I have active low relay board with optocouplers that is driven by an arduino.
All is well unless untill I take the power away from the arduino. Then the relays are suddenly becoming active.
The indicator led for that relay also lights up dim.
The internal resistance i measured between a datapin and the ground pin when powered down is about 12 M Ohm. This means about 0.4 µA is allowed to flow to ground... is this enough to trigger the optocouplers?
I would have thought that this would not light the LED.
Can you give a drawing of the input to the board and power supply connections?
Weedpharma
For this to happen the Vcc of your arduino must be getting power from somewhere. (Assuming you have the relay board wired correctly.)
All is well unless untill I take the power away from the arduino. Then the relays are suddenly becoming active.
Yes what on earth did you expect?
An output with no power looks like a diode to ground so you are going to activate the relay.
NEVER connect two systems together when you only power one of them. It is one of the best ways of destroying electronics.
Yes what on earth did you expect?
Hindsight is always 20/20 but you could have prevented this if you had made a Truth Table for the
inputs and outputs of your system and included a column for Power On (YES/NO).
This would show that when the system responsible for generating the active LOW signal is powered OFF,
the OUTPUT for that system will obviously be LOW. This can be corrected by adding a transitor (or logic
inverter gate) on the input of the relay system so a LOW on the input of the inverter generates a HIGH
on the input of the relay circuit. I would think that the relay would contacts would consist of a COMMON,
a Normally Open (N.O.) contact and a Normally Closed (N.C) contact and you could prevent the problem
by simply using the opposite contact than you are using now (N.O. if you are using N.C. or N.C. if you are
using N.O. By your description, if the relay is active LOW, that means that when the relay is OFF, your
load is ON. Relay modules don't generally use active LOW. Most of them are active HIGH. If your load is ON
when your relay is OFF , then you must be using the N.C. contact, which is rarely used in most applications.