What to do when you suspect a relay is faulty?

[leydenjar]

Thanks Enzo.

An example would be RY3a and RY3b on the power supply schematic for a Maverick (attached).

Ok. So there's no optocoupler here so I'll address that in a subsequent post.

1. How many volts is actuating that relay and where is it coming from?. I'm assuming the RHY/LD switch switches the relay but it takes a voltage to actuate it.

[km6xz]

It helps if the 6th page of the schematic was shown, the rail is +3, and -3volts, taken from the heater DC supply for the heater of V1.
I fully agree with Enzo, a schematic is not a wiring diagram, it is showing the electrical, not mechanical relationship between components, sub-circuits and reference potential. Often there are no voltages printed but seldom is that needed. Looking at a circuit, a pretty good idea of what would be an expected reading ought to be. A diagram is cleaner and easier to follow if lines not included that could be simply represented by a signal name or bus name.
I too, am at a loss as to why you are troubled by relays and optocouplers in schematics. When working on digital circuits, there is ever much less relationship between wiring diagram and schematic where almost everything is a buss or data line that does not show it connected to anything on one side. If you looked at early digital schematics you would see quickly why named busses and signal/control lines are not displayed now, it would be too confusing to show or follow it from page to page. The replacement named line is a Node and everything that connects to a node is at the same potential. "Clk1" sure is simpler to understand what it expected on that line than seeing a maze of lines running all over the page that is at the same potential.

[Dave Mac Farlane]

Not trying to hijack a post - I promise, but I worked on relay logic too, then leapt into diode, then dtl and ttl, - what incredible leaps.
What I find splendid about this forum is that everyone is so, so helpful, and km6xz, not everyone thinks at the same rate or ability - ever tried working with army techs? before anyone shouts bias!, let me say there were brilliant ones, who carried the woodenheads.
Your additions to the article on relays are sterling - all of you.
Thanks for a good forum.
Dave.

[Enzo]

Yes, your posted schematic is only a partial. I don't have the complete Maverick myself, but compare to this Nomad schematic set as simlar. Pages 8 and 9 having relay control circuits. You can't blame the manufacturer because there are incomplete schematics bouncing around the internet.

http://www.webphix.com/schematic%20h...ie_nomad45.pdf

And optocouplers or LDRs are really no different. Just as in a relay, there is an element in an opto that is on or off to control the signal path. Unlike the relay, they are analog and can vary in resistance. But either way, there will also be an electrically unrelated control element - a light bulb or LED usually - that activates the first part. Just as a relay, we need to draw current through that control part - light the lamp. SO similar circuits can be used. A transistor doesn't much care if it controls current through a relay coil or an LED.

[Enzo]

Yes, your posted schematic is only a partial. I don't have the complete Maverick myself, but compare to this Nomad schematic set as simlar. Pages 8 and 9 having relay control circuits. You can't blame the manufacturer because there are incomplete schematics bouncing around the internet.

http://www.webphix.com/schematic%20h...ie_nomad45.pdf

And optocouplers or LDRs are really no different. Just as in a relay, there is an element in an opto that is on or off to control the signal path. Unlike the relay, they are analog and can vary in resistance. But either way, there will also be an electrically unrelated control element - a light bulb or LED usually - that activates the first part. Just as a relay, we need to draw current through that control part - light the lamp. SO similar circuits can be used. A transistor doesn't much care if it controls current through a relay coil or an LED.

[leydenjar]

This was very informative. Thanks everyone for the help.

Concerning relays, they have various voltages printed on them and listed in the data sheets.

I'm attaching the data sheet for the relay in a mesa maverick. I have the exact relay indicated with color arrows.

How do I determine the minimum voltage the relay will actuate with?

The data shows maximums but is "Nominal Operating Power" how am I supposed to determine minimums?

I'm asking because if a -/+ 3vdc rail will actuate these relays and it says "2A 30 VDC" on the relay (maximum?).

Also, are relays always actuated with a diode across the coil?

[km6xz]

The +/- 3 volts(6 volts) is well within the 3.5volt Must Operate Voltage rating so it will deliver the rated contact resistance or better. 6volts is over the nominal rated voltage so a series diode would drop your 6 volts enough to keep the coil cool.
The drop out voltage is the voltage they spec for which will release the armature even when the spec .25 volts is present across the coil. Any more than that and they won't guarantee that spec... It take more voltage to pull the armature fully than voltage needed to maintain the contact. The release voltage is lower, than the pull in voltage. If you lower the voltage gradually from when it is fully activated at 5v, the voltage were the armateur no longer going to providing a low resistance contact will be a lot lower value, say 2.5 volts. The break voltage is the highest voltage that still allows the relay to open the contacts. In this case, for any current flow up to 2 amps, .25 volts across the coil will be essentially the same as 0 volts. They do not guarantee that it will release with 1 volt across the coil or any level above .25 volts.

The 30 volts/2 amp rating is contact maximum for make and break.
When turning off a relay coil current flow, the collapsing magnetic field induces a reverse polarity voltage that can be quite high, called Back EMF. If a transistor switch is used to operate the relay coil, the peak inverse level generated by the collapsing field can be far in exceed of the transistor's breakdown voltage. The diode is used to short out the back EMF so the drive circuit is protected.