Power Protection Systems for NOOBs

When you think of power protection, you first think of fuses or circuit breakers. While these are the most common devices used in both home and industry, they are far from being the only type of protection device.  Normally the fuse or circuit breaker will only be part of a larger protection system.
Fuses are an over current protection device, designed to interrupt the flow of electricity when the current flower exceeds the rating of the fuse.  Fuses are rated based on the time it takes to blow and the level of current that they can interrupt.  Of the different types of fuses we have:
  • Slow Blow
  • Standard Fuse
  • Fast Blow
  • High Rupture Capacity (HRC)
  • Semiconductor Fuse.
The most common use of a fuse is to protect the wires from the power source to the end device or distribution device; however fuses can be used in conjunction with other protection devices 
Slow blow fuse are normally used where momentary overload of current can occur during normal operation, such as with electric motors that have start currents up to 5 times their rated current then drop back to normal run currents.
Standard fuse are only interested in protecting the cable under normal type loads
Fast blow fuse are used where any level of overload is bad for the equipment or the supply source.
HRC fuses are used when the possible current supplied can be very high such as from a large high voltage battery bank or mains supply
Semiconductor fuses are similar to the HRC but with a very fast blow times, normally used in high power electronic devices such as mains powered Variable Speed Drives or in smaller sized on motor controller circuit boards.
Circuit Breakers have both a current rating and a trip class rating. A - D.
The class is all about the trip time curve, at 100% of the current rating the circuit breaker will not trip, but even 1% over and the breaker will trip, but may take quite a bit of time. The length of time the breaker will take to trip at different amounts of overload it called the trip curve, the higher the overload, the faster the breaker will trip.  The most common trip class used in power distribution systems such as those used in your house are the Class C trip curve.
The last of the common over current devices is known as the Polly Fuse or Positive Temperature Coefficient (PTC) fuses. These devices heat up when too much current passes and become a high resistance almost stopping the power but not quite. When the power source is removed, the Polly will cool down and reset by its self.  These devices are typically used for speaker protection but have been used in high power electronics in some 3D printers.
Over current is only one of the things we need to protect against. Over Voltage can be even more damaging than over current. For the over voltage protection, the humble Zener Diode can be our friend. Using I high wattage Zener Diode at a voltage slightly above the desired voltage and placed across the supply, if the voltage exceeds the rating of the Zener, it will conduct the power to clamp the voltage down.  If the power supply is able to supply a lot of current then the Zener may not last long, so a current limiter such as a fuse or circuit breaker is advisable to disconnect the power source if the voltage goes too high.  This simple method is good for lower power circuit such as the power feed to a computer board or LCD Display. 
One of the most common over voltage problem is called the Transient or Spike.  These are problematic as they can easily damage our delicate electronics.  To remove the Transient spike, one of the most common methods is the L-C Filter Network. This normally consists of an inductor in series with the supply line and with capacitors both and after the inductor across the supply lines. This won't provide protection against continuous over voltage at all, but can be used in conjunction with MOV's to provide an excellent level of protection.  These networks are not suitable for use with motors.
For larger over voltage surges from the likes of static discharges, we can use Metal Oxide Varistors (MOV).  These work on both AC and DC but are rated for much higher voltages than we typically use in out robotics systems, a MOV ratted o 25Vac will clamp the voltage down to 77V and a 275Vac MOV will clamp the voltage down to 710V. These devices will absorb the energy and will quite often self-destruct during an event. They are rated by operating voltage, Clamp voltage, peak current and max energy.  Normally you would put a fuse in front of the MOV to disconnect the supply during a surge.  I have seen a case where the energy that was absorbed by the MOV in a Microwave Oven was so high the MOV, half the HRC fuse holder and the wire between the two was vaporised.  Believe it or not, the rest of the Microwave oven with its electronic timer control survived the incident.
Finally the next most common form of power protection is the under voltage.  In motor circuit, under voltage can be as damaging or more so that over voltage.  In order to turn a shaft with a load on it a required amount of power is needed. Power is Volts X Amps, if you reduce the voltage, the motor will increase the amps to achieve the correct power. All wires currently in general use have a resistance, including the windings in an electric motor, if you pass a current through a resistance, it will heat up, the more current that flows the more heat will be created, in a motor, this is bad as the heat build-up will burn away the insulation around the copper wires causing them to short out. This results in a burnt out motor.
