2 Amp Electromagnetic Chaser

Thread Starter

William Schneider

Joined Apr 21, 2018
6
I am trying to make a series of electromagnets that cycle like an LED chaser. I am going to use it on a maglev model train. I am going to make a test track ten magnets long to make sure my concept will work and then I am going to make a track ~ 300 electromagnets long. I was thinking I would do this by taking an LED chaser circuit and replacing the LED with a NPN transistor like these https://www.adafruit.com/product/976?gclid=EAIaIQobChMIrbDKjp-L2gIVEJN-Ch3FUQ_-EAQYAyABEgIA8PD_BwE and wiring the electromagnets with that. I was going to use this power source https://www.amazon.com/dp/B01M3372K...olid=1BCVNFYS0SCVN&psc=0&ref_=lv_ov_lig_dp_it for the train and wire it so that the chaser circuit is wired low amps by placing a resistor before it. The collector on the transistor would be wired in parallel to the chaser so the chaser would have low eneough amps running through it but the electromagnets would have all two amps. Will this work or is there an easier way? Any help is appreciated. The goal is to have one circuit that will work at a length of around 10 but that can be extended to ~300 long effectively.
 

wayneh

Joined Sep 9, 2010
16,400
I am trying to make a series of electromagnets that cycle like an LED chaser. I am going to use it on a maglev model train. I am going to make a test track ten magnets long to make sure my concept will work and then I am going to make a track ~ 300 electromagnets long. I was thinking I would do this by taking an LED chaser circuit and replacing the LED with a NPN transistor like these https://www.adafruit.com/product/976?gclid=EAIaIQobChMIrbDKjp-L2gIVEJN-Ch3FUQ_-EAQYAyABEgIA8PD_BwE and wiring the electromagnets with that. I was going to use this power source https://www.amazon.com/dp/B01M3372K...olid=1BCVNFYS0SCVN&psc=0&ref_=lv_ov_lig_dp_it for the train and wire it so that the chaser circuit is wired low amps by placing a resistor before it. The collector on the transistor would be wired in parallel to the chaser so the chaser would have low eneough amps running through it but the electromagnets would have all two amps. Will this work or is there an easier way? Any help is appreciated. The goal is to have one circuit that will work at a length of around 10 but that can be extended to ~300 long effectively.
I’d use a MOSFET instead of a darlington because they can switch larger currents with essentially zero base (gate) current and very little heat dissipation. I keep IRF540 on hand but there are many more modern parts. Just about any MOSFET can handle the 2A load.
 

Colin55

Joined Aug 27, 2015
519
The first thing you have to do is produce a coil around a soft-iron core and then have a soft iron material attached to a base-plate.
Then you have to activate the coil and make sure the base-plate lifts.
Then you have to see how much linear motion your can obtain.
 

wayneh

Joined Sep 9, 2010
16,400
The MOSFETs should be between ground and the load, not between +6V and the load. With only 6V for a gate voltage, you should probably choose a logic-level MOSFET so that it turns fully on. If the load is very small compared to the current rating, you may be ok with a regular MOSFET. Finally, you need a snubber diode in reverse parallel with each coil to prevent a voltage spike from damaging the MOSFET when the coil current is shut off.

One last thing. There is such a thing as a MOFET array. If it has a current rating that works, it could be a good choice.
 

AnalogKid

Joined Aug 1, 2013
8,675
You have the MOSFETs connected as source-followers (a parallel to an emitter follower). With a much higher battery voltage this might work, but in you case the MOSFET threshold voltage decreases the voltage available across the solenoid coils. The available voltage would actually be larger if you stuck with the TIP120, but there still might not be enough voltage to insure reliable operation.

As above, the "right" way is to connect all of the MOSFET sources to GND, all of the drains to solenoids, and all of the solenoids other ends to Vbat. In this way the transistor can act as a saturated switch with a much lower voltage drop across it.

Also, you *must* add a suppression diode across each solenoid. A MOSFET can turn off very quickly, and the resulting magnetic field collapse will generate a voltage spike across the solenoid coil that can reach hundreds of volts. This is the same principle that helps turn 12 V into 20 kV across a spark plug. A power MOSFET has an intrinsic zener diode internally; don't rely on it.

