help needed with triggering relay chain circuit

Using Arduino General Electronics
#1

Hi. I have this final attempt at a relay chain circuit. Could someone check on the component values and circuit to make sure it will work? Here is the idea:

The micro-controller will use two digital out pins that will trigger a SSR. I calculated that the current limiting resistors would be 1050 ohm. This is from ohms law with 2.1V across the resistors and 1.2V across the leds inside the SSR. Is this right? Do I need anything else at this part of the circuit?

The outputs of the SSR will act as a SET momentary button and a RESET momentary button to turn on/off an OMRON relay. It is a latching circuit setup. The SSR outputs are switching +30V DC. Does this circuit part look ok? Are my values ok? Can someone recommend a diode part number for the flyback diode across the OMRON relay?

Let me know what you think of this. Thanks a lot! See attached picture and SSR spec sheet.

Pba150.pdf (102 KB)

circuit_question.jpg
circuit_question.jpg1115×503 69.6 KB

#2

Fly back diode such as 1N1004 or similar will do.

The circuit appears OK as long as the coil current does not exceed the SSR specifications.

Weedpharma

#3

Cool. Thanks!

1) I think the Omron relay is about 650 ohms, so not more than 50 mA from measuring. Should be ok.

2) Someone was saying that the SSR inputs won't get enough current with those 1050 ohm resistors. The leds need 5 mA?

#4

I would use a 470 ohm. I have not calculated this value. It is simply a value that will allow enough current without being too much.

Weedpharma

#5

rrboyd: 1) I think the Omron relay is about 650 ohms, so not more than 50 mA from measuring. Should be ok.

2) Someone was saying that the SSR inputs won't get enough current with those 1050 ohm resistors. The leds need 5 mA?

It's not a relay coil, so resistance not relevant. According to the datasheet, the IR diode needs 5mA, worst case. That, all factors worst case, calculates to ~330ohm. 470ohm might also do. 1050ohm probably not. Leo..

#6

Wawa, the R was the Omron coil being switched by the opto.

Weedpharma

#7

Ahhh, missed that. Leo..

#8

Not sure about the circuit of a single coil latching relay. I thought coil current had to be reversed to reset. Then a diode across the coil is not possible. Leo..

#9

The circuit uses one set of contacts to apply coil volts, via the NC opto, there is 30v to the NO relay contact.

When the opto NO is closed, it applies 30v to the coil and the contact then locks it on. To turn it off, the NC opto cuts the voltage.

A mechanical latch relay operates and latches via a magnet. The reverse pulse releases the latch.

Weedpharma

#10

Yes exactly, Weedpharma, that is how the Omron relay circuit is suppose to work.

The Omron relay area is a latching circuit. The relay itself isn’t latching. The circuit makes use of one of the relays 4PDT outputs by routing it to the input. If the relay is turned on momentarily by the input, it then stays on because that one output switch is connected to the input. The entire relay turns off by breaking the NC path. No negative volts.

Thanks!

#11

@OP, The fact that you don't recognize the dual N-channel mosfet symbols on the output of the relay probably explains why you continue to refer to it as a "coil" even after being told that it is not. Please get with the program. It is NOT an inductive coil. It is a dual mosfet output.

We use a similar scheme with actual coils and an "auxiliary relay" that snaps onto the side of the contactor. When you push the ON button , the contactor energizes, changing the state of the N.O. and N.C. contacts on the auxiliary relay which is used to route the contactor coil current through the N.O. contacts, resulting in a latching circuit.

#12

"@OP, The fact that you don't recognize the dual N-channel mosfet symbols on the output of the relay probably explains why you continue to refer to it as a "coil" even after being told that it is not. Please get with the program. It is NOT an inductive coil. It is a dual mosfet output."

@raschemmel - Well I'm the original poster and I never referred to anything on the SSR as a "coil." I know it has mosfet outputs. I can see them on the picture.

This circuit does have a second relay. The Omron one. Is that what you are talking about? This relay definitely has a coil. This is what Weedpharma and I were discussing.

