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Design Challenge : relay spike protection

Catwell
This is a common problem with relays. The relay coil acts like an inductor when power is removed, and it sends a spike of power back into your circuit. And let's say you are originally putting 5V through the coil. When the power is removed, in some cases, the coil could send a spike of any level, even in the range of 400V, back into your circuit. And the problems cause by that are many, from burning out circuitry to tremendous electromagnetic interference.
 
 
I have seen spikes that oscillate between hundreds of volts positive and negative. Like below:
 
image
The challenge is to eliminate the problem in the cheapest way, with as few components as possible. Let's just say space and money are scarce for this project. And the relay is connected to very sensitive circuitry.
 
 
Use the relay drawing here to draw your ideas.
image
The relay drawing here has NC & NO contacts.
 
Cabe
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  • 0 in reply to Catwell

    Well, yes, thats a whole load of theory there.

     

    In practice, one needs to find a balance between cost and complexity - and find a solution which is cost effective, whilst at the same time providing a measured approach to protection.

     

    I'm not denying that the research in these docs is valid and true, but one needs to design to a level which provides an acceptable level of protection, not necessarily FULL protection.

  • 0

    Hi Cabe,

     

    Apparently I missed the discussions on the relay coil switching. I don't have a lot to add, the most is said already but you never know...

    The suggested diode across the coil is absolute textbook proof. The type of diode however might make a difference. A mains rectifier diode (1N4001) is really slow and would not be my choice. I would go for a fast signal diode, like 1N4148 or similar (or with higher rating if needed). The diode is mainly needed to protect the switch that drives the relay. A semiconductor device (transistors are the worst) are killed by high voltages. Mosfet's are more robust, they have a avalanche diode (zenerdiode) protection the drain .

    Your oscilloscope plot might be influenced by the ground loop wire which pickes up a lot of noise. If you measure HF ringing use a short tip probe (or at least wrap the ground wire around the probe tip to have the smallest magnetic loop).

    Interference in your circuit is more likely induced by contact sparks (wide broadband noise) or the coupling between the wires (or tracks) of the switching contact to the rest of the circuit (high dI/dt). Placing an RC across the contact damps the spark and the emitted noise.

     

    Best regards, Enrico Migchels

  • 0 in reply to Catwell

    Hi Cabe,

     

    Thanks for the articles, they were really informative. I do think some clarification is in order, as you can see the oscilloscope plots look nothing like the one you obtained so like enrico mentioned it's likely your probes were picking up some sort of interference.

     

    As you can see in the oscilloscope plots and the articles provided both the reverse rectifier diode and reverse rectfier diode with zener suppress the transient voltage spike successfully. The disadvantage of using just the single diode is that it takes a long time for the CONTACTS not the coil to de-latch which can cause the tack soldering effect. The addition of the Zener diode forces the coil inductance to build up a greater voltage that wil increase the current flow once the diodes breakdown. This translates to a faster response from the contacts hence no welding effect.

     

    I think the zener/rectifier solution is still a pretty cheap solution so I don't think it would be a big issue to use it as the standard suppresion method.

     

    Thanks again for the articles.

     

    Jorge Garcia

  • 0 in reply to Jorge_Garcia

    A friend of mine had a problem with relays a long time ago, and it was all due to the coil discharging. Adding a single diode helps, but only block half of the wave form. Since it ocillates in both the positive and negative. I am thinking this zener solution might at least clamp the issue.

     

    Or just go with a transorb. The bidirectional one acts as two opposing zener diodes and block current in both directions. Where the text book solution of one diode only block one direction.

    Picking a transorb with a low breakdown voltage might do the trick best.

     

     

    Cabe