Increase VR, R1, and R2 by 10x and decrease C1 by 10x to have the same timing range with less stress on the 555 guts.

Add a decoupling capacitor across each IS's power and ground pins, as close as possible with short leads.

ak
 

AnalogKid

Joined Aug 1, 2013
8,675
Also the timing system was based off of this schematic
I hope it's *only* the timing. That schematic has a critical error, and the 555 will fail shortly after it starts operating.

What is the purpose of R3? It is between the battery minus, which normally would be the circuit ground, and the ground connections of everything except the MOSFETs. This will not work.

ak
 

Thread Starter

William Schneider

Joined Apr 21, 2018
6
What is the purpose of R3?
I intended to place that there so the 555 and 4017 would not need to handle all two amps. I think I placed it in the incorrect spot. I wired the MOSFETS in parallel with it so the coils would still run off of a full two amps. Is this better?
That schematic has a critical error
Also, I based most of this circuit off of that schematic, will this work or does it have the same flaw?
Capture.3.PNG
 

AnalogKid

Joined Aug 1, 2013
8,675
There is no need for R3. The 555 and 4017 do not "handle" the 2 A; only the MOSFETS do that.

A cleaner way to draw a schematic is to let non-connecting traces cross with straight lines, and use connection dots for connections. Never have two crossing traces that are supposed to connect form a 4-point connection. Stagger one of the traces so there are two adjacent 3-point T connections with dots. This is unambiguous and will survive photocopying, scanning, faxing, etc.

For the values in your schematic, what are the minimum and maximum operating frequencies, and the min and max ON times for the MOSFETs?

A schematic is not an assembly drawing; its primary function is to show signal relationships, not physical ones. It almost always is better for the information flow in a schematic if you do not use a component's actual pinout as its decal. Feel free to rearrange the pin locations to improve clarity. Signals flow from left to right; power from top to bottom. For example, put the 555 timing pins on the left and output on the right; 4017 clock and control pins on the left, and all outputs on the right (0 on the bottom, 9 on the top). In this way, the signal flow is from left to right through each component and across the sheet. Also, you can eliminate a lot of crossing lines if you use Vcc and GND symbols.

ak
 

AnalogKid

Joined Aug 1, 2013
8,675
The 555 is operating as an astable multivibrator, an oscillator. Its frequency is adjusted by changing the value of VR. Using the equation in the datasheet (or an online calculator site), what are the max and min frequencies of oscillation as VR changes from its min to max values?

ak
 

wayneh

Joined Sep 9, 2010
16,400
There is no need for R3.
Ditto.

The only possible advantage I can think of, and I'm not even sure of, is that R3 and C1 (which is oriented backwards in the drawing) can act as an RC filter to protect the two ICs against noise on the power supply. Switching those solenoids will potentially be generating a lot of noise. But I think 24K Ω is too large a resistance, and that there may be better ways to isolate the ICs from solenoid switching noise.
 

Tonyr1084

Joined Sep 24, 2015
5,191
Looks like you're on track for the circuit. But isn't a MagLev supported by opposing magnetic poles (north to north for instance)? If so, how do you plan on making the train move from magnet to magnet? Unless you have two perm magnets opposing to provide the lift and the coils to provide the traction force to move the train.

Gotta be honest here, this sort of looks like the beginnings of a rail gun. Hope I'm wrong. Rail guns are a prohibited topic.
 

Tonyr1084

Joined Sep 24, 2015
5,191
Two more considerations in your schematic: It's not necessary to draw an IC with its pin-out. It's OK to have pins 2 and 6 side by side on your 555, and it's OK to draw the 4017 with all its outputs on one side with the inputs and controls on the other side. The second consideration is that you don't have to draw out the entire common ground (negative line) throughout your circuit. A simple pointed down triangle can suffice to indicate a common connection of ground points.

[afterthought] Power and ground points on many IC's don't need to be drawn in because they're just assumed to be there. You don't see gates drawn with power and ground. Same is true of OpAmps.
 

Thread Starter

William Schneider

Joined Apr 21, 2018
6
Unless you have two perm magnets opposing to provide the lift and the coils to provide the traction force to move the train.
That is essentially what I was planning on doing and no I am not making a rail gun.

On a different note. is this circuit, as drawn, correct other than the resistor values for controlling the 555 IC and the C1 value and orientation?
 
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