SNAP-OMR6 vs SNAP-ODC5R. Mechanical relay ouput modules

It still surprises me how many people work with Opto 22 hardware and software and yet are not signed up to OptoNews and so miss out on hearing all the cool stuff that happens in the world of Opto 22… Another topic for another day perhaps, right now I just want to answer a question about our new digital output modules which got announced in[URL=“”] last week’s OptoNews.

SNAP-OMR6. 4 channels of 6 amp mechanical relay goodness, more than any other SNAP module to date. The other cool thing, is because they are mechanical relays, whether you have one channel turned on or all 4, they are still 6 amps per channel (remember, for most of the SNAP output modules, it’s 3 amps per module, not per channel).

Ok, Ok, yeah, you’re right, this is a technical forum, not a sales forum, so here is the real reason I’m taking up your time… We have already had a question about the new modules……

Middle of page 1 of the new modules doc there is this statement ‘ NOTE: These modules may not be suitable for low level switching.’
The question I got was ‘What does this mean?’.

Here is my answer (and I’m interested in your answer or comment on my answer)…… Put simply it means that they have some contact resistance.
Page 3 of the same doc has the specifications. You can see that they have less than or equal to 100 milliohms contact resistance. In the same table, the minimum load is quoted as 5 VDC at 10 mA. This is the load that they will reliably switch and not ‘alter’ too much.
In a nutshell, if you try and switch a low level signal with these relays, you are not necessarily going to get out what you put in.

Compare that if you will with our other mechanical relay, the SNAP-OCD5R. The [URL=“”]doc quotes no minimum load, or contact resistance, but what it does quote is a maximum load, that is 10 VA.
People have been using these relays for a lot of years; some have had trouble with the contacts welding shut because they have been pushing a little too much current through these relays. Where the ODC5R relay excels is switching signals.

Two customers spring to mind. The first uses a lot of them to switch composite video signals between video decks; perfect application. The other uses them to switch automotive computer signals (exhaust gas O2 levels, temperatures, etc.) between test ECU’s. Again, you could not get a more perfect application for that module.

With the addition of the new 6 amp modules, we now have a wide range of signal to power switching covered. It’s up to you to select the correct module for what you need to switch.

Have you got any applications that you could use new modules in?
Got any signal applications that have been running just fine with the reed relays?

Switch on!


I was looking through these the other day, and was surprised to see that we have 2 models.

[B]SNAP-OMR6-C[/B] (SNAP Isolated 4-channel Mechanical Power Relay Output Module, Form C)
[B]SNAP-OMR6-A[/B] (SNAP Isolated 4-Channel Mechanical Power Relay Output Module, Form A)

OK, one is for normally closed contacts and the other is for normally open contacts. I get that, but the [B]-C[/B] version allows you to either connect to NC or NO contacts by using the relevent screw contact with the common middle contact. The rest of the specifications are identical. Maybe I am missing something, but with that flexibility, why does the SNAP-OMR6-A actually exist?

The silence is deafening! Anyone out there?


I always try and give others a little time to answer before I jump all over a thread…

From what I understand, we had a good numbers of customers ask for a ‘simple to wire’ mechanical relay.
There is no mistaking where to wire each output on the form A.

Of course, you are quite right, why do you even need both models, and I know which one I would buy… but in the interest of those who want to KISS (keep it super simple), well, we have the -A version.

(Personally, I am really interested in looking at the sales numbers in about 6 months time and seeing which we sell more of).



I think we both agree on this one Ben. The [B]SNAP-OMR6-A [/B]is going to get pwned unless you are dumbing down U.S. customers!

Can the channels be tied in parallel to increase the switching capacity? In other words, would a module capable of switching 4 channels of 6 A be capable instead of switching 2 channels of 12 A?

Hi Sensij,

The problem with putting relays in parallel is that one relay will always switch faster than the other, and so it will carry the full load until the other one closes.
Given that inrush current is the highest load, that one relay will burn out its contacts before the other…

In short, if you need to switch higher current, use a relay rated for that current.
(The OMR6 can always switch the coil of that larger relay).


When using these modules, don’t forget that they use more power from the rack than our other digital out relays…
They are a mechanical relay, this means they have a coil driving the contacts, this coil uses more power than the optical circuitry in other digital output modules.
If you are just using 1 or 2 of them, its probably not a big issue, but if you are getting banks of them, you really need to look at your 5v PSU that will be powering the rack…