I'm putting together a project for school which uses a bank of electromagnets to interact with a bath of ferrofluid placed above them, and I was wondering if some of you on this forum might be willing to give me some guidance as I learn (the basics of) design for these circuits. The end goal is to control these with Arduino and be able to influence the topography of the ferrofluid surface in response to user input. Interactivity will be accomplished by using photoresistors to feed values to the Arduino. I'm in a good place with the programming, but I'm inexperienced with building physical electronics; this is why I'm here.
I've been building and testing electromagnets for the past few days and I'm at a point where I have the result I want, but I don't know how to implement it in a way that is safe and efficient.
This post is organized into three sections: Background Info, Test Results, and Questions.
Background Information
Here are two magnets I recently built:
Two hand-made electromagnets, each wound once down and then once back (two layers of turns). The wire is 24 AMG copper magnet wire on both. The core on the left one is a stainless (I think) hitch bolt, perforated along its length.The core on the right is a ~1 cm iron (I think) bar, cut to ~ 8 cm length. The latter gives me a slightly better result in its effect on ferrofluid, perhaps because its longer shape is causing more of its field to pass through the air around it.
Here is a diagram of my plan so far:
A diagram of the planned approach to the circuit. The two electromagnets shown will need to expand to a bank of nine in a 3x3 configuration. Regarding the power way the power from the PSU is handled, please see question 3 below.
Test Results
Note that I have not built the above circuit yet, I've only tested the electromagnets in a simple circuit to ascertain their function. I have seen good results hooking the electromagnet pictured on the right (above) to the following power supply:
Class 2
Model SY-13180
Input: 120V AC
Output: 13.5V DC : 1800mA (measured at 15.9V DC)
I also tried it with batteries but this was much better because it made the result I am seeking in the fluid, where the effect under battery power was too subtle. However, using the PSU made the magnets warm after just a few seconds, and also got rather warm itself. I'm sure this is because:
The Circuit draws ~ 5.5 amps (measured with multimeter)
And this exceeds the PSU spec of 1800 mA.
Resistance across the electromagnet wires (measured unhooked from the circuit) is ~ 0.7 ohms.
Questions
1. How does one calculate the resistance and wattage values for resistors in a circuit?
For instance, I have a power supply that delivers 16 V, and a device which has 0.7 Ohms of resistance. Is it proper to apply Ohm's law, and say that pairing this device with this power supply, the circuit will always try to pull:
(16 V) / (0.7 Ohms) = 22.86 Amps?
Does Ohm's law work this way? Is the Amps draw for a given device solely dependent on its resistance (a physical constant?) and the voltage supplied?
If so, would it also be true that if this same 16 V power supply is rated for 1800 mA then the minimum resistance any circuit attached to it would need is:
(16 V) / (1.8 A) = 8.89 Ohms?
If so, would lowering the current in a circuit connected to this power supply to 1800 mA mean that the maximum watts of any circuit it powers would be:
(16 V) x (1.8 A) = 28.8 Watts?
To sum it up, is it true that to make a 16 V / 1800 mA PSU safely power a device with 0.7 Ohms of resistance, one would need to add a resistor rated at [8.89 Ohms - 0.7 Ohms] = 8.19 Ohm (or greater?) // 28.8 Watt (or greater?) resistor in series with the device? Is this how I would spec a resistor to protect a power supply in this situation?
2. In general how does one work with the physical specifications of an electromagnet to manage heat, strength of field and power requirements?
I understand that the resistance of the magnet will be dependent on the length and gauge of the wire in the winding. So it would seem that if my calculations in Question 1 are correct, I simply need to either place, in series with the electromagnet, a resistor with the appropriate specs. Or I would need to increase the turns in my winding until I have enough resistance in the electromagnet itself. *
But I'm also curious about how I might accomplish more with less. I see on Wikipedia that using less current by doubling the number of turns in the winding will dramatically reduce the power loss (presumably making the magnet run cooler).
