Controlling repulsion of a magnet with electromagnet and SCRs.

Thread Starter

atferrari

Joined Jan 6, 2004
3,487
I intent to do some tests with a small electromagnet repelling a hanging magnet (imagine a pendulum) in a controlled way.

With the switch open, I plan to discharge the cap on the inductor via SCR1 and to cut the discharge at a certain instant after, through SCR2. Both tyristors under control of a micro so I can adjust the timing finely.

For the moment I am planing to work with no more than 30V (and probably even much less) and if at all possible with low current, whatever that is in this context.

1 - Do I need a diode in parallel with the inductor? If so, how it should be connected?

2 - A bigger magnet against the same coil will result in a more effective repulsion?

Comments, caveats, suggestion are appreciated.

Disparo acotado20191111_03.jpg
 

shortbus

Joined Sep 30, 2009
7,075
I intent to do some tests with a small electromagnet repelling a hanging magnet (imagine a pendulum) in a controlled way.
Comments, caveats, suggestion are appreciated.

Can't comment on the circuit, but a question on the set up. Your only going to get one repulsion out of it the way it looks. As the magnet swings back it will crash into the electromagnet. That will severely disrupt it until it comes to rest again from gravity. Or am I missing something? And a second thing is, if you are using an electromagnet with a steel core, what will keep the magnet from being attracted to that core?
 

Thread Starter

atferrari

Joined Jan 6, 2004
3,487
Can't comment on the circuit, but a question on the set up. Your only going to get one repulsion out of it the way it looks.
That is what I need. One at a time.
And a second thing is, if you are using an electromagnet with a steel core, what will keep the magnet from being attracted to that core?
Learnt that the hard way...and even posted about it some time ago. No core.
 

cmartinez

Joined Jan 17, 2007
6,593
I intent to do some tests with a small electromagnet repelling a hanging magnet (imagine a pendulum) in a controlled way.

With the switch open, I plan to discharge the cap on the inductor via SCR1 and to cut the discharge at a certain instant after, through SCR2. Both tyristors under control of a micro so I can adjust the timing finely.

For the moment I am planing to work with no more than 30V (and probably even much less) and if at all possible with low current, whatever that is in this context.

1 - Do I need a diode in parallel with the inductor? If so, how it should be connected?

2 - A bigger magnet against the same coil will result in a more effective repulsion?

Comments, caveats, suggestion are appreciated.

View attachment 191158
I'm not sure if you need the diode... although I would definitely place it as a precaution. Place it in parallel with the coil, in a direction opposite to ground. That is, with the diode's "arrow" pointing towards SCR1.
 

profbuxton

Joined Feb 21, 2014
418
How are you going to charge the cap? Also need to ensure the hanging magnet is being repelled in direct line, not swinging round at angle. Ensure that swinging magnet cannot get close enough to attach itself to solenoid. What will stop hanging magnet just turning around on suspension? A lot depends on the mechanical arrangement.
A reverse connected diode across solenoid is recommended.
Dont know if microswitches will be suitable controls, may need some sort of electronic switch(photocell etc)(reread, you mean micro controller I guess).
 

AlbertHall

Joined Jun 4, 2014
8,516
I'm not sure if you need the diode... although I would definitely place it as a precaution. Place it in parallel with the coil, in a direction opposite to ground. That is, with the diode's "arrow" pointing towards SCR1.
That is, with the diode's "arrow" pointing towards away from SCR1.
 

Thread Starter

atferrari

Joined Jan 6, 2004
3,487
Thanks to all replying.

What is the purpose of D1?
@Alec_t @AlbertHall @shortbus

D1 went in the wrong place, sorry. But then, where and between what points? Catode to B and anode to A?

I intend to repell a magnet by pulsing an air core solenoid. That simple; no DNA

Any comment on my second question?

2 - A bigger magnet against the same coil will result in a more effective repulsion?
Disparo acotado - toma 202.jpg
 

cmartinez

Joined Jan 17, 2007
6,593
On
That is, with the diode's "arrow" pointing towards away from SCR1.
Now you got me confused, Albert. It's always been my understanding that a snubber diode should always be installed in inverse-parallel to the load... unless there's a confusion between "towards" and "away", which is quite common when translating Spanish to English.

Anyway, here's what I meant:

1573574252534.png
As for the differences in language that I just mentioned, if you tell an English speaking person to draw a smiley face in front of the sentence "Here pretty kitty", here's what that person would do:

:D Here pretty kitty​



But if you ask a Spanish speaking person to do the same thing, the result would most likely be this:

Here pretty kitty :D
 

Bernard

Joined Aug 7, 2008
5,285
Might review pendulum in Search Threads. In some cases a reflective IR sensor was used for position, others used single or dual coil.
 
