3
\$\begingroup\$

I have a transistor, driving a normally open reed relay (protecting transistor with flyback diode), driving a solenoid. The solenoid is actuating at about 3 Hz.

The relay is good for a few million actuations. And then, it mysteriously stops working. Replacing the relay works, for another few million actuations. I conclude that the problem is not the transistor or the solenoid.

The nature of the failure is somewhat peculiar. At first, it appears to be a stuck closed failure; the relay will not open when the transistor is not driven. When power is removed from the relay, it then opens. When power is re-applied, the relay stays open, until the transistor drives it, at which point it becomes stuck closed again.

I tested one of the "dead" relays. The coil exhibits the correct resistance, 250 ohm. The reed switch is also open.

I would have expected this to be a failure where the contacts are welded together, but the fact that a "dead" relay is open and that it only sticks after it has first been energized lead me to believe there is something else going on. Does anyone have an explanation for this failure?

EDIT: Addressing some comments

Using the reed relay because that's what we have already, this is a small one-off project. The relay is a Radio Shack relay, SPST 5VDC 20 mA nominal, rated for 500 mA 125 VAC/60VDC. I believe the equivalent relay (OMR-106H) can be found on digikey here http://www.digikey.com/product-detail/en/OMR-106H,V000/PB886-ND/1095211. The DC power supply for the solenoid (12V) shows about 400 mA peak current. The datasheet lists 5 million "operations" at 24V 400 mA. Two relays have now died around 3 million at 12V.

The diode is across the coil, not the switch. The literature I have read indicates that the diode is usually for the protection of whatever drives the reed relay's coil.

\$\endgroup\$
  • \$\begingroup\$ Cut it open, post a picture. Short of that, everything is just conjecture. \$\endgroup\$ – Passerby Nov 14 '13 at 0:59
  • 2
    \$\begingroup\$ Its a reed relay, that is what. Do you have a flyback diode for the solenoid? If not, the reed points will die. Its obviously not an accident, so why not use a ssr or the like? \$\endgroup\$ – Erik Friesen Nov 14 '13 at 3:26
  • \$\begingroup\$ What is the relay switching? What's the switched current & voltage? \$\endgroup\$ – Connor Wolf Nov 14 '13 at 12:00
  • \$\begingroup\$ 12V @ 400mA ... A simple job for a MOSFET and you don't need to worry about wear. And of course as mentioned, you need a flyback diode across your solenoid regardless of how you drive it. \$\endgroup\$ – Tut Nov 14 '13 at 20:32
  • \$\begingroup\$ 12v 400mA is 4.8W Switched. It might be more due to inrush current. And do you also have a diode across your solenoid as well? The datasheet shows that max current regardless of power is 500mA. You might be exceeding that, if the solenoid inrush current is higher than the holding current (400mA). \$\endgroup\$ – Passerby Nov 15 '13 at 8:40
4
\$\begingroup\$

Every relay has a life expectancy... they are not infinite. You are saying that at 3 hertz you activate it a few million times, that is a lot of activations for a relay.

As you have not provided a part number, for illustrative purposes, I have used a generic one. The datasheet of your relay should have a graph like this one...

enter image description here

As you can see, in this relay particularly, the maximum life operations it can have no more than a million and a half operations...

That type of failure is normal in relays because the contact material starts wearing away with normal operation, and if you operate at the maximum rated load, as you can see in the table, the relay will have no more than 30,000 operating cycles.

I hope this information is clear enough.

\$\endgroup\$
  • 1
    \$\begingroup\$ There are some reed relays that claim up to 10^9 actuations. \$\endgroup\$ – Connor Wolf Nov 14 '13 at 12:01
  • \$\begingroup\$ Through some experimentation (i.e. searching digikey's reed relay catalog using some of the markings), I think I have managed to find the equivalent part number. OP has been updated. \$\endgroup\$ – ajs410 Nov 14 '13 at 17:21
2
\$\begingroup\$

Did you protect the reed relay from the solenoid Back-EMF? When you energize a coil such as the one in your solenoid, and then suddenly cut the power, a very high voltage, like tens time the energizing voltage, appear a the coils ends. That voltage surge is likely to cause an electric arc in your reed relay and will accelerate its aging process. There are many ways to protect a relay from back-EMF surges, one of the cheapest/easiest is to bridge the solenoid ends with a TVS diode.

Transient Voltage Suppressor (TVS) diodes are designed to be blocking below a predefined voltage, and conductive otherwise. They also can withstand pretty high voltage. When you clamp your solenoid ends with one of these, back-EMF surges are shorted by the diode and stays "trapped" in the coil and cannot damage your Reed Relay.

You need to choose a TVS diode with a reverse standoff voltage > your installation voltage and a Clamping Voltage < your reed relay max voltage.

\$\endgroup\$
  • \$\begingroup\$ OP is having Contact issues, not coil activation issues. The EMF would only affect the coil side. \$\endgroup\$ – Passerby Nov 14 '13 at 6:05
  • \$\begingroup\$ Op also mentioned he has a diode in the very first line. \$\endgroup\$ – Passerby Nov 14 '13 at 6:25
  • \$\begingroup\$ ooops, my bad, didn't see that. \$\endgroup\$ – martinm Nov 14 '13 at 7:07
  • 1
    \$\begingroup\$ I can second that. Switching inductive loads means, when switching off, you will get large transient voltages, which create an arc on the contacts. I have a relay here at home switching a 12V motor, and I can see the arcs in the relay when it turns off. And I see the same problem there - sometimes the contacts just stick together, until power is turned completely off. The data sheets for the relay also shows that higher voltage reduces the life time. \$\endgroup\$ – hli Nov 14 '13 at 19:51
  • \$\begingroup\$ @Passerby the EMF can create an arc which can weld the contacts \$\endgroup\$ – teambob May 28 '15 at 1:51
1
\$\begingroup\$
  1. To avoid arcing, add a diode across your 12VDC solenoid's contacts too. This "freewheeling diode" allows the solenoid's coil to dissipate its pull-out energy without damaging other parts.

  2. 3Hz is a fast, constant switching speed for a relay & is probably causing premature wear due to accelerated fatigue of the 'reed.'

  3. replace the relay with an N-channel, enhacement-mode power MOSFET. Then you won't need the diode across the (no longer existant) relay coil & you won't have the physical wear on the reed/contacts. You will, however, still need the diode from #1 in this answer, keeping the solenoid from blowing up your MOSFET.

Replacement

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.