Since the magnetic field is proportional to the product NI, the number of turns in the windings N and the current I can be chosen to minimize heat losses, as long as their product is constant. Since the power dissipation, P = I2R, increases with the square of the current but only increases approximately linearly with the number of windings, the power lost in the windings can be minimized by reducing I and increasing the number of turns N proportionally, or using thicker wire to reduce the resistance. For example, halving I and doubling N halves the power loss, as does doubling the area of the wire. In either case, increasing the amount of wire reduces the ohmic losses. For this reason, electromagnets often have a significant thickness of windings.
Am I to understand that by simply adding more layers of turns I could both cool the electromagnet down and increase the resistance while keeping the strength of the magentic field the same?
* Because this question is oriented toward my general knowledge about how resistance works, I'm putting the option of changing power supplies aside, although this is also a possibility.
3. What types of components should I seek for distributing power to an array of electromagnets as shown in my diagram above?
Assume that I have nine electromagnets, each with 50 Ohms of resistance (this is a different spec than my hand-made ones; this is a spec for an industrially-made electromagnet which I have access to), and that hooking each of them individually to a 12 V source accomplishes what I need in terms of strength of field and effect in the fluid (which it doesn't quite, but I'm using them here as an example).
How would I handle distributing power to each of them so that
Each has access to exactly 12 Volts, and...
They can turn on and off independently without overloading the circuit, and...
They can turn on and off independently without affecting the strength of the others' fields?
To whit, if I were to power these electromagnets through a series of 12 V regulators wired to the PSU in series (I believe that's what I show in my diagram above), would I need a power supply capable of giving:
(12 V) x (9 electromagnets) = 108 Volts?
Or can I just use a 12V supply?
Would wiring the voltage regulators in series be another option?
What kind of device should I look at for taking a 108 V power supply and dividing it into nine sources limited to 12 V? In the diagram I show voltage regulators available in my school's lab (LM7809C. These only give 9 V but the lab has a similar component that delivers 12), but looking at the specs for this part, I see that it can only take 11.5 – 24 V as input. Would wiring nine of these in series to a 108 V supply (as shown in the diagram above) blow them up, or can I look at each of them as receiving (108 V) / (9 electromagnets) = 12 V because they're all drawing from the PSU? What about wiring them in parallel?
Thank You
I know this post is long, but I've tried to be careful to be exhaustive and to place the information in an easy-to-access format. This way I'm clear and not wasting people's time. I don't anticipate that any one person will answer every question (even though that would be great), so even if you have just a quick comment, please feel free to post a "quote" reply. It's most appreciated. Thanks.
I've been building and testing electromagnets for the past few days and I'm at a point where I have the result I want, but I don't know how to implement it in a way that is safe and efficient.
This post is organized into three sections: Background Info, Test Results, and Questions.
Background Information
Here are two magnets I recently built:
Two hand-made electromagnets, each wound once down and then once back (two layers of turns). The wire is 24 AMG copper magnet wire on both. The core on the left one is a stainless (I think) hitch bolt, perforated along its length.The core on the right is a ~1 cm iron (I think) bar, cut to ~ 8 cm length. The latter gives me a slightly better result in its effect on ferrofluid, perhaps because its longer shape is causing more of its field to pass through the air around it.
Here is a diagram of my plan so far:
A diagram of the planned approach to the circuit. The two electromagnets shown will need to expand to a bank of nine in a 3x3 configuration. Regarding the power way the power from the PSU is handled, please see question 3 below.
Test Results
Note that I have not built the above circuit yet, I've only tested the electromagnets in a simple circuit to ascertain their function. I have seen good results hooking the electromagnet pictured on the right (above) to the following power supply:
Class 2
Model SY-13180
Input: 120V AC
Output: 13.5V DC : 1800mA (measured at 15.9V DC)
I also tried it with batteries but this was much better because it made the result I am seeking in the fluid, where the effect under battery power was too subtle. However, using the PSU made the magnets warm after just a few seconds, and also got rather warm itself. I'm sure this is because:
The Circuit draws ~ 5.5 amps (measured with multimeter)
And this exceeds the PSU spec of 1800 mA.
Resistance across the electromagnet wires (measured unhooked from the circuit) is ~ 0.7 ohms.
Questions
1. How does one calculate the resistance and wattage values for resistors in a circuit?
For instance, I have a power supply that delivers 16 V, and a device which has 0.7 Ohms of resistance. Is it proper to apply Ohm's law, and say that pairing this device with this power supply, the circuit will always try to pull:
(16 V) / (0.7 Ohms) = 22.86 Amps?
