Relay coil currents

Hello everybody,

I’m building a solar powered arduino Mega-based system and using ArduinoConnect ipad App for remote control.

One of the things I do is turn on and off a fan from my iPad (with the app above) in addition to graphs and meters (pic below).

Since it is a solar powered system, I’m very concerned about current draw and have put INA219s through out the system so I know just where the power is going.

Here’s some information:

Arduino Mega 2560 - 100ma typical.
5V computer fan - takes about 80ma when it is on.

I was very concerned about current draw from the relays so I bought a selection and measured the currents. Here are the results:

Name / Measured Current Draw with Relay On / Specification

SainSmart 2 Signal Relay / 30ma / 8.7ma
SainSmart 2 Relay / 70ma / 20ma
Evil Mad Scientist Relay Shield / 70ma / 72ma

I’m using the SainSmart 2 Signal Relay as it saves me 40ma. Note the huge difference in spec to measured on the SainSmart 2 Relay and the big difference between the spec and measured for the SainSmart 2 Signal Relay. 30ma is better than 70ma however.

A picture of the control panel is attached.

Note that while I am reading the currents from INA219s connected to the Arduino I2C bus, I put the data in a MySQL database on a Raspberry Pi to crunch the data and build the graph using matplotlib in python. So in the picture below from my iPad, the graph is connected up to a Raspberry Pi and the meters and buttons are connected to the Arduino directly (across the internet of course). ArduinoConnect allows me to mix end computer devices on the same page.

Best,

Randall

Are the Sainsmart relays "relay modules"? I mean with the transistor drivers on board. Then the spec current could be what the module input draws form the Arduino digital output.

What do you intend to switch with relays? There are other options to switch just about anything.

Yes, the Sainsmart relays are modules. Driven by 3.3V or 5.0V inputs. I understand from other forums that the Pi has problems driving these, but my reading of the specs, it should work. The Arduino has no problem.

I'm actually measuring what the module takes from the 5.0V supply.

To clarify exactly what devices I measured:

SainSmart 2 Signal Relay - SainSmart 2 channel Signal Relay Module Board SainSmart 2 Relay - Sainsmart 2-Channel 5V Relay Module

I'm currently switching a fan that is connected to a 6.5V power supply (actually directly off the solar cells to make things more efficient since if it is hot, the sun is probably out....) to ventilate my box.

Randal.

randallsussex: I'm currently switching a fan that is connected to a 6.5V power supply (actually directly off the solar cells to make things more efficient since if it is hot, the sun is probably out....) to ventilate my box.

Then it sounds to me that you should be using and NPN transistor or small N-channel mosfet to switch the fan, avoiding the relay current.