Bigger flyback surge ... relay with fixed core or relay with moving core? - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: March 02, 2020, 04:28:32 pm
content: Two relays with coils of same DC voltage, one with fixed core, one with moving core. Inductance for moving core coil, with core inside, equal to that of fixed core coil. Assume no flyback suppression. Which will have the highest peak voltage on flyback? How will the voltage vs time profiles of the flyback surges differ?Fixed core relays are more common and consist of an electromagnet moving a ferrous switch-bar against the force of a spring. Moving core relays consist of a solenoid moving a switch-bar against the force of a spring.I suppose answering the questions with regard to the moving core relay might require additional data such as inductance with core withdrawn, dimensions of coil and core, mass of core and velocity profile of core movement. The situation is one of a coil with a core and current flow. And then having the core withdrawn by a spring immediately after the current is stopped.Perhaps an empirical comparison has been done, but I haven't been able to find one.Mike in CaliforniaEdit: If you want more specificity on the circuit connected to coil, let's make it simple: A 1000 ohm, non-inductive resistor in parallel with the coil. 12 VDC coil supplied with that voltage.

author: trobbins
author_rank: Frequent Contributor
timestamp: March 03, 2020, 11:24:37 pm
content: Perhaps if you lead the way, identify and purchase 2 relays that are directly comparable, and test them wit a scope or peak hold meter.

author: calzap
author_rank: Frequent Contributor
timestamp: March 04, 2020, 12:41:31 am
content: I'll think about it. Right now my lab is disassembled due to a new floor being installed. It will probably be quicker to wind my own coils than to use off-the-shelf relays. The reason is manufacturers seldom, if ever, provide inductance specs on relay coils. They spec things like voltage, AC or DC, inrush current, sealed current, must close voltage, must open voltage, etc., but not inductance. I've used my LCR meter to measure inductance on a few, but not found a good match between a fixed core and moving core relay.Mike in California

author: trobbins
author_rank: Frequent Contributor
timestamp: March 04, 2020, 07:00:35 am
content: I've used relay coils for tone/equalisation inductors - some 12VDC like Omron G2R gave about 190mH, although that was just the coil with the bar effectively glued to the face of the coil core to maximise inductance, as the application is AC not DC.The energy in the coil will be the current and inductance when energised. That energy will then transfer to a voltage transient that won't be related to inductance, but rather coil self-capacitance, and any other circuit capacitance (unless clamped by another means).For a DC relay, the operating DC current, and inductance measurement can be set up using the technique in the linked doc below.https://www.dalmura.com.au/static/Choke%20measurement.pdf

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2020, 11:03:53 am
content: Measuring the inductance of the coil, when the relay isn't energised, will not tell you the full story, because the inductance changes, when the armature is attracted to the core.A reasonable estimate of the energy can be gained by connecting a small capacitor in series with the usual anti-parallel diode and monitoring the peak voltage. E = 0.5CV2Of course there will always be some losses in the diode, so the true figure will be slightly higher.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 04, 2020, 01:31:49 pm
content: Drive is undefined, so I choose to drive it with a constant voltage; setting the voltage to zero to turn off the relay results in exactly zero flyback in the circuit.What is a "fixed core" relay, anyway? All the relays I can think of are reluctance based... Magnetized latching relays, perhaps?In any case, the mechanics almost always move slower than the electrical circuit -- and if you're concerned about flyback, that implies an unclamped switch which will discharge quite quickly -- so it makes little difference what the core is and how much it's moved, because it's moved hardly at all in that time.The most common case where it does matter, is the worst case, when using a diode clamp (or as above, with a clamped zero coil voltage). Here the turn-off is limited by the L/R time constant of the coil itself, and contact opening can be several to tens of milliseconds later than it would be otherwise. That is, the mechanics are actually moving while the coil is still discharging. But, this only happens because the flyback is (aggressively) clamped in the first place...In time-critical applications, a modestly clamped flyback voltage is a good idea. Practical example: automotive injectors. They're also lower mass, so the electrical and mechanical time constants can overlap.So, to summarize --1. Driver isn't defined, so we have no idea what the flyback might be, if any at all.2. The armature probably isn't moving much during discharge anyway, so there isn't likely to be a difference due to core, if the electrical parameters are the same.Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 04, 2020, 02:18:09 pm
content: Tim;Please illustrate me, what do you mean by a "modestly clamped flyback voltage"?Like using a resistor in series with the clamping diode as the snubber?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 04, 2020, 04:04:56 pm
content: Sure, or a straight up parallel resistor if you don't mind the DC loss, or a diode into a zener/TVS for a flatter peak, or two-switch inverter to return the flyback to the supply, etc. Anything where Vfb ~ Von.Tim