Relays with a diode
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Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=28260
Printed Date: May 10, 2024 at 3:08 AM
Topic: Relays with a diode
Posted By: kaffeene
Subject: Relays with a diode
Date Posted: March 12, 2004 at 3:25 PM
Hey I am a little confused about something so I appreciate help. If you have a relay you would connect a diode in parallel with the coil between teminal 85 and 86, say 86 is the positive side so the cathode is on that side. RIght?? Now you flip your switch off and you have a reverse voltage spike from the relay coil. Since its a reverse voltage I assume it come back from the relay coil through terminal 86 (positive) back down your wire. I'm confused about this, how does connecting it in parallel help? It would seem wiring the diode inline with the positive terminal with the cathode facing the relay would work. That way the cathode prevents voltage from passing unless of course it is more than the diode can handle. Also what would be the best way to connect wires to a relay? Would you use female disconnects? If you soldered how would you insulate the bare wire so it didn't touch the other contacts? Help
Replies:
Posted By: xetmes
Date Posted: March 12, 2004 at 4:22 PM
Posted By: kaffeene
Date Posted: March 12, 2004 at 4:32 PM
Well thats good and all but I understand what it does just dont understand the path it takes. Someone in that thread said anode to positive side, thats wrong no.. you would have a short. I basically dont understand why the path of voltage would chose to come out of the terminal and turn to the relay .. instead of going straight down the wire to the components or switch.
Posted By: markcars
Date Posted: March 12, 2004 at 4:40 PM
the diode is only there to suppress any reverse voltage given back from the relay coil when it stops from being energized (in order to protect sensitive devices).
and positive goes to anode, that is correct. check the relay section. side with stripe is cathode and other side is anode. voltage flows through the diode from anode to cathode and stops any voltage the other way.
Posted By: xetmes
Date Posted: March 12, 2004 at 4:52 PM
Ok ill try to clear it up if possible. If we apply a voltage to a relay we build a magnetic field in the coil of the relay. This produces the magnetic field to operate the relay. (im sure you understand this part). Since the relay coil has an impedence it will have a voltage drop on it (the supply voltage) and a current through it. The current through an inductor (like the coil of a relay) has a current flowing through it that can not change instantaneously. So even if voltage is removed there will still be a current through the inductor. The current through and inductor is equal to L (di / dt), the time derivative of the current times the inductance. Since the inductance is constant it really doesnt matter here. The derivative however shows that the voltage will be proportonal to the change in current. When we remove the voltage from the coil the current decreases drastically. Meaning the voltage across the coil will become a very large negative voltage. So now we know that when power is removed from the relay we will cause a very large negative voltage. Now why is a diode used?? Capacitors used to be used to lower the voltage a bit since the voltage on a cap can not change instantaneously, now most people would rather use diodes. Why? It takes a little understanding of the diode. Silicon diodes mostly used today have an electric field potential inside of around 0.7 V, this voltage is not a constant however, it will change depending on the current through the diode. The thing is it changes very little, to rise the voltage just a few millivolts may require amps of current. So the voltage across a forward biased diode will never change much from 0.7V. On a reverse biased diode the diode appears to be an open so the voltage drop is zero and the current is zero. Tie it all together: We now see that if we place a diode across the coil opposite of the supplied turn on voltage the diode will be reversed biased and not alter the circuit. But we see that the reverse voltage caused by the decrease in current is going to forward bias the diode. Since the diode becomes forward biased the voltage across it will not exceed around 0.7 V, and the diode will pass the current from the magnetic field. So the reverse voltage stays below 1V and doesnt harm electronics. Hope that helps, sorry it was so long...
Posted By: markcars
Date Posted: March 13, 2004 at 7:44 AM
xetmes, good explanation.
would it not be good idea to have the quencher diode across all relays? since diodes are so cheap, why not use them for all relays? Just a question.