Does Ohm's law work this way? Is the Amps draw for a given device solely dependent on its resistance (a physical constant?) and the voltage supplied?
If so, would it also be true that if this same 16 V power supply is rated for 1800 mA then the minimum resistance any circuit attached to it would need is:
(16 V) / (1.8 A) = 8.89 Ohms?
If so, would lowering the current in a circuit connected to this power supply to 1800 mA mean that the maximum watts of any circuit it powers would be:
(16 V) x (1.8 A) = 28.8 Watts?
To sum it up, is it true that to make a 16 V / 1800 mA PSU safely power a device with 0.7 Ohms of resistance, one would need to add a resistor rated at [8.89 Ohms - 0.7 Ohms] = 8.19 Ohm (or greater?) // 28.8 Watt (or greater?) resistor in series with the device? Is this how I would spec a resistor to protect a power supply in this situation?
2. In general how does one work with the physical specifications of an electromagnet to manage heat, strength of field and power requirements?
I understand that the resistance of the magnet will be dependent on the length and gauge of the wire in the winding. So it would seem that if my calculations in Question 1 are correct, I simply need to either place, in series with the electromagnet, a resistor with the appropriate specs. Or I would need to increase the turns in my winding until I have enough resistance in the electromagnet itself. *
But I'm also curious about how I might accomplish more with less. I see on Wikipedia that using less current by doubling the number of turns in the winding will dramatically reduce the power loss (presumably making the magnet run cooler).
Since the magnetic field is proportional to the product NI, the number of turns in the windings N and the current I can be chosen to minimize heat losses, as long as their product is constant. Since the power dissipation, P = I2R, increases with the square of the current but only increases approximately linearly with the number of windings, the power lost in the windings can be minimized by reducing I and increasing the number of turns N proportionally, or using thicker wire to reduce the resistance. For example, halving I and doubling N halves the power loss, as does doubling the area of the wire. In either case, increasing the amount of wire reduces the ohmic losses. For this reason, electromagnets often have a significant thickness of windings.
Am I to understand that by simply adding more layers of turns I could both cool the electromagnet down and increase the resistance while keeping the strength of the magentic field the same?
* Because this question is oriented toward my general knowledge about how resistance works, I'm putting the option of changing power supplies aside, although this is also a possibility.
3. What types of components should I seek for distributing power to an array of electromagnets as shown in my diagram above?
Assume that I have nine electromagnets, each with 50 Ohms of resistance (this is a different spec than my hand-made ones; this is a spec for an industrially-made electromagnet which I have access to), and that hooking each of them individually to a 12 V source accomplishes what I need in terms of strength of field and effect in the fluid (which it doesn't quite, but I'm using them here as an example).
How would I handle distributing power to each of them so that
Each has access to exactly 12 Volts, and...
They can turn on and off independently without overloading the circuit, and...
They can turn on and off independently without affecting the strength of the others' fields?
To whit, if I were to power these electromagnets through a series of 12 V regulators wired to the PSU in series (I believe that's what I show in my diagram above), would I need a power supply capable of giving:
(12 V) x (9 electromagnets) = 108 Volts?
Or can I just use a 12V supply?
Would wiring the voltage regulators in series be another option?
What kind of device should I look at for taking a 108 V power supply and dividing it into nine sources limited to 12 V? In the diagram I show voltage regulators available in my school's lab (LM7809C. These only give 9 V but the lab has a similar component that delivers 12), but looking at the specs for this part, I see that it can only take 11.5 – 24 V as input. Would wiring nine of these in series to a 108 V supply (as shown in the diagram above) blow them up, or can I look at each of them as receiving (108 V) / (9 electromagnets) = 12 V because they're all drawing from the PSU? What about wiring them in parallel?
Thank You
I know this post is long, but I've tried to be careful to be exhaustive and to place the information in an easy-to-access format. This way I'm clear and not wasting people's time. I don't anticipate that any one person will answer every question (even though that would be great), so even if you have just a quick comment, please feel free to post a "quote" reply. It's most appreciated. Thanks.