Last edited:

Bernard

Joined Aug 7, 2008
5,285
Subject to review, I would put SCR cathode & one side of coil @ N, neutral, anode to C+ & current limiting R, C- to coil. + supply , charging switch, to R. May add diode, k to N for charging current to bypass coil. SCR should self reset if SW is open or R is large enough.
 

TeeKay6

Joined Apr 20, 2019
450
Thanks to all replying.



@Alec_t @AlbertHall @shortbus

D1 went in the wrong place, sorry. But then, where and between what points? Catode to B and anode to A?

I intend to repell a magnet by pulsing an air core solenoid. That simple; no DNA

Any comment on my second question?



View attachment 191220
I suggest that this is a better approach:
Emagnet.PNG
Operation: Assume that R2 is greater than 30/U2HoldingCurrent (and also > 30/U1HoldingCurrent). Rmag is the DC resistance of the magnet coil. Close S1. Nothing happens. Trigger U2. C1 will charge due to current through the switch, through R2, through U2. Charging will continue until either the current drops below the holding current of U2 or the switch is opened. Assume that C1 has charged to about 30V, the U2 trigger is removed, and U2 has turned off. The top terminal of the C1 symbol will be + relative to the bottom terminal. Nothing further happens other than self-discharge through C1. Now, while C1 is still charged, trigger U1 for at least a minimum time as described below. When U1 turns on, current will flow from C1 through Lmag(net) and Rmag, through U1, and through D1, then returning to C1. In addition, if the switch is still closed, current through R2 will also flow through U1. The electromagnet will be energized gradually as its inductance limits the rate of rise of its current (while at the same time C1 is discharging). At some point* the discharge current through U1 will drop below the holding current rating for U1. If the U1 trigger signal has been removed prior to this point, then U1 will then turn off. If the U1 trigger signal is still present, then U1 will remain on (unless the resonance of the RLC circuit has caused the current through U1 to reverse). To recharge C1, the U1 trigger signal must first be removed and then U2 must be triggered to allow C1 to recharge via R2. *This point is difficult to calculate as C1 and L1 are part of an RLC resonant circuit. I am aware that this circuit does not yet meet all the requirements you stated in your first post. If you are interested, we can look at appropriate mods.
 

Thread Starter

atferrari

Joined Jan 6, 2004
3,487
Hola @TeeKay6

I much appreciate your reply.

My original circuit (and the second!!) are pure nonsense because the capacitor has to be somehow in parallel with L1 to finally discharge through it.

My idea of two SCRs tried to replicate this: 20 years ago I implemented a shortcircuit protection of a series PSU where a SCR was triggered to cut the pass transistor, diverting the base current to ground. To reset the PSU I simply used a NO pussbutton in parallel with the SCR.

When pressed down, the pushbutton also diverted the base current to ground but now cutting the SCR. Pretty much what I tried to get by using a second SCR in my horrible circuit.

The timing (when and how long current will flow through the inductor) would be between the triggering of both SCRs.

This brings another point: both SCRs should be before the inductor to effectively control the current through it.

Since my tests are focused on the pulse duration (timing defined with both SCR as said above) I planned to switch manually to load the cap.
 

TeeKay6

Joined Apr 20, 2019
450
Hola @TeeKay6

I much appreciate your reply.

My original circuit (and the second!!) are pure nonsense because the capacitor has to be somehow in parallel with L1 to finally discharge through it.

My idea of two SCRs tried to replicate this: 20 years ago I implemented a shortcircuit protection of a series PSU where a SCR was triggered to cut the pass transistor, diverting the base current to ground. To reset the PSU I simply used a NO pussbutton in parallel with the SCR.

When pressed down, the pushbutton also diverted the base current to ground but now cutting the SCR. Pretty much what I tried to get by using a second SCR in my horrible circuit.

The timing (when and how long current will flow through the inductor) would be between the triggering of both SCRs.

This brings another point: both SCRs should be before the inductor to effectively control the current through it.

Since my tests are focused on the pulse duration (timing defined with both SCR as said above) I planned to switch manually to load the cap.
I do not understand all of what you wrote. Be aware that you cannot turn off an SCR or Triac by removing the gate drive; once triggered the device will conduct current until the current drops below its holding current rating--regardless of how/why the current drops. That is, a second SCR cannot turn off the first SCR unless the second SCR stops the current flow through the first SCR (and that second SCR will remain conductive until its current drops below its holding current rating).
 

cmartinez

Joined Jan 17, 2007
6,593
Agustín, my friend, I too am having difficulty understanding what you're trying to do. Could you describe in the simplest manner possible what you're physically trying to accomplish? Maybe we could come up with better alternatives than the one you're currently pursuing.
 
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