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I'm building a circuit to measure both voltage and current given to a solenoid valve 24V DC 15W. In parallel to the valve is a voltage divider with 470k + 100k Ohm resistors. In series with the valve is a 1 Ohm 4W resistor. In parallel with both valve and 1 Ohm resistor is a 1n4007 Diode correctly positioned in order to actuate as a Flyback Diode. But yet, whenever i remove the +24V contact of the valve, simulating a opening of a Relay, I still see some sparks flying around. They are easily noticeable. I've measured the peak voltage and indeed it has lowered: from -10V (minimum readable value) to -250mV. But there are still sparks! Is it ok or will it damage a operating relay?

schematic

simulate this circuit – Schematic created using CircuitLab

thanks!

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    \$\begingroup\$ I feel unwell! Please draw schematics positive to the top, negative to the bottom. It's a good convention, everybody will thank you for it, and you will grow to love it too. \$\endgroup\$ – Neil_UK Sep 25 '16 at 6:40
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    \$\begingroup\$ We also don't know what's sourcing 24V. If it has an inductive component, then breaking the circuit may generate a flyback kick at the driving end. A reverse biased diode or an R-C snubber across the switch contacts may be called for. \$\endgroup\$ – Brian Drummond Sep 25 '16 at 9:50
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If you are simulating the relay by simply pulling a wire out (of a breadboard for instance) there will be a period of connect/disconnect as the wire moves. This will give a series of forward connections to the circuit, each time the inductance will cause an inrush current of some magnitude.

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It is preferred to place the flyback diode directly over the valve coil connections. Remember the flyback diode only protects the valve coil from high induction voltages. Verify the health of the diode. If it is ok then the remaining spark does not harm the valve coil.

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Sparks are... maybe fine! It depends.

What kind of relay is it? Do you see sparks inside your relay? In my experience if you can see sparks inside, your relay contacts are probably going to melt together eventually. Your relay for driving a solenoid valve may be strong enough to prevent this from happening.

You can limit the initial current with a soft-start circuit. This page from Elliot Sound Products has more than you ever wanted to know about soft-start circuits. http://sound.whsites.net/project39.htm


Side note: The forward voltage of the 1N4007 is 1V. This may be fine for your application. I have seen other situations where 1V is too high. Enough noise was created to cause a microcontroller to reset! In this case, a schottky diode (with a forward voltage of ~0.3V) fixed the resetting problem.

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    \$\begingroup\$ Shottky diodes are not suited as flyback diodes for heavy inductive loads in general, because their max peak reverse voltage is fairly low: ~20V..60V is typical, whereas the 1N4007 can stand 1000V. If the OP uses a Schottky he'll have to carefully asses the worst case peak voltage across the coil and then select the diode. It may turn to be expensive (as Schottky diodes with high reverse voltage are). \$\endgroup\$ – Lorenzo Donati Sep 25 '16 at 7:07
  • \$\begingroup\$ Moreover, Schottky diodes have a lower voltage drop than plain silicon rectifiers only with currents quite lower than their rated maximum. Go near their rated maximum and they could be on par, especially those with higher reverse voltage rating. This is worse for current spikes: an 1A Schottky could well stand 10A spikes, but at that current level it could have a 1.2V forward drop. \$\endgroup\$ – Lorenzo Donati Sep 25 '16 at 7:11
  • \$\begingroup\$ Bottom line: a Schottky may be better, but it could be fairly more expensive and the OP will have to read the datasheets carefully. \$\endgroup\$ – Lorenzo Donati Sep 25 '16 at 7:12
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24V sparks, what can you do? It's fine. I mean, if this is a small spark. If you see something like 1mm bright purple spark, it's something else, like no diode. Normal spark from 24V is very (length) short and visible only if you are staring at the contact point. As per spark period- depends on the contact. With good contact it happens once for short period, with bad one- it's a burst of dozens of sparks for 20msec.

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If the contact is arcing I would investigate a snubber circuit across the contacts. If the arcing is just when the contacts open, I would start with a 10 or 20 ohm resistor in series with a 1uF cap.

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