SPDT relay is not switching

Question

I have a 8 channel, 10A SPDT relay, which I'm having problems getting to work. Here are its specifications. I've attached a few images below for reference.

So, initially, I connected a 6V power supply to VCC and GND pins and supplied a 5V input signal to input 8. I checked the continuity with a DMM, and the Normally closed (NC) and Common (C) terminals are showing continuity before and after giving the control signal.
Then, I increased the control voltage to 18 V, but there's still no switching sound and no continuity between normally open (NO) and common (C).
I also tried increasing the the voltage to VCC to 12 V, still doesn't switch
I tried jumping the two GND pins and repeated the procedure, still the same results
Finally, I increased the VCC to 48 V and jumped the VCC and input 8 pins, the relay switched then. I heard the click and checked the continuity. But, the specs say that the control signal HIGH voltage is 2.5 V+. So why can I not get it to work at lower voltages?
I thought the relay might be bad and repeated this process with another relay, but even that's not being energised for conditions 1 to 4. (I did not check condition 5 for it)

I think there's a mistake in my understanding of how to wire/power relays. Where am I going wrong?

Thanks a lot!

P.S.: I did try all the 8 channels on the relay, but with same results.

You bought 48V version. So it needs 48V nominal for the coil(for the control input). You need the version with lower nominal coil voltage, — user1245, Jan 17 '19 at 21:43
Here is the 5V model https://www.aliexpress.com/item/10PCS-lot-Power-relays-SRU-05VDC-SL-C-SRU-5VDC-SL-C-5V-sru-05vdc-sl/32800136696.html But you might still need buffer check the nominal current — user1245, Jan 17 '19 at 21:45
It's worth pointing out that this is properly called a "relay module"; the little blue boxes on it are the relays themselves. — Hearth, Jan 17 '19 at 22:00
Hello Chris, the transistor part# is 2N5551. I don't know much about electronics, but I checked the image of the relay module on their website, and the transistors seem to be connected in the same way as they are on the part which I have with me. — Ved Soni, Jan 17 '19 at 22:59
The link you provided is for a 24 volt version - and the relays in the photos on that site say "24V". Your pictures show 48 volt relays. Are your photos showing the actual board you have? If not, what voltage do the relays on your board show? Whatever it is, that is the Vcc voltage required to operate the relays. The required control voltage may be different. — Peter Bennett, Jan 17 '19 at 23:24

Tony Stewart EE75 · Answer 1 · 2019-01-22T02:02:32.057

5

These are 48V coil relays.

You can drive the LED with 3~5V but the coil needs 48V on Vcc and Gnd.

Maybe you bought the wrong relays.

(not exactly as later found on website)

schematic

^{simulate this circuit – Schematic created using CircuitLab} Above was my guess.

below is from website

8ch 48V coil boards

53831

Looks like I guessed wrong on the LED connection from your photo, but close enough on the driver level. Your link was for a different board. (24V)

So Vcc is 48V with common ground with driver = 3.3V to 5V

So yes it looks like your connections were incorrect, perhaps ground or one port transistor is damaged with significantly reduced hFE if the logic input threshold is now 29V instead of <3V

edited Jan 22 '19 at 02:02

answered Jan 17 '19 at 21:42

Tony Stewart EE75

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Hey! So if I use 48V on VCC and GND, and use a 5V for the control signal input, will that work as well? – Ved Soni Jan 17 '19 at 22:49
@VedSoni - No, it won't work. 2N3904s are only rated for 40 volts. – WhatRoughBeast Jan 18 '19 at 00:09
The transistors on my relay are 2N5551 – Ved Soni Jan 18 '19 at 00:30
@VedSoni It should work fine. 2N5551s are rated for something like 140V, so you have plenty of margin. – Hearth Jan 18 '19 at 00:34
This schematic is what I see from the part values on your photo and it works and agrees with my web searched specs. – Tony Stewart EE75 Jan 18 '19 at 04:40
I tried powering the relay at 48 V, and set the control signal input to 5V, it didn't work. Later, I jumped the power supply +ve and the control signal wires and I wanted to check at what voltage the relay gets energised. It switched at 29V. Can I classify this relay as bad? – Ved Soni Jan 18 '19 at 23:52
@VedSoni Do you have the proper specs for the 48V board. You linked the 24V board – Tony Stewart EE75 Jan 19 '19 at 00:17
Thge Vcc ground MUST be connected to the 5V ground. – Tony Stewart EE75 Jan 19 '19 at 00:32
I'm sorry. Here's the correct link: http://www.electronics-salon.com/index.php?route=product/product&path=4946_7561&product_id=53831 – Ved Soni Jan 19 '19 at 00:45
I am thinking now the transistors are in backwards and they never tested them – Tony Stewart EE75 Jan 19 '19 at 00:46
I did try jumping the grounds of both power supplies earlier, but in that case the power supply voltage was 6V and not 48V. I'll try what you're suggesting now. Thanks! – Ved Soni Jan 19 '19 at 00:47
We can verify with better pictures or actual PN of Q. Some are EBC and CBE and if Vbe goes more than -10V pfft – Tony Stewart EE75 Jan 19 '19 at 00:47
Okay, if it doesn't work in this weekend, what specific pictures do you want of the relays? I cross-checked the arrangement of the transistors on my relay module and the picture on the website, it seems correct. – Ved Soni Jan 19 '19 at 01:03
transistor part number – Tony Stewart EE75 Jan 19 '19 at 01:04
Okay.. i did check the transistor number after reading your comment yesterday. It is 2N5551, but I don't have a picture of that. – Ved Soni Jan 19 '19 at 01:06
I agree looks correct , and they changed parts so silkscreen is backwards . Can you test another port? shared grounds on 5V – Tony Stewart EE75 Jan 19 '19 at 01:13
Okay.. I'll try testing after jumping the grounds and post an update here. – Ved Soni Jan 19 '19 at 01:22
Hey, its working, after I jumped both the grounds. Thanks a lot! – Ved Soni Jan 21 '19 at 16:37
Some relay boards are isolated with optoisolators, but NOT these – Tony Stewart EE75 Jan 22 '19 at 02:03

score 0 · Answer 2 · answered Jan 18 '19 at 06:25

As per the circuit diagram you need to provide above 2.5V between control signal and ground and rated operated voltage as per the module number between Vcc and gnd. Please check the supply from which you are sourcing, some time it is set on a zero current and that is why not able to provide sufficient current.

SPDT relay is not switching

2 Answers2