This circuit has 2 relays :

IXYS brand SSR - mosfet outputs OMRON brand relay - YES!!! has a coil http://www.digikey.com/product-detail/en/MY4-02-DC24/Z2634-ND/369710

Try reading and opening the jpg circuit before you jump down people's throats. Try that program pal.

#13

weedpharma: A mechanical latch relay operates and latches via a magnet. The reverse pulse releases the latch.

I understand all of this, but then you can't use a diode across the relay. A "bridge" can reverse polarity, and the bridge can have diodes. Omron also uses a circuit with a capacitor for these type of "reversed coil polarity" relays.

http://www.digikey.com/product-detail/en/MY4-02-DC24/Z2634-ND/369710 This links to a NORMAL relay with INBUILD diode. NOT a latching relay.

So I don't understand why this complicated circuit is needed. As I see it, all of this isn't any different from a BC547 and a 1k base resistor. Leo..

#14

My apologies. I did not look at the drawing close enough.

So I don't understand why this complicated circuit is needed.

Set & Reset buttons are required when you don't have uP. With the uP, all you need is boolean flags. I don't see the point of a hardware latching circuit when you can do a S/W LATCHING RELAY I haven't actually tried writing code to simulate SET & RESET but I would think it would be trivial.

#15

no prob. it’s the internet, happens all the time.

i think you guys are right. the whole latching circuit is not needed. it can all be handled in the micro-controller.

i do think the reverse diode is needed though. i’ve read when the relay coil on the OMRON is turned off you get the back spike discharge which could damage the micro-controller or SSR, which ever is hooked up to it.

the reason i chose the SSR is because of the opto isolation. i was trying to protect the micro-controller from this back spike.

you prefer the micro-controller switching a BC547 on, right into the OMRON and get rid of the SSR?

#16

Well, actually, I don't understand why you are sourcing a relay. Standard convention is to sink relay current not source it. The relay (cathode end of diode) should be connected to the dc relay power and the SSR should sink the current. Then the diode would make sense because it would protect the SSR. In the current arrangement , it serves no purpose because there it isn't protecting anything.(Wawa's point I believe)

#17

the cathode is connected to the dc power. right? you can see the anode go right to ground. see attached pic for the yellow path - cathode to power.

cathode to pwr.jpg
cathode to pwr.jpg1115×503 71.2 KB

#18

A raschemmel pointed out, it’s normal to connect power to the relay coil, and switch the other side of the coil to ground.
Then the driving electronics can all be ground related.

As said, the datasheet of THIS relay states an inbuild kickback diode.
So it must have positive/negative markings on the coil.

You can still use this opto SSR (instead of a simple BC547/1k).
Connect the positive pin of the relay coil (cathode of the back EMF diode) to 24/30volt.
Connect the negative pin of the coil to e.g. pin 6 of the SSR.
Connect pin 5 of the SSR to ground (relay supply ground).

The other half of the SSR can be used for a second relay.
Leo…

#19

I understand now. I'll try that. Thanks for all the help guys!

I'm not sure my exact Omron relay has that diode built in. Mine is from the late 70s or early 80s. I can't find the spec sheet anywhere. I contacted Omron and they said that other one (the spec sheet i attached) is the closest they have now. The old one was special for high end audio circuits. The contacts are probably a different metal type. I'm not sure of the other differences.

I don't want to replace it, because it is already soldered to the board and not even sure there is a suitable replacement.

Any way to test to see if there is a diode in there?

#20

Try a DMM, set to diode measurement. Or a range that measures the resistance of the coil. If there's a diode inside, flipping the leads will have a different reading. I doubt there will be a diode inside an old relay, so better add one.

Is this a speaker protection relay or an input selection relay. There were plenty of problems in the eighties with speaker protection relays. Wrong contact material. Intermittend contact when the volume (speaker current) was very low. You had to give the amp a blast to get the sound back. Have replaced many of them. It was a cure for a few years. Leo..