Using a collapsing magnetic field to explain the effects of inductance is fine but you are putting the cart before the horse. You say "if this field is able to collapse very quickly" as if it were an independent action, but it is not. How rapidly the field collapses is determined by the external circuit. If there is no path for the current when a switch is opened then the coil current must rapidly stop which causes the external back emf to build up very quickly (as determined by external capacitance) and that is what causes the field to rapidly collapse (V = L di/dt).[FONT="]I explain the “back EMF” generated by a collapsing magnetic field in a completely different way. [/FONT]
[FONT="][FONT="]When a coil is de-energised, the magnetic flux (magnetic lines of force) (magnetic field) collapses and if this field is able to collapse very quickly, it cuts the turns of the coil and produces a voltage in the opposite direction that can be 10, 100 or even 1,000 times greater than the original applied voltage. [/FONT]
[FONT="]You could use a MOV to limit this reverse voltage but many transistors can only withstand a very small reverse voltage , so by using a diode to suppress the voltage , only a very small voltage is allowed to be produced (less than 1v). [/FONT]
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The above is true if there is no contact bounce.The voltage spike can occur if the coil is suddenly de-energized (as by another relay contact or switch) when the coil is at other than the zero current part of its cycle. The nearer to the peak current this occurs, the larger the spike.
This voltage spike does not occur when you energize the coil (although you may get a current surge).
Can you post a schematic of your circuit as you do not say if the SSR is between the transformer and the bridge or between the bridge and the load. Can you also geve details of the nature of the load. An SSR sould not have the same problems as a normal relay as it does not cut the power to the load as soon as the drive signal is removed. It remains conducting until the next zero crossing of the current waveform.
Les.
If you can't trace out the circuit and will not give the nature of the load I have no idea what the cause may be.
Les.
If you can't trace out the circuit and will not give the nature of the load I have no idea what the cause may be.
Les.
Hi WWT,I understand the concept behind using MOV's (Metal Oxide Varistors) to surpress voltage spikes on a collapsing coil.
My problem is this, I need to know, specifically with AC relays at 120 VAC if a spike can be created when the relay is first fired depending on the part of the cycle the coil is first engergized in. e.g switched on right at the point of zero crossing of the AC line vs turning on at the peak.
Additionally, as the coil of an AC relay in essence charges and collapses 60 times a second, what keeps the coil from spiking with every cycle?
Help! I need to understand this in detail.
Hi WWT,
May I suggest simply turning on the solid state relay (SSR) at zero volts. You can use a standard chip to do this or do it yourself.
The SSR, if it is an AC type, can only turn off at zero volts, so you will not get a voltage spike at turn off.
By the way the voltage generated by an inductor is, -L * dI/dT, where, L is the inductance of the inductor in Henries, dI is the change of current in Amps, and dT is the time in seconds. It, thus follows that, if you disconnect an inductor from its current source instantly the voltage generated by the inductor will be infinite, in theory that is. Because the inductor is not perfect, and because of parasitic capacitances and resistances, the voltage will not be infinite, but it could be 2 to 5 times, typically for a relay, of the input voltage (that is causing the current flow).
spec
Hello
The SSR is a Zero Point crossover type and the control circuit already does this. However there are still spikes getting through !. The Inductor is not being suddenly disconnected from its source. The voltage into the inductor primary coil remains constant. However it is the load which is being disconnected suddenly and thus the spikes are being generated within the secondary coil.
I really did not want to get into an in depth conversation about the circuit design. And I don't want anyone to try and figure out ways to stop the spikes in the first place. I simply wished to know if I am using the MOV correctly by placing it across the output of the secondary coil. Or should I be using two MOV's linked together at an Earthing point ?.
OK GGEL, got the message,
I think that it has been explained to you that the normal sine wave voltage supply does not create voltages higher than itself.
So on to the voltage spike suppression that you inquired about:
It is normally best to suppress any transient voltage generators at source, if possible. In your case the only source, as you state, is the secondary coil, so it would be best to connect the voltage suppressors across the secondary coil, remembering to use as short leads as possible and also baring in mind that large currents will flow (for a short period) when the suppressor is doing its job so sturdy conductors and joints are necessary.
There are better suppressors than MOVs, which I was going on to describe, but you are obviously are not interested.
spec
See my revised post 15I also have a CR Snubber circuit across the secondary side and have seen with my scope that it does help with the smaller spikes. However the larger spikes which are causing the problem are rare and may occur a couple of times a day. Unfortunately I am not going to sit an stare at a scope screen all day to catch it.
See my revised post 15
OK GGEL, got the message, but I was not answering your post- you have hijacked this thread which is against the rules of ETO- you should start your own thread!
spec
Whoops, sorry GGEL- my error. I have modified post #15.I have hardly hijacked the post, you are seriousley over reacting !.... I had one simple question about the installation on an MOV, which should only have taken "one" simple reply. Plus this thread is quiet old with its last post in 2011 and was dormant when I posted to it. As I said, my question was so similar to the original poster, it made sense to see if I could get an answer here.
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