.JPG|1385x679

There have been some very conflicting posts in this thread.

With regard to the OP, relays can stay on 24/7 for years, that’s not a concern. But, if it’s battery powered, a conventional relay is the wrong solution due to the continuous power required to keep the relay energized. Your make-shift latching relay is a bad idea as it provides zero benefit with additional parts cost and failure points.

That said, forget relays for dc switching, there are much better ways.

The application just screams for a simple, zero power, high side switch with a logic level p-channel mosfet per the image below. The load shown would be your Rpi and no heat sink would be required with the mosfet shown although a small one may not be a bad idea in high ambient temperatures. With the heat sink, all the components will fit on a pc board with less area than just the songle relay.

|500x413

WattsThat: There have been some very conflicting posts in this thread.

With regard to the OP, relays can stay on 24/7 for years, that’s not a concern. But, if it’s battery powered, a conventional relay is the wrong solution due to the continuous power required to keep the relay energized. Your make-shift latching relay is a bad idea as it provides zero benefit with additional parts cost and failure points.

That said, forget relays for dc switching, there are much better ways.

The application just screams for a simple, zero power, high side switch with a logic level p-channel mosfet per the image below. The load shown would be your Rpi and no heat sink would be required with the mosfet shown although a small one may not be a bad idea in high ambient temperatures. With the heat sink, all the components will fit on a pc board with less area than just the songle relay.

|500x413

Great advice and circuit, and I will follow this instruction! I don't completely understand how it works, but that's on me so I'll be looking at that today. I mentioned potentially using the ESP which has 3.3V GPIOs, would this circuit still be relevant or would I need to change a few things? The datasheet for the recommended MOSFET FQP47P06 datasheet

I would use N-channel vs P-channel, and switch the Gnd connection. Lower on resistance, and you can find parts that will turn full from with 3V on the gate, such as AOD424 https://www.digikey.com/en/products/detail/alpha-omega-semiconductor-inc/AOD424/3152449

AOD424 Rds at 2.5V.JPG|911x83

I agree with CrossRoads as I do not see the FQP47P06 as a viable solution for high side switching. Your load is minimal at less than 500 mA. You are only switching 3.3 volts with that low current. With only 3.3 volts applied as source bringing the gate to zero I doubt will work let alone work well. In the interest of simplicity I still suggest a DC SSR which has a 3.0 volt control voltage.

Ron

CrossRoads: I would use N-channel vs P-channel, and switch the Gnd connection.

Thanks for the suggestion. I am not sure what the second half of this comment means though. Could you clarify? Thanks

Normally a P Channel MOSFET is used for High Side (Load +) switching while a N Channel MOSFET is generally used for Low Side (Load -) switching. That in keeping it simple. So we switch either High or Low side of the load.

Ron

I swear, if I had posted a low side driver using an N channel device, the usual suspects on this site would say you could never switch an Rpi using low side. I can only image the OP’s frustration with the sudden turns in this thread, it’s enough to give one whiplash. Well, at least mechanical relays appear to be off the table. If that’s progress, I’ll take it.

Gets off soapbox...

With regards to my reply #12, I regret I didn’t credit Nick Gammon with that schematic and I overlooked the fact that the OP has a 3.3 volt, not 5 volt controller. In that case, the NDP6020P mosfet would be a more appropriate choice for 3 volts.

If the OP is willing to add a NPN bipolar transistor as a level shifter to the 5 volt supply voltage, the original mosfet part number is quite capable of the task. This is detailed on Nicks web site in the link below. It’s all explained there in great detail.

If the actual load of the Rpi is an amp or less, no heat sink would be required with either circuit arrangement.

Low side switching is the correct solution for single devices like motors and lamps with a single ground point connection. High side switching is the correct solution for devices that may have other, secondary grounding sources that you cannot or do not want to isolate.

Switching the power ground of a Rpi that has other things connected to it that have their own ground connections and potentially their own supplies is a sure fire recipe for failure.

Nick Gammons switching tutorial

NDP6020P datasheet

Bald Engineer low and high side switching

Ron_Blain: Normally a P Channel MOSFET is used for High Side (Load +) switching while a N Channel MOSFET is generally used for Low Side (Load -) switching. That in keeping it simple. So we switch either High or Low side of the load.

Ron

Ahh I see probably should have noticed the N and P for - and +. I found the circuit below online, but I imagine it is similar to what you are suggesting. But take the 12V and sub in 5V and the pin will be at 3.3V, does that sound right? [img alt=See the source image]https://protosupplies.com/wp-content/uploads/2017/11/N-CH-MOSFET-Logic-Level-Circuit.jpg[/img]