I was told that when a relay is used for DC (+24V)to the coil, you should also put a diode across (parallel to) the coil. Why is this? The relay is to control a 120VAC circuit.

The diode is placed across a relay coil that is powered by DC, to absorb voltage spikes generated by the coil inductance when the current through the coil is shut off. Without this diode, the high voltage spike generated by the collapsing magnetic field of the coil ("inductive kickback") can damage solid state components, particularly a transistor or other semiconductor used to control the relay coil.

The diode is installed so it is normally reverse biased (cathode to the positive end of the coil). The voltage generated by the inductive kickback will be of the opposite polarity, and will cause the diode to conduct, absorbing the spike. I usually use a diode with a PIV rating at least 5X the applied coil voltage. Something in the 1N400x series is suitable for most situations.

I accidentally discovered inductive kickback when i was a teen. I was holding the wires on a relay, wired it up so that the coil was in series with the normally closed part of itself. (Tired to make it like a buzzer) Hooked it to the battery and got little shocks. Sometimes I've also seen a 0.1uF capacitor in parallel with the coil and diode. I often see the capacitor in parallel of small motors, such as in toy RC cars. The capacitor helps smooth out the inductive kickback because (i hope i got this right) a diode can take a small fraction of a second to work. I've even seen the caps on a bridge rectifier, although I'm not sure if it's for the same reason.

The capacitors perform a similar function, but not exactly the same.

The capacitor across a diode acts as a short circuit to any high frequency AC created when the diode turns on and off. The capacitor across a DC motor armature (and often from either end of the armature to ground, as well) helps filter out noise from the brushes, and prevents it from interfering with the control electronics.

Just for the record,
These things are used alot with PLC's that use relays, they usually use a series RC network called a "snubber circuit", to minimise the inductive effects of the coil Back EMF.

That happens to many electricians when they use a flashlight continuity tester on a transformer.
Don

Or even some types of ohmmeter.

A cap on a small DC motor can also help the brushes last longer by reducing arcing.

Larry Fine;

Welcome to ECN.

I have gotta ask ... your User Name ... is it who I think it is modeled from (part of an "Older" Trio), or is it your real name.

I am thinking "Larry Fine" as in "Howard, Fine and Howard".

If needed, feel free to E-mail me at the Phoenetically listed address below:

setelectric at pacbell dot net (without the spaces or text for the symbols).

Scott35

BTW, the "UnCap'ed" Brushes of DC Motors generate a lot of RFI - in the form of harsh pulses, which shows up big time on TV sets (mostly in the AM video part, but have seen in both AM video and FM audio parts), create havoc in other areas of Radio use (AM, HAM, etc.) , and may cause control signal errors being received on Carrier Current applications (or RF controls).

Damping the RFI is another reason to bypass the Brushes / Commutator with Non-Polarized Capacitors.
Non-Polarized to allow the passage of "AC-Like Noise" ... I think that's correct (been some time!)

To add some more to the Clamped DC Coil thing, one very common application where a Capacitor was used in conjunction with a DC driven Coil, was the Automotive Ignition System.

The Coil was controlled by the "Breaker Points", which opened/closed the "Primary" circuit of the Coil, so a high voltage DC pulse was achieved at the "Secondary", and Distributed to the approporiate Spark Plug.
The Points were clamped with a Capacitor (using the nomeclature "Condenser"), so the points would not become prematurely Barbequed from the surged DC - as result of opening the circuit.

It was a simple Parallel clamp method, using something like a 4.7 µF 125 VDC Cap., connected between Line side of points and Chassis "Ground" (the distributor)

Just some additional FYI.

Scott, thanx for the welcome. It's both: one of the 3 Stooges and my real name.

ECN 7 Topic: Topic: diode on relay {CRW} ../Forum7/HTML/000402.html

A bit of O.B. yappin’ here.. {not under the immediate influence of headache pills}

This may be sortta’ gameshow-wrappup or color-commentary, but never any crested-suit with gold buttons, or even denm oxferd opncoller buttondwn with clipon mic or fake·pred’stressd a&f/o·n/gupy·jeenz. More jerrygarcia@1130am·wid·abowluhcornflayks·wardrowb..

• 1N400x-type {simple silicon—PN-junction, epoxy-potted <offshore>97¢-a-pegboard-bag} diodes are DC-mode, id est; polarity-sensitive devices that make extry-fast “fuses” if reverse connected.
  
  Else, lower-microfarad {±±±~~±~±±1 &#956;fd nonpolarized electrolytic <or> nonpolarized tantalum) or series resistor-capacitor networks are genrally considered oprating in the (DC or) AC mode, which means they will snub or ‘absorb/transcend’ a few low-nrgy, fast transients of both polritys.
  
  As mentioned by all in this thread it is not a trivial matter. and should not be passed up, for you or some of your peers will be faced with self snot-nosed self-important production MBA-wannabe puissants fretted with personally justified [surely sloppy·lortis-v2.11-extrapolated-over-stale-twwinkies] sophomoric-debated J·i·T and ever-looming carrot / mangeral ‘threats’ 2 theiyr precius vision o’ th’ anuual bonus pot.]
  
  Eboneezr Scroooge and his integagaltic corporate metacila Hävðrrd Bizînsss Shcool would kave newer beean so Platinum–Berkelium-Iridium GPS-sans–SA Magna—Cüum-Ladæ doern Successful.. [/i]

***BUMP***

Bumping the thread to life again!

Scott35

Scott35 brings up an interesting concept in his last "full" posting.
Having a capacitor as part of the load and drawing vis Current makes it hard on a set of contacts.
Considering that a relay coil could be fed via the Collector of a Power Transistor, makes the transistor vulnerable to over-currents from the coil.
Cascading relays/contactors,for more or heavier contacts makes this situation worse.

Hi, does anyone knows what is the consequence of accidently reversing the polarity of the diode (i.e. the cathode connecting to the negative side of the relay)?

Hello "Greenr" - welcome to the Forum!


This will result in the Diode being connected in the "Normal" Forward-Biased direction.
Unless there is a Resistor, or some other means of limiting the Current (AKA: "Ballasting"), the Diode will draw high Current - acting as a Short Circuit.

Results will be:
<OL TYPE=A>

[*] Relay's Coil unlikely to latch closed - due to the extreme voltage drop,


[*] Diode will lose smoke, and after all smoke is lossed, it will become an Open Circuit - or,


[*] High Current level will either/or:
- melt fuse / open thermal cutout,
- damage DC power supply.


[*] Or all of the above - depends on how sturdy the Diode is.
</OL>

The reason to Reverse-Bias the Diode, is so clamping may be achieved during the time when the "Saturated" Coil reacts inductively to the rapid change in current flow.
The "Inductive Kick" flows in the opposite direction - and such, a Reverse-Biased Diode may work effectively in an active circuit, as a clamp during the inductive kick, and still block current while the Coil is in normal "On" operation.

Make any sense?

Let me know if Yes / No

Scott35