Under voltage detection is more difficult to detect, but can be achieved by using a simple comparator to test a voltage derived from a resistor voltage divider against a known voltage from a Zener diode with a limiting resistor.  While the test point is above the Zener reference voltage, we can say the power supply is ok and turn on a relay to allow power to flow, if the voltage drops too low, we can turn off the relay cutting power from our circuit. In most case the test circuit will have a latching function to prevent the relay from closing once the voltage comes back up as can be the case when batteries are getting low.
So how do we know what type and size protection device do we need?
Well that will depend on what you are protecting.
In most cases you will need to use a combination of protection devices.
Let us work through a hypothetical example of a power system used for a robot.
In our Robot we will need Batteries, a charger, and different voltages for larger DIY servos or wheels and lower voltages for small servos and even lower voltages for the computer control systems.
In this hypothetical robot the control system will be four Raspberry Pi 3 computers with 10 Arduino Nano an Arduino Mega 2560 and 4 Adafruit 16 channel servo drivers based around the PCA9685 PWM driver chip.
For this example we will start with 24Vdc SLA Battery pack, this battery pack consists of 4 X 12 Volt 7 Amp Hour batteries arranged in a series parallel combination to give a 24V 14AH Battery pack.
As a rule of thumb and not at all accurate if we work off 20 times the Ah rating of the battery pack we could potentially have a fault current of 280 Amps.
There are two things we need to look at around the battery pack, Charging and Discharging.
To charge a 24V SLA battery we will need a power supply with both voltage and current regulation. We will set the max voltage to 27 Volts which equates to around 2.25 Volts per cell, and a max current of 5.6 Amps.  This will give you a charge time of around 16 hours. Fast charger can reduce this time to around 8 - 10 hours but can reduce the life of the battery, and you will be looking at a charge current of about 11.2 Amps.  On the charging circuit I would advise two HRC fuses of 12 Amps in series with a 30V MOV across the supply between the two fuses. Since 12A fuse are not easy to obtain, the closest being either 10A or 15A, I suggest the 15A HRC. The MOV after absorbing more than 10 joules of energy can fail in one of two ways, it can go open circuit with very big fault currents, or it can go short circuit more common with moderate fault currents. If it fails open circuit, then most of the rest of our Robot will most likely be toasted normally the result of a direct lighting strike to the charger.  If it fails short circuit, it will blow both the incoming fuse from the charger, the source of the problem, and then also the fuse from the battery.  If the input voltage from the charger is not quite enough to trigger the MOV the battery will absorb it at a higher current blowing one or both fuses.  In any case, the battery and the rest of the robot will be protected from a failure in the charger.  The HRC type fuse is recommended because of the high prospective fault current that can flow from the battery in the case of the charger becoming a short circuit or the MOV becoming a short circuit.  For added safety you could also add a diode in series with the charger to prevent discharge from the battery via the charger, a STPS1545F Schottky Diode rated at 15A and 45V should do the job nicely and a Zener Diode across the supply rated at 30 Volts should trigger before the 30V MOV which will trigger at around 70V.
 It is recommended that for maximum battery life that you keep the discharge current to less the 0.05 x the rating of the battery, for this pack that's 0.7 Amps, or 16.8 Watts.  Should be enough for a small computer and associated control gear, an Arduino Nano or Mega 2560 is around 0.1 Watt's, a Raspberry Pi is around 2.5 Watts.  We will be looking at a standby load of around 11.1 Watts, well within the 16.8 Watts of the battery for max life. We will of course go well beyond this when the robot starts moving around.
We need a very stable 5V power supply for the 4 Raspberry Pi 3's but our battery pack is 24 Volts and when charging will be up at 27 Volts.  We need to drop the voltage down to 5 Volts with a current of 0.5 Amps per Raspberry Pi 3, that's a total of 2 Amps or 10 Watts.  For this we will us a Switch Mode Power Supply (SMPS) which has an efficiency of between 80% and 90%.  Working on the worst case of 80% the power drawn from the battery will be around 12.5 Watts, at 24V this is around 521 mA.  As the voltage goes up to the 27 Volt charging voltage the current will drop to 463 mA.   The possible fault we could see here are the SMPS failing internally and either shorting out, going open circuit or passing the battery voltage through to the Raspberry Pi's, or a short circuit could occur in one of the raspberry Pi's.  