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Relay coils polarity, why some matter and others dont ?
Relay coils polarity, why some matter and others dont ?
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Old 22nd October 2019, 02:12 AM   #21
davidsrsb is offline davidsrsb  Malaysia
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dI/dT=V/L
V=0.7V, L is fixed for a given relay coil, so dI/dT is low, the current takes a long time to go down. This decay time can be long enough to be noticed.
Make V=12V with a zener and the current decays roughly 20x faster
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Old 22nd October 2019, 03:24 AM   #22
mandu is offline mandu  Singapore
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Most of the DC operated relays will have a +/- marking for the coils. When the coils are released, the back emf induced will be in the order of a few hundred volts. A reverse connected diode wired will absorb the back emf protecting the driver transistor or the contacts that energize the relay from arcing or in sensitive circuits prevent the spike travel back to other circuits. Since the back emf is opposite in polarity, the diode (conducts) absorbs the energy only when coil is released. This causes an increased delay in magnetic mechanism release time by another 10 milli seconds. In industry all dc relay wiring are polarized even if the relays do not use diodes. This is to prevent accidental shorting of DC power supply if one relay is replaced with a diode model. Some manufacturers provide external diode/RC snubber accessories that can be fitted to the relay bases. RC networks are used in relays with AC operated coils. Diode with resistor will shorten the mechanism release time than a diode itself. Regards.
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Old 22nd October 2019, 03:35 AM   #23
nigelwright7557 is online now nigelwright7557  United Kingdom
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A trick I was taught was to put a Zener in series with the back emf diode.
This helps release quicker.
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Old 22nd October 2019, 08:09 PM   #24
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by mchambin View Post
The diode is the usual well proven way to protect the transistor.
What is the delay value because of the diode ?
I do not see why it has some damaging effect on the relay contacts. Is this known and proven ?
Nor do I, here's my understanding:

In most relays the magnetic force is at its peak when the armature iron contacts the pole piece of the solenoid. A spring is trying to pull the armature back. When the current gradually drops the magnetic force reduces until the point the spring wins, Then the armature accelerates away from the pole as the magnetic force drops sharply with distance even if it is changing slowly with time.

The contacts are on another spring which means the contact force doesn't reduce until the armature is already accelerating away from the pole.

So the contacts are unaware of how slowly the energizing current falls, and the circuit is broken just as cleanly with a diode or not.

All assuming the relay is engineered well.

You can test a relay coil using a variable voltage supply and vary the voltage up and down, seeing both the hysteresis in operating voltage and the clean snap of the contacts when closing and when opening. If it doesnt switch cleanly in either direction its likely to be faulty.

Note that you should use some means to protect the supply from back EMF if it doesn't have large decoupling caps to absorb it.
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Old 23rd October 2019, 12:57 AM   #25
Mark Johnson is offline Mark Johnson  United States
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Relay coils polarity, why some matter and others dont ?
Sometimes folks want the relay to "drop out" as quickly as possible, i.e., with no delay at all. The typical example is a Turn Off Thump preventer / output muting relay. When it's time to mute, you want to mute RIGHT NOW and not 20 milliseconds later. The circuit tricks which maximize the ramp-down of coil current (engineers would say: which maximize |dI/dt| ), give the quickest shutoff.

Since the coil inductance has no other choice but to obey the inductor differential equation |dI/dt| = V/L ...

if you seek to maximize |dI/dt| you can either increase V or decrease L (or both). One of these requires you to be a relay designer, the other only requires you to be a circuit designer.
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Old 23rd October 2019, 04:30 AM   #26
daqvin_carter is online now daqvin_carter  United States
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You may want to get some peak detector before hoping that a 300 volt transistor will work for something like a 12 volt relay. Kickback can be more than you think for a brief moment. I only know from measuring the DC resistance of assorted transformers and coils with an old ohm meter and getting a good kick because I was smart enough to hold the leads with my fingers.
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Old 23rd October 2019, 06:47 AM   #27
nigelwright7557 is online now nigelwright7557  United Kingdom
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Quote:
Originally Posted by nigelwright7557 View Post
A trick I was taught was to put a Zener in series with the back emf diode.
This helps release quicker.
If the relay drops out quicker there is less chance of contact arcing on DC systems.
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Old 23rd October 2019, 07:17 PM   #28
Mark Johnson is offline Mark Johnson  United States
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Relay coils polarity, why some matter and others dont ?
I just put together this experiment on the solderless breadboard.

It's a 300V NPN transistor "MPSA42" driving the coil of a 12V relay (33 mA coil current) with no flyback protection whatsoever. No diode, no zener diode, no resistor, no capacitor, nothing. I monitored the collector voltage with my least expensive {most expendable} oscilloscope. Data below.

The scope (100 MHz bandwidth) says the collector shoots up to about 600 volts, and dwells in the Super Duper High Voltage realm for about 1.5 horizontal divisions on the scope, namely, about 15 microseconds. "236V" is where I told the scope to trigger, on the rising edge.

Interestingly, the "300V" transistor keeps right on working. I ran the experiment for about 100 on/off cycles of the relay, i.e., about 2 minutes, while I adjusted the instruments and snapped a photo. It was working just fine with no apparent ill effects, when I shut it off.

Resistor R2 ensures that the relay driver transistor Q1 is turned off extremely quickly; R2 sucks out the base charge of Q1 in nanoseconds. Predriver transistor Q1 buffers the input, because the 1 Hz oscillator is made of CD4000 CMOS logic gates (Schmitt trigger inverters) that are barely able to source or sink half a milliamp.

_
Attached Images
File Type: png schematic_relay_flyback.png (13.1 KB, 92 views)
File Type: jpg breadboard.JPG (559.5 KB, 86 views)
File Type: jpg scope.JPG (344.2 KB, 87 views)
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Old 23rd October 2019, 07:23 PM   #29
nigelwright7557 is online now nigelwright7557  United Kingdom
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I did a bit of work with car ignition coils and electronic ignition systems.
The primary can get to about 400 volts when the points open.
I had to use a 800 volt mosfet to switch the primary.
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Old 24th October 2019, 06:33 AM   #30
Vovk Z is offline Vovk Z  Ukraine
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Relay coils polarity, why some matter and others dont ?
Quote:
Originally Posted by Mark Johnson View Post
The scope (100 MHz bandwidth) says the collector shoots up to about 600 volts, and dwells in the Super Duper High Voltage realm for about 1.5 horizontal divisions on the scope, namely, about 15 microseconds.
_
Thank you! Picture is very clear now (I don't need, but for society).
I'm shure transistor works as zener here, but only we dont know its avalanche parameters. If we take higher rated voltage transistor - peak voltage could be even higher.
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