Solenoid Flyback protection

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Marcodavid

Joined Aug 1, 2022
39
I am planning to make solenoid:

*Coil = 3.5 Ohm Dc - 3.5 mh inductance

*Capacitor = 450v- 1000 uf

*Dc Battery = 12v

*Dc Booster = up to 400v

*Push button switch

Iam willing to operate it on 150v or 200v

I am worrying about the switch to get damage by the back emf

I did some research and found some people are using scr to switch the solenoid and other use a diode ( confused which one to use)

Anyway my question

which is the simplest solution to avoid the back emf and protect the circuit?

IMG-20220831-WA0000(1).jpg
 

DickCappels

Joined Aug 21, 2008
9,337
A diode connected across the solenoid in the direction that prevents current from flowing while the solenoid is being driven. Back EMF from the solenoid will reverse the voltage across is and flow through the diode, limiting the back EMF voltage.

Make sure that all of your components are rated at the expected currents and voltages. For example, that little push-button in the picture may not be ok for 400 volt use, and not much current.
 

Tonyr1084

Joined Sep 24, 2015
7,109
DickCappels is saying this below: Notice the diode is in opposition to the current when powered. When power is removed via any means the coil (solenoid or any type of coil, choke, whatever) the collapse in the magnetic field will induce a current in the same direction it was flowing. The diode captures that BEMF and safeguards whatever switching method you use, whether a switch or transistor or other electronic component. Notice the diode is as close as possible to the coil. If you're using a boost circuit then the circuit itself will protect the switch (assuming 12V) but there's nothing to protect the boost from BEMF. I didn't draw out all the possible switching and control scenario's I just used the 200V you said you may use. Just make sure the diode is rated for 1 1/2 to 2 times the voltage you're going to be using. The BEMF will be much higher than the source voltage, but it's quite short lived as long as you choose one that can handle a period of time for the BEMF to dissipate, which will happen quickly. And no, I can't define "quickly". Just saying it's not going to take several seconds to dissipate the BEMF.
1661944866141.png
But it only works for DC. You need a snubber for an AC circuit.
 

strantor

Joined Oct 3, 2010
6,112
FYI this coil will will draw nearly 60A at 200V. A cute little pushbutton like you have drawn will not work. You will need something much more substantial.
 

Thread Starter

Marcodavid

Joined Aug 1, 2022
39
DickCappels is saying this below: Notice the diode is in opposition to the current when powered. When power is removed via any means the coil (solenoid or any type of coil, choke, whatever) the collapse in the magnetic field will induce a current in the same direction it was flowing. The diode captures that BEMF and safeguards whatever switching method you use, whether a switch or transistor or other electronic component. Notice the diode is as close as possible to the coil. If you're using a boost circuit then the circuit itself will protect the switch (assuming 12V) but there's nothing to protect the boost from BEMF. I didn't draw out all the possible switching and control scenario's I just used the 200V you said you may use. Just make sure the diode is rated for 1 1/2 to 2 times the voltage you're going to be using. The BEMF will be much higher than the source voltage, but it's quite short lived as long as you choose one that can handle a period of time for the BEMF to dissipate, which will happen quickly. And no, I can't define "quickly". Just saying it's not going to take several seconds to dissipate the BEMF.
View attachment 275227
But it only works for DC. You need a snubber for an AC circuit.
The selonoid will be powered for few seconds, so you recommend a diode rated for 1 1/2 to 2 times the voltage between solenoid and the switch, right?
 

DickCappels

Joined Aug 21, 2008
9,337
What is also important is the current rating of the diode. The initial current during back EMF will be approximately equal to the current through the solenoid at the time the voltage to the solenoid is switched off.
 

Tonyr1084

Joined Sep 24, 2015
7,109
you recommend a diode rated for 1 1/2 to 2 times the voltage between solenoid and the switch, right?
Yes, just as shown. GENERALLY!
Like this:
1661958197293.png
There are some recommendations I'd like to make: First, though, it would be helpful to fully understand your application and the goal. What I'm seeing is a battery that is always powering the Buck/Boost converter. Personally I'm not aware of a BB that can take 12VDC and output 150VDC. But if you know of one then I don't need to dwell on that. As you said
The selonoid will be powered for few seconds
I'll assume you also intend to put a switch between the BB and the battery. Otherwise you're going to be unnecessarily draining your battery. Especially if the assembly will only be used "for a few seconds" every once in a while, ranging from maybe once every 5 minutes to once or twice a week - or longer.
Also, I'd recommend the switch being put on the plus side as opposed to the negative side. That would be a more proper circuit of this type.

