Is it safe to use two pole DPDT relay to disconnect one side and connect another?

Thread Starter

nardev

Joined Feb 26, 2018
24
I have a situation where i'm supposed to turn of one line and turn on another line at the same time. Actually, it would be great if there is slight delay between those switching to let the device be protective and to do it bit more safe.

If i use two relays, i would not have that problem, i would simply make 0.5s delay between on/off of the relays and everything ok. But for the sake of using only one two pole DPDT relay, i need this.

The power lines that are used for switching could be 12VAC and 12VDC. Both things could happen.

Any idea? Any suggestion? How to protect the circuit? Etc?
 

pmd34

Joined Feb 22, 2014
368
Hi Nardev, it sounds from your description that 1 relay would be OK for your application. The contacts in the larger style of power relays move a fair distance, so it is not likely to be a problem with both lines being activated at the same time. But you can check the data sheet on a relay to see if it states it is "break before make".

You should be a little careful however, as if are switching high currents there may be some sparking which might cause some slight overlap effect.
 

AlbertHall

Joined Jun 4, 2014
8,387
The time gap between one contact making and the other breaking would be very small - a few milliseconds - so if this would be a problem for your circuit then you will need to use two relays.
 

Thread Starter

nardev

Joined Feb 26, 2018
24
This configuration a relay can NOT supply both AC and DC at the same time.

View attachment 185018
Definitely it would not be at the same time ;)

p.s. this would be double side relay. So 8 terminals in total.

1. side would be disconnected

2. side would be normally connected

In the event of activating the relay, the roles would change.

First side would be connected and the second one would be disconnected
 

Tonyr1084

Joined Sep 24, 2015
3,584
Can you provide a drawing of what you want to accomplish? From your description of two sources, a single DPDT (as drawn) would do the job.

You mentioned a time delay between deactivation and activation. Is that just so you don't hammer two different inputs to the load at the same time? In that configuration you can not have two inputs at the same time. Since the armature is physically connected (dotted line) one side can not move unless the other side does too. Between the time the DC shuts off and the AC kicks in - on the order of 50/60 cycles, 1/50th (or 1/60th) of a second represents a single sine wave of the AC, your load will hardly notice a difference. If at all.
 

Tonyr1084

Joined Sep 24, 2015
3,584
So 8 terminals in total.
I see six. Not eight. Two common, two NC (Normally Closed) (as drawn) and two NO (Normally Open). NO closes when power is applied to the relay control coil. At the same time the NC goes open.

Am I not on the same page as your thinking? A drawing would be beneficial for us all.

[edit] OK, eight if you include the two contacts for the relay control coil.
 

Thread Starter

nardev

Joined Feb 26, 2018
24
I'm sorry i didn't make a sketch.

It's two completely separated PSW devices.

It sounds bit strange but i actually need exactly that.
 

Thread Starter

nardev

Joined Feb 26, 2018
24
I see six. Not eight. Two common, two NC (Normally Closed) (as drawn) and two NO (Normally Open). NO closes when power is applied to the relay control coil. At the same time the NC goes open.

Am I not on the same page as your thinking? A drawing would be beneficial for us all.
well it's 6 + 2 for controlling the relay.
 

Tonyr1084

Joined Sep 24, 2015
3,584
well it's 6 + 2 for controlling the relay.
Yeah, caught that. Edited my comment to include the coil.

As for a drawing, grab pencil and paper and sketch something. Snap a picture with your phone. Send the picture to your e-mail, then drag the picture to your desktop. Then start a reply and click "Upload a File". Then click on the picture. You can load it as a file or as a full image. We can work with it from there.

What's a PSW?
 

Thread Starter

nardev

Joined Feb 26, 2018
24
However, the real question is "Would that be safe for the circuit and power supply?" Is there enough the delay. The device which is connected is simple coil. Can tolerate 8-24 VAC or VDC. But again, is this safe for the board?
 

Thread Starter

nardev

Joined Feb 26, 2018
24
Exactly.

Well the board is straight forward MCU Atmega328P, isolated and powered with buck converter and it is supposed to run this relay. Also, there is 2 simple 1 wire sensors. but i doubt that they would have problems.

I'm more worried
What is the board?
A simple coil will not care about a delay between the two power systems, but a circuit board of some kind might.
that the MCU could reset because the buck converter or so.
 

AlbertHall

Joined Jun 4, 2014
8,387
Do you have a rectifier and smoothing capacitor between the switched power input and the buck regulator?
If so then the smoothing capacitor could maintain the input voltage to the buck regulator during the switchover and there would be no problem.
 
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