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Topic: Low power replacement for a mechanical relay in a solar powered Arduino project (Read 319 times) previous topic - next topic

HSWD

Hi! Can you please help me finding a good low power replacement for a 5V mechanical Relay in the following diagram.



This is a solar powered (with a backup Li-ion battery) project and supply is managed (using MCP73871 chip) and regulated to make it steady.
Timer is generating square pulsers. Every T minutes arduino is switched on for few seconds (~25s) using the Relay. But relay is using quite amount of power. Because this circuit is powered 24x7, relays power consumption (~70mA) really matters.

Therefor is there any substitute that is equal to a mechanical relay that has very low power consumption?
I found that SSR ICs, OptoMOS Relay ICs and Analog Switching ICs can be used but i'm not sure about there power consumption.

JonesyWolf

Hi HSWD,
While I am no expert on the subject, from my limited experience, I'd recommend using a MOSFET for turning on and off your Arduino as they are pretty power efficient. I'd also recommend watching this video from GreatScott! to learn more: https://www.youtube.com/watch?v=o4_NeqlJgOs
Best of luck,
John

jremington

You could use a high side MOSFET switch. Pololu manufactures a nice collection of small modules.

However, there is no need at all to turn on and off the power to an Arduino or to use the 555 timer IC. In sleep mode, power consumption of a "bare bones" Arduino is negligible, and it can wake itself using the watchdog timer to do something at any desired interval.

Have a look at this solar powered Arduino project that uses only a supercap for backup power.

MarkT

Every T minutes arduino is switched on for few seconds using the Relay. But relay is using quite amount of power. Because this relay is powered 24x7, it's power consumption really matters.
No, the relay is on for a few seconds every T minutes.  The 555 is powered 24x7, so the first
idea I have is to change it for a CMOS 555 like the 7555 which use less current.

You also need a freewheel diode across that relay winding to stop it frying the '555.

The other approach is to make the Nano use sleepmode.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Robin2

Timer is generating square pulsers. Every T minutes arduino is switched on for few seconds using the Relay. But relay is using quite amount of power. Because this relay is powered 24x7, it's power consumption really matters.
If the relay is really drawing power 24/7 then you have it wired up incorrectly. It should only need power for the few minutes that the Arduino is on.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

Paul_KD7HB

I am looking at a DC SSR I pulled from a junk box. Grayhill 70m-odc5. Will switch up to 60 volts DC at 3 amp. Has 1.5ma leakage, which a real relay will not have.

Paul

HSWD

I'd recommend using a MOSFET for turning on and off your Arduino as they are pretty power efficient. I'd also recommend watching this video from GreatScott!

Video is great. MOSFET seems to be a great solution. What do you think about using LCA717 IC?

You could use a high side MOSFET switch.
However, there is no need at all to turn on and off the power to an Arduino or to use the 555 timer IC. In sleep mode, power consumption of a "bare bones" Arduino is negligible, and it can wake itself using the watchdog timer to do something at any desired interval.
MOSFET is a good choice. How about LCA717? And i'll chek sleep function also. Seems to be the better solution.

No, the relay is on for a few seconds every T minutes.  The 555 is powered 24x7, so the first
idea I have is to change it for a CMOS 555 like the 7555 which use less current.
You also need a freewheel diode across that relay winding to stop it frying the '555.
The other approach is to make the Nano use sleepmode.
its a mistake to say relay is powered 24x7.  Thanks for mentioning 7555. I have already using a freewheel diode. Ill try sleep mode.

If the relay is really drawing power 24/7 then you have it wired up incorrectly. It should only need power for the few minutes that the Arduino is on.
...R
Its my mistake to say so Robin2. I have corrected it.

I am looking at a DC SSR I pulled from a junk box. Grayhill 70m-odc5. Will switch up to 60 volts DC at 3 amp. Has 1.5ma leakage, which a real relay will not have.
Paul
Seems this SSR is not a gud solution. But how about something like LCA717?

MorganS

The LCA717 looks like it might work. Those modules aren't talked about much, so there must be good reasons not to use them in other situations. The datasheet says it is intended for medical and aerospace applications, so they must be confident that it's reliable. 

0.15 ohms is a rather high on-resistance, much more than a relay. But for just powering an Arduino, it's OK.
"The problem is in the code you didn't post."

HSWD

The LCA717 looks like it might work. Those modules aren't talked about much, so there must be good reasons not to use them in other situations. The datasheet says it is intended for medical and aerospace applications, so they must be confident that it's reliable. 

0.15 ohms is a rather high on-resistance, much more than a relay. But for just powering an Arduino, it's OK.
Thanks for checking it out MorganS. Btw 0.15ohms is for a  AC/DC Configuration. For DC Configuration max resistance is 0.04ohms. If my calculations are right it will save lot of power compared to using a ~60mA relay.

hammy

I would question why you are not just letting the nano sleep and wake up  via the watch dog timer every 8 seconds , increment a counter then sleep
- after so many counts the nano can stay awake do whatever task you wanted , then go back to sleep.No relay , no 555 timer .  Have a look at Arduino sleep, and other low powr options. Also older Nanos need optiboot installs if you want to use the watchdog

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