There is also the potential of the battery being disconnected and full uncontrolled charge power being sent to the SMPS. The Zener Diode in our charge protection circuit should stop the voltage from exceeding 30V.  The SMPS failing Open Circuit is not a problem we need to worry about with circuit protection.  For the SMPS failing Short circuit, a HRC fuse is required, rated at the capacity of the SMPS, assuming the SMPS is rated for 3A at 5V that give us about 780 mA so an 800 mA fuse will be plenty. Just a quick note, with the output of the SMPS being only 3A we will need one for each Raspberry pi, each with its own circuit protection, if you know you won't be using the USB ports, you could run all four off the one SMPS.  An M205 Ceramic (HRC) fuse will be fine.  For the problem of the Raspberry Pi going short circuit we need to look first as the max load the Raspberry pi can draw, the Pi it's self is about 500 mA, but it also supplies power to it USB ports.  There are 4 USB ports each rated to 500 mA, the micro USB input is rated to 2.5 Amps.  Since we can get 2.5 A M205 fuse we can use that.  This brings us to the last SMPS failure type, Battery voltage applied to the Raspberry Pi.  The 5.1V Zener Diode is the best protection we can get here, placed across the supply lines after the 2.5A HRC fuse.
The same setup can be used for the Arduinos; however I would suggest a larger SMPS with an output of 6V and a 100ma fuse for each Arduino with a following 6.2 Volt Zener Diode.
The next we have our 24V motors. Motors are an interesting problem because by design they will and do overload every time they start.  The average motor will draw between 5 and 10 time their full rated load during start-up.  A slight over voltage here of 27 Volts on the 24V motors will not be a problem. More dangerous to a motor is an under voltage condition or a significant overload of the motor, the other problem that can occur with a motor is insulation failure normally caused by overheating.  For this example, let's assume our motors are rated at 6 amps with a locked rotor current of 20 amps. We will be using a PWM full bridge controller for both speed and direction. It is not that unusual for a motor on say a wheeled robot to get stuck so an overload will most likely happen, in this case, a circuit breaker is going to be the best choice. 
Most DC circuit breakers will hold their rated current indefinitely and 135% of their rating for 1 hour, and 200% of their rating for 1 minute, the trip time gets much shorter as the overload goes up. If we use 8 Amp DC circuit breaker for our motors, then locked rotor will be 250% of the circuit breaker rating, as the motor should be working well below the circuit breaker rating it will cool down from the start-up. That covers overload and short circuit, but what about under voltage?  This one is a bit more of a challenge, but an under voltage switch as describe earlier is the solution here.
Finally we have our small servo's, these are normally 6V but contain both a motor and sensitive electronics.  Again we will use a SMPS, but this one will be rated at 12A.  To cope with the expected start current of the servos, I would add a couple of large capacitors after the SMPS but before our output circuit protection, I would also use 3Amp HRC fuse as the input power protection for the SMPS The large HS-805BB servos will normally run between 520mA and 1 A depending on load, however they will peak at 8A when they first start to turn or if they get jammed. The large capacitors will help supply the surge the servos need to get going. Depending on the number of servo's you are planning on running, I would suggest no more than 8 per SMPS and even then may be too many if you're overloading your servos.  Over Voltage protection should be added, but when higher currents are being supplied, can be difficult. In this instance a voltage window circuit may be required; a voltage window device will turn on a relay only if the incoming voltage is above the minimum voltage and below the maximum voltage.  These units when you can find them can be very expensive, so protection here may be limited to just overloading and just where the cost if the SMPS passes the high voltage through. The more SMPS you use to split up the servos and their load can help reduce the potential damage bill if you do get a failure. 
I hope this helps you out when looking at power protection systems for your robot.

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Kakadu31's picture

Nice blog Ray, this pretty

Nice blog Ray, this pretty much resumes every possible danger for the sensitive parts of our robot and how to do best to protect them. Do you think think you can add some quick schematics? Best would be with equations isntead of fix values for example a 24dc system so everyone could just calculate the the needed speccs of the fuses etc as need. I think If you could add this this should definately have atleast a mention on the InMoov site or forum.

Keep going!

Ray.Edgley's picture

Your quite correct.

Damn, i did all the maths and forgot to add the way i did it.

Put simply i converted the power requirements of the low voltage to watt, devided by the efficiency of the SMPS and converted the power back to amp by dividing the watts by the battery volts. 

The selection of fuse is based on the closest rating i could find in a catalog of fuses above the required current.