I'll bang out a schematic for what you show along with my recommended additions in a few minutes.
 

Tonyr1084

Joined Sep 24, 2015
7,109
OK, here's a proper schematic:
1661959627202.png
SW1 can be either a push button or a toggle switch. Most simplest type would be either a "Momentary SPST push button" or a "SPST Toggle" switch. (Just in case you don't understand some terminology, SPST is "Single Pole Single Throw" meaning there's just two points of connection on the switch and the switch itself either makes or breaks contact between those two points).
With SW1 closed (upper diagram) BB Conv. 1 will power up and try to charge the cap (C1). At the same time it is energizing the solenoid, making C1 basically useless in this application. That's not a good idea because the solenoid could keep C1 from charging.
With SW1 closed (lower diagram) BB Conv. 1 will power up and charge C1 to full voltage (depending on the size of the cap and the time it takes to charge). Once charged is is ready (via SW2) to deliver a charge to the solenoid. This is a better approach because with two switches you can fully control the circuit.

I've thought of something similar to launch model rockets. Only instead of a solenoid I'd use an electric match to light the hobby rocket motor. And I wouldn't be using the BB Conv. 12V is more than enough to fire off an electric match, and there wouldn't be a need for any diodes or capacitors.

Could you share with us what you intend to do with your circuit? The "rail gun" idea would require far more solenoids and much different circuitry. And rail guns is a forbidden topic in this forum. So please clarify your goal and approach to achieve it.
 

Thread Starter

Marcodavid

Joined Aug 1, 2022
39
After some research about relays, i saw a video using car relay 12v/40A for solenoid 390v



Screenshot_20220901-021715_YouTube.jpg

Connected to micro switch 15 A

images (40).jpeg
I think it is recommended to use car relay and micro switch instead of my cute push button

But will this relay stand with about 60A while it reated for 40A? Also will protect the components from bemf?
 

strantor

Joined Oct 3, 2010
6,112
A relay is a good candidate for replacing the cute pushbutton. You can even keep the cute pushbutton if you want it, it can power the relay coil no problem.

As for which relay though, the 12V relay you showed is an automotive relay and isn't designed for 200V. You see the other guy putting 390V through one though. It will not last. Does it need to last? Is this device some toy project you're working on? Or is it something that your life might depend on? If you need it to be reliable, forget about that automotive relay and prepare to spend more money.

https://www.mouser.com/c/electromec... Voltage|~Contact Current Rating&sort=pricing
 

Thread Starter

Marcodavid

Joined Aug 1, 2022
39
Ofcourse i need it to be last stand but most relays in markets are rated for 12vdc!

If i want more voltage it will cost more and more money

So i think thyristor should be considered here, cheaper and can protect the circuit and use as switch

But i am not sure about how to choose the right thyristor rating, should it be 200v and 60A or i need higher rating ?
 
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DickCappels

Joined Aug 21, 2008
9,337
Unless you are going to operate the solenoid at a high frequency a diode across the solenoid should be all of the back EMF protection you need.

If you use an SCR to fire the solenoid the SCR needs to be rated to at least the highest voltage it will see and the peak current. Since the peak current will be a short burst the current rating you should pay attention to is the peak-nonrecurring current.

SCRs do not turn themseles off but will remain on until the current drops below the minimum holding current. For this to work the current to charge the capacitor must be low enough so as to not sustain the latching of the SCR. Alternatively, you can open the circuit, put a reverse voltage pulse through the circuit or use a relay or push-button to turn off the SCR.

Some SCRs, referred to as Gate turn-off SCRs can be made to turn off reliably by applying a suitable reverse voltage on the gate. Don't know if those are available today.
 

Tonyr1084

Joined Sep 24, 2015
7,109
Something that may need clarification: A 12 volt relay is one that uses 12 volts to activate the coil. Their contacts have a max voltage rating and a max current rating. You can get a relay that can handle 390 volts, but for gaud's sake - use one with the correct type of current rating. For instance, a relay (contacts) that can withstand 200 volts may stand 200 volts DC but not AC. Or vice versa. One reason why, if you've ever noticed, a relay may be rated for 20 amps at 240VAC but only 5 or 10 amps at 24 volts (didn't forget the zero). Why? Because once contact is made and current is flowing, breaking contact may result in a fiery arc that can burn (flames) the relay. I've seen videos on YouTube showing that exact phenomena, where high current DC was flowing through a knife switch and when the switch was opened an arc jumped across the gap. Obviously at the best outcome would be simply shutting off the circuit. But a worse outcome would be an arc that continues to conduct current to the circuit you needed to be shut off - but it doesn't shut off. Worst of all, the relay could burn down the house.

