Voltage drop when using relay

Using Arduino Motors, Mechanics, Power and CNC
1

Hi!

I'm having a issue using arduino, relay and sensors.

Everything works as expected from the logic point of view: sensor reads, relays close/opens, etc.

The problem is the current draw of the relay and the voltage drop in the microcontroller, which impacts on the sensor readings drastically.

When the relays are closed i see almost 5.1V on arduino vcc pin.
If I turn on the relay, the VCC voltage drops to 4.3 v...

I'm learning about electronics so the questions are:

what is the best electric design to fix this problem with voltage drops and sensors?
perhaps gating the relay using a NPN transistor switch?

Any ideas?

Thanks in advance

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You should not draw a significant amount of power from the Arduino 5V pin. The pin is intended only for things like low power sensors, or a couple of LEDs.

Use a separate power supply for the relay.

3

What relay module do you actually have? Some have on board transistor drivers, and some don't. If yours doesn't, there are lots of relay driver circuits online, you can just Google them. Even just find a relay module schematic, lots of those online too.

@jremington just reminded me, the relay control input drive is not the only problem. But if your module has no transistor, driving it from an Arduino pin isn't okay, either.

1 Like
4

The picture shows the weak side of Fritzings. No power supply. I've never managed to run a controller without supplying power somewhere.

5

this one

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It's seems no driver on it.

I see, i have to find a good relay, i was stucked, thank you.

6

:slightly_smiling_face:

I was lazzy, it powered with a 9v 1 amp.

But, i suspect that the issue is that i need a relay with a driver. Do you think that i can i use a PNP switching transistor instead?

Thank you.

7

ok, so i have to put a transistor as driver, isn't it?
Any pnp would do the trick ?

8

Use an NPN or N channel MOSFET in common emitter or common source configuration.

From post #3:

there are lots of relay driver circuits online, you can just Google them. Even just find a relay module schematic, lots of those online too

Or the easy way, obtain a relay module with built in driver.

9

From the pictures you've posted - it already has one.

10

Yes, that's right. Don't know how I missed it.

11

The 5V output can supply more current, or at least supply it more safely, when the Arduino is powered from the USB connector. Barrel jack powering is the worst. But as a benefit, you discovered the weakness...

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12

That's the issue. Don't use the not heat sinked 5 volt board regulator to supply motor or relay current.

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13

I have a Uno (Official product) and a 4-relay board.
I am using 9V via the barrel jack.
I am click-clacking one relay, its output is running an LED like the OP's.
Uno 5V measures 5V when the relay is actuated and when it's not.
The switched output via the relay measures 5V, too.
I can make a vimeo if necessary.

I believe there's an OP wiring problem.

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14

Good point. I'd like to see OP photos. Some clones have an inferior 5V regulator though...

15

Which Arduino (I have learned not to trust Fritzing pictures).
How do you power that Arduino.
You mention 9volt.
Smoke alarm battery?
That will struggle to provide the current for an Uno + relay (125mA), and could cause VCC to drop.

Which sensor. I don't see one on that toddler drawing.
Leo..

2 Likes
16

OP stated "9V, 1 amp". Regulated or Unregulated?

My total current draw is 50mA w/ relay 'off' and 110mA 125mA w/ relay 'on' (coil current, relay board LED, output circuit LED).

1 Like
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Also, "real" or "advertised"?

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