ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

ECN Shout Chat
Top Posters(30 Days)
Admin 19
Recent Posts
Anyone hiring inspectors?
by HotLine1. 03/27/17 08:03 AM
Old decora style outlets
by Admin. 03/25/17 11:40 AM
ESA Arc flash course
by TheShockDoctors. 03/24/17 10:15 AM
fuse rejectors
by HotLine1. 03/24/17 07:53 AM
Another Forum Update
by Admin. 03/22/17 03:04 PM
New in the Gallery:
SE cable question
Popular Topics(Views)
231,626 Are you busy
166,455 Re: Forum
160,717 Need opinion
Who's Online Now
0 registered members (), 68 guests, and 11 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
Page 1 of 2 1 2
#129531 - 04/13/05 07:02 PM diode on relay  
CRW  Offline
Member
Joined: Feb 2001
Posts: 159
Bethlehem, PA USA
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.


Tools for Electricians:

#129532 - 04/13/05 10:12 PM Re: diode on relay  
NJwirenut  Offline
Member
Joined: Sep 2001
Posts: 806
Bergen County, NJ
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.


#129533 - 04/15/05 01:39 AM Re: diode on relay  
PEdoubleNIZZLE  Offline
Member
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
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.


#129534 - 04/15/05 08:06 AM Re: diode on relay  
NJwirenut  Offline
Member
Joined: Sep 2001
Posts: 806
Bergen County, NJ
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.


#129535 - 04/16/05 08:58 AM Re: diode on relay  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
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. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#129536 - 04/18/05 07:24 AM Re: diode on relay  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Quote
I accidentally discovered inductive kickback when i was a teen.

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


Don(resqcapt19)

#129537 - 04/18/05 06:28 PM Re: diode on relay  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
That happens to many electricians when they use a flashlight continuity tester on a transformer.

Or even some types of ohmmeter.


#129538 - 04/24/05 05:05 PM Re: diode on relay  
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
A cap on a small DC motor can also help the brushes last longer by reducing arcing.


Larry Fine
Fine Electric Co.
fineelectricco.com

#129539 - 04/24/05 10:25 PM Re: diode on relay  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
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 " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#129540 - 04/24/05 11:15 PM Re: diode on relay  
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
Scott, thanx for the welcome. It's both: one of the 3 Stooges and my real name.


Larry Fine
Fine Electric Co.
fineelectricco.com

Page 1 of 2 1 2

Member Spotlight
Posts: 165
Joined: March 2007
Show All Member Profiles 
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.020s Queries: 15 (0.004s) Memory: 0.8179 MB (Peak: 0.9938 MB) Zlib enabled. Server Time: 2017-03-28 06:22:17 UTC