Posted By: kaffeene
Date Posted: March 13, 2004 at 3:29 PM
thanks... whats the best way to hook diodes up to the terminals of a relay along with the other wires?
Posted By: xetmes
Date Posted: March 13, 2004 at 4:24 PM
markcars, I use diodes on all relays because as you said they are very cheap, dont see why you wouldnt. the easiest way is usually just to get a socket for the relay that already includes the diode, watch your polarity though. Partsexpess has them: https://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=330-076
Posted By: markcars
Date Posted: March 14, 2004 at 10:02 AM
as xetmes said, you can buy them readymade (however make sure you connect 85 and 86 with right polarity.
If you have to connect a diode to a relay, still same thing, pins 85 and 86 always are the ones needin a quenching diode. Make sure the polarity of the diode is right whereby you dont short it.
+ve goes to diode's "ring end" also called cathode. -ve goes to other end.
Posted By: bradleonard
Date Posted: March 23, 2004 at 2:55 PM
Here is my question regarding diodes and relays All the diodes that I have seen and used have an amperage rating. The biggest amperage rating I have been able to find is a 3.0 AMP diode. Will a diode rated at 3.0 amps only block 3 amps before it does nothing anymore? I soldered a 3 amp Diode across the 85 and 86 pins of a relay that was attached to a circuit with a 25 amp circuit breaker. When I applied power to either side of the relay coil, the diode caused power to show up on the other side of the relay, therefore causing a short when ground is applied. Why would this happen, does it have to do with the rating of the relay? thanks ------------- werd
Posted By: xetmes
Date Posted: March 23, 2004 at 3:47 PM
the current rating is not very important here, 1A diodes should work fine for this purpose. It sounds like you may have the diode in backwards, when the relay coil is powered the side of the diode with a band (the cathode) should be at the positive side of the relay coil. This is backwards to what many think but that is the whole purpose,only to shunt the negative voltage...
Posted By: bradleonard
Date Posted: March 24, 2004 at 3:20 AM
I guess it makes sense that the diode is simply to block the voltage spike, but as you described the cathode side of the diode should be on the same side as the positive power input. Since a relay can be powered with the positive on either side I put the cathode on the 86 terminal where I also hooked up the positive. This caused a Positive 12volts to show up at the 85 terminal. I tried putting the postive on ht 85 terminal and the exact same thing happened. In this test the diode stayed in the same place the whole time. I will have to try this test again with just a relay and not use a diode to see what happens with voltage is applied. ------------- werd
Posted By: JasonL
Date Posted: March 25, 2004 at 4:30 AM
xetmes wrote:
the easiest way is usually just to get a socket for the relay that already includes the diode, watch your polarity though.
Actually, the simplest way is to purchase relays that have built-in diode or resistive suppression. If you get the latter, you don't have to worry about coil polarity either.
Many Bosch relays have this built in, but you generally need to ask for them specifically. It will we drawn on the schematic on the side of the relay casing.
Posted By: its_radio_shop
Date Posted: March 25, 2004 at 10:29 PM
BradLeonard, dont worry about seeing voltage on both sides of the diode. As long as your Neg lead is NOT on the striped side (cathode) you will be fine. Think of the striped side as a one way "gate" that allows whatever is on the striped side to pass through to the other side. The reason it is not a short circuit when you connect NEG to the other side is this. Electrons will only flow from NEG to POS. So although the POS side sees an open gate to the NEG side, it wont go through because POS electrons will not flow to NEG. Therefore when you put NEG on the striped side and POS on the other side, NEG sees an open gate right to POS...you guessed it, a short circuit. That might be a little too simplified of an analogy but oh well. Ever wonder why they say having a good ground is so important? Electrons actually enter your equipment from the ground cable and flow back to the battery on the POS cable. Oh and by the way, the reason your seeing +12v on both sides of the diode when its soldered to the relay and the ground is not hooked up is because the relays internal coil is connected to both terminals and is very little resistance.
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