Using the proper relay rated for the service you intend is the minimum safety suggestion I can make. Using a low current charger to charge a capacitor then have the relay dump that high current into your solenoid would be a safer approach in that once the cap is depleted the current would drop to a point where the current can no longer jump across the contacts. HOWEVER! Arcing contacts will burn out quickly, depending on how badly they're arcing. So choose the right relay for the task.

And I'm still wondering why you need a solenoid with high voltage capability. From the "Ball Kicker" you want a strong punch. OK, I get that. But it looks like in the video (kickball) even at such a high voltage doesn't produce much of a kick. Yes, it kicks. But how well will that work on the grass? Just my opinion here - but I think this project will be a waste of time and resources, and the payoff will be less than satisfying.

Next year I'm going to assemble a compressed air rocket launcher using a couple sprinkler solenoids to hold back a charged cylinder of air pressure then released through a tube with the rocket fitted over that tube, launching the rocket dozens of feet into the air. No need for details because this is your thread and we're discussing a kicker. I just don't see much satisfaction in the results.
 

Tonyr1084

Joined Sep 24, 2015
7,109
Here - watch this video and consider carefully what you're doing. Also note that the video is using 220 volts AC and then DC. You're talking about 390 volts DC. An even greater concern.
 

Thread Starter

Marcodavid

Joined Aug 1, 2022
39
Something that may need clarification: A 12 volt relay is one that uses 12 volts to activate the coil. Their contacts have a max voltage rating and a max current rating. You can get a relay that can handle 390 volts, but for gaud's sake - use one with the correct type of current rating. For instance, a relay (contacts) that can withstand 200 volts may stand 200 volts DC but not AC. Or vice versa. One reason why, if you've ever noticed, a relay may be rated for 20 amps at 240VAC but only 5 or 10 amps at 24 volts (didn't forget the zero). Why? Because once contact is made and current is flowing, breaking contact may result in a fiery arc that can burn (flames) the relay. I've seen videos on YouTube showing that exact phenomena, where high current DC was flowing through a knife switch and when the switch was opened an arc jumped across the gap. Obviously at the best outcome would be simply shutting off the circuit. But a worse outcome would be an arc that continues to conduct current to the circuit you needed to be shut off - but it doesn't shut off. Worst of all, the relay could burn down the house.

Using the proper relay rated for the service you intend is the minimum safety suggestion I can make. Using a low current charger to charge a capacitor then have the relay dump that high current into your solenoid would be a safer approach in that once the cap is depleted the current would drop to a point where the current can no longer jump across the contacts. HOWEVER! Arcing contacts will burn out quickly, depending on how badly they're arcing. So choose the right relay for the task.

And I'm still wondering why you need a solenoid with high voltage capability. From the "Ball Kicker" you want a strong punch. OK, I get that. But it looks like in the video (kickball) even at such a high voltage doesn't produce much of a kick. Yes, it kicks. But how well will that work on the grass? Just my opinion here - but I think this project will be a waste of time and resources, and the payoff will be less than satisfying.

Next year I'm going to assemble a compressed air rocket launcher using a couple sprinkler solenoids to hold back a charged cylinder of air pressure then released through a tube with the rocket fitted over that tube, launching the rocket dozens of feet into the air. No need for details because this is your thread and we're discussing a kicker. I just don't see much satisfaction in the results.
I still confused about how that guy used a car relay 12v/40 to power this solenoid without relay got damaged.

The second point is
My set is 3.5 ohm at 200v which is about 60A
So Should i choose relay that rated for 200v at 60A or should i choose one stand with the back emf voltage and current?

I am planning to make a kicker solenoid for a medical device (sorry can not reveal more about it)
This kick that solenoid is producing is high than what i need that is why i am sure this kicker will work for my project.
 

BobTPH

Joined Jun 5, 2013
5,809
If the relay is discharging the capacitor through the coil , when it releases there will be no voltage left, so no arcing, and it will survive higher voltage and current than its rating.
 
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