simple Adams motor concept 亚当斯电机原理01

These first simplified circuits are here to show and explain some fundamentals pertaining to Hall IC's, Transistors and the reactive characteristics of Induction coils in pulsed motor systems. In particular, they are designed to show a couple of simple steps that can be taken to prevent components from "blowing" due to current surges and over voltage problems. This site was started to help experimental enthusiasts to keep their component budgets within acceptable limits.

This site is dedicated to electronics beginners and I will try to keep all explanations as simple as possible. If you have an intermediate to advanced knowledge of electronics then you will already be familiar with the circuit concepts and explanations presented here.

In relation to Fig 2 above, first lets examine the characteristics of the Hall IC. Whilst it looks very similar to a transistor, there are significant differences in its mode of operation. Like transistors though, they come with varying degrees of recommended maximum voltage and current ratings. Most Hall IC's are designed to operate on voltages between 5 + 10 Volts. And most have Source to Emitter junctions that only allow up to 100MA current flow before they are strained beyond their operating parameters. At this stage, we will neglect the internal resistance of the Hall IC and the voltage drop across the Transistor Q1 Base to Emitter junction and concentrate on the Hall and the Motor Induction Coil only.

In Circuit A, when the motor is first connected and a magnet passes by the Hall IC for the first time to activate the Hall IC's "on" state, the only limiting factor on the current flowing through the Hall IC's Source to Emitter junction is the actual DC resistance of the motor coil.

Lets assume that the supply voltage is only 1.5 volts, and the DC resistance of the motor coil is 10 ohms. This means that 1.5V divided by 10 ohms resistance gives us an initial maximum current of 150 MilliAmps. Already, the circuit is in potential trouble because we are 50 MilliAmps above the nominal current rating of the Hall IC. Now even if we subtract the .6V (for silicon) drop across the Transistor Q1 base to emitter junction, we are left with .9V divided by 10 ohms, giving us 90 MilliAmps current flow. Which would still be too much for some Hall Ic's which are often only rated at between 30 to 50 Milliamps. Fortunately there is an internal resistance in the Hall IC, even when it is in the "on" state, and some of the voltage drop in the whole circuit is also actually across the Motor Coil.

At just 1.5 Volts supply, the Hall IC will probably survive the surge from the initial turn on. As the motor starts to run, and picks up speed, the likelihood of the Hall IC blowing will actually decrease. Why? Well now we have to look at the characteristics of the motor coil, which has not just a DC resistance due to the length of wire, but also exhibits a characteristic called Inductive Reactance. When a coil is subjected to straight out DC, it only has DC resistance. But when it is subjected to Pulsing DC or AC, it also exhibits Inductive Reactance, (also known as impedance), and this is also measured in Ohms. As the motor spins faster and faster, the frequency of the pulses increases, and while the DC resistance of the coil remains steady, the impedance increases with the frequency. So the coil exhibits an ever increasing total Ohmage which reduces the current flow through it and every other component which is in series with it. Because the coil is in series with both the Transistor base to emitter junction and the Transistor Collector to Emitter junction, the current will be reduced in both current paths through Transistor Q1.

On top of that, normally, the fast rotating magnets that sweep past the coil, also induce an ElectroMotive Force back into the coil which is in the opposite direction to the incoming supply current. This opposing direction of EMF is what is known as Back EMF and happens in all conventional motors regardless of motor type. In the circuits shown above however, this BEMF has no extra ability to reduce the incoming current, but I will explain on Page 2 why this is so. It is suffice to say the increasing impedance of the coil due to the increasing pulse frequency will be enough to lower the running current to acceptable component limits.

Now imagine the initial starting current in the Hall Ic's Source to Emitter junction if the supply is 3 Volts or 4.5 Volts or 6V or 12 Volts. Remembering that Current is Volts divided by Ohms, it is easy to to see that the currents in the Hall IC may well go beyond their recommended maximum range and blow out the Hall IC before the rotor has a chance to reach a speed that provdes enough Inductive Reactance to lower the running current to acceptable levels.

In Circuit B, when the motor is first connected and a magnet passes by the Hall IC for the first time to activate the Hall IC's "on" state, the only limiting factor on the current flowing through the Hall IC's Source to Emitter junction is the Resistor R1, because the Motor Coil L1 is actually now isolated to the circuit path formed in the Collector to Emitter junction of the Transistor Q1. Again we will initially neglect the internal resistance of the Hall IC in the "on" state and the voltage drop across the Transistor Q1 Base to Emitter junction and concentrate on the Hall and the Resistor R1.

Again lets assume that the supply voltage is only 1.5 volts and the Resistor value is 100 ohms. Remembering that Current is Volts divided by Ohms, this means that 1.5V divided by 100 ohms resistance gives us an initial maximum current of 15 MilliAmps. Now this a much more acceptable current flow through both the Hall IC and the Transistor Q1 Base to Emitter junction. Now the transistor base current allows a much higher current to flow through the Collector to Emitter junction of Transistor Q1, due to the current amplification characteristics of all transistors. But the initial current through the motor coil at startup will once again be limited by the the DC resistance of the coil itself which is 10 ohms. So the startup current through the Motor coil and the Collector to Emitter junction of Q1 will be 1.5 V divided by 10 ohms which is 150MA. This is perfectly acceptable to the transistor because the Collector to Emitter junction of Q1 is designed by its very nature to handle higher currents. That is exactly why transistors are used to amplify and what they were designed to do; use a small current in one junction to control a larger current in the other junction.

Once again, as the motor spins faster and faster, the frequency of the pulses increases, and while the DC resistance of the coil remains steady, the impedance increases with the frequency. Again the coil exhibits an ever increasing total Ohmage which reduces the current flow through it and the Transistor Q1 Collector To Emitter junction. But the Transistor Q1 Base to Emitter junction now has no Inductive Reactance component, consisting only of the Resistance R1, so the current flowing through it will never exceed 15 mA during running, nor will it constantly decrease with the rising impedance of the coil.. The advantage of this is that a cleaner, squarer, more consistent controlling pulse will be delivered to the Transistor Base to Emitter Junction, regardless of the increasing impedance of the coil at rising frequencies.

Now imagine the initial starting current in the Hall Ic's Source to Emitter junction if the supply is 3 Volts or 4.5 Volts or 6V or 12 Volts. Remembering that Current is Volts divided by Ohms, it is easy to to see that the currents in the Hall IC will still be within the recommended maximum range up to 3 Volts for Halls rated at 30 mA and 5 Volts for Halls rated at 50mA and 10 Volts for Halls rated 100mA and so on. By Changing R1 to a value of 200 Ohms, most Halls will still be within their rated nomimal current range.

I usually use a value of 1000 ohms for the Hall IC's in my circuits running on 12 Volt supplies, and use what is known as diode clipping in conjuction with "bias" resistors to ensure that the base to emitter voltage and current is maintained at the right levels of the Transistor Q1 for proper operation. I will explain this further when I introduce you to slightly more sophisticated circuits in the upcoming pages.

I hope this information is understandable to you "the reader", as there is a balancing act between brevity and simplicity of explanation and the need for enough information for you to envision what I am trying to impart to you.