March 4, 2018

Can I use a bigger battery

This is a common question which people starting out in Electronics struggle with. Batteries have ratings in volts, and mAh. Sometimes they’re rated in C or A too. Trying to understand what you can safely change, and what won’t work is difficult.

Batteries come in multiples of 1.2, 1.5, 2, 3 and 3.7 volts (actually, there are a lot more, but these are the most common ones). The rectangular 9 V batteries are actually 6 of the 1.5V type packed together in a small case. A car battery gets to 12V by taking 6 of the 2V type. In a laptop, you’re likely to have 3 of the 3.7V, giving 11.1 volts. Given this, it might be easy to see that sometimes you will want to swap out one battery for another in a given circuit.

In the other side, you’ll have a circuit so some sort, rated to take a specific current (or power) at a defined voltage. A small computer board (like a Raspberry Pi) will take about 200mA at 5 volts. Actually, this varies quite a bit, and when it’s working really hard, it might take 500mA. Circuits usually specify a maximum current, and sometimes will also specify a typical current. What matters most with a circuit like this is the voltage range which the circuit is designed to operate on.

In the case of the raspberry pi, this specification is 4.75-5.25V. What we can assume from this is that providing less that 4.75V might cause the circuit to stop working (temporarily, possibly causing problems depending what you’ve using it for). Between 4.75 and 5.25, you’re guaranteed to be safe and reliable. Above 5.25V, there is a risk of permanant damage. Not always immediately, and not always for every part. We can’t make any guesses about how safe 5.3V would be, even if we have tried it already on one part and not seen any problems. Based on this, we can’t safely use a combination of 1.5 V (alkaline cells) or 2V (lead-acid), but we could try 1.2V (NiCd) because four of them would give 4.8V. Unfortunately, as soon as we start to use a battery, it’s voltage drops a little (like a small engine under load), and we’d see that the computer might start to work, but fail after a few minutes or when it takes a bit more power than the average.

In the case of powering a small computer from batteries, we probably need to use a circuit called a voltage regulator. This either absorbs some of the extra voltage (a linear regulator), or actually translates the voltage directly to what we need (a switching regulator, more efficient, but more complex).

Lots of circuits already have built in regulation (so they are more tolerant of input voltage). Maybe a circuit can accept anywhere between 3V and 7V. In this case you can safely use any combination of batteries which give this voltage.

The current or charge capacity of a battery is usually only critical in determining how long a circuit will operate for. The voltage is what drives the operation, and the circuit only takes as much current as it needs. This means that there is no harm in providing a battery of the same voltage, but with a bigger capacity. On the other side, it’s not quite as simple. If your circuit is a motor which takes 5 amps (in a robot, for example) then a small battery won’t be able to provide it’s rated voltage when you draw such a large current. Different battery types vary, but if the battery would be completely discharged in less than an hour, you need to check if the battery is suitable for the application. High current batteries can be designed to discharge completely in 5 minutes or less, but these are very specialised applications and risk both overheating when there is any small fault, or damaging the battery (if any part of the battery overheats internally).

In summary:

  • Battery voltage must be close (depending on the specification of the equipment.
  • Higher capacity batteries are OK as a replacement.
  • Lower capacity batteries might not work.

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March 2, 2018

Voltage, Current and Resistance in simple terms

One knowledge area which you are expected to be familiar with for ‘Engineering Operations’ is the basic function of electronic components, and some simple strategies that can be used to test them. You can find many books on basic electronic theory, but these tend to be aimed at people who will be designing circuits – and they rapidly reach the point of needing a good understanding of the theory. My aim here is to explain enough to understand the concepts, and to leave the complete theory to ‘The Art of Electronics, 3rd Edition’. If you can read and understand  Horowitz and Hill, this post will probably be too simplified.

The challenge is to relate voltage and current in an electronic circuit with some concepts that are already familiar, without getting tied up in equations, power, energy and electrons. Particularly not conventional current, holes, etc.

You will probably have seen how a resistor is needed when you connect an LED and a battery together (otherwise, depending on a few things, your LED might go pop). This simple example doesn’t help to explain the concepts that we care about, but I might come back to it later.

The ‘classical’ analogy is to compare electricity with the flow of water in pipes of different sizes. Although this is quite an accurate comparison from a technical point of view, it might be a bit too complicated as a first step. Comparing electricity with water assumes you understand how water works under pressure, or in narrow pipes.

Starting with voltage, this is  the driving force. It can vary in size, and you will come across voltage as the main rating of a battery (1.2, 1.5 4.2, 9, 12 volts being typical for some common batteries). This doesn’t say how long the battery will last, just how strong it is. Imagine a weight lifter, and a long distance runner. The weight lifter can perform a harder task, but does this for a short time. Actually, we can see that how strong they are doesn’t tell us too much about what they can do at all, it’s only part of the story. Depending on the task, you know you need the right sort of person though. If you carry a rock up a hill, you give it more ability to do work.

Current is part of the measurement of going a job. It’s like asking how fast someone is running, or how many people are in a taxi, or how long it takes to read a page in a book. With the rock we carried up the hill, current is a way of looking at how fast it rolls down the hill. Relating current to a battery is complicated.

Resistance is the magic that links voltage and current. For any part of a problem, if you know two, the other is already known. Resistance is a measure of how easy it is to do something. A direct comparison would be to look at crossing a room. When the room is empty, its easy to cross. Replace that empty room with a tight crowd of people, and it will take longer to cross. If our rock is round, it will roll more easily. If we give our runner a heavy weight to carry, they will run more slowly.

Circuits always have some resistance, but it’s not clear what the reason to add the ‘resistor’ component is. To understand that, we need a purpose for our circuit, and this is where things get ugly. Modern circuits hide most of what is going on inside a chip, and use digital logic. The first time that the purpose of a resistor becomes clear is where we need to make a comparison within the circuit. Coming back to the athlete analogy, we can even up any competition by handicapping one of the entrants. Applying resistors in a circuit allows us adjust parts of the behaviour, or start to decide when we reach a certain level. If you want to split a voltage in two, the two resistors will do the job for you. Here, we’d chose high value resistors so that we didn’t waste more current than necessary. If the voltage we have is just a bit too high to do a certain job, we might add a resistor to make the job a bit harder (or maybe more predictable). In some cases, you can view resistors as a kind of marshal, making sure the circuit is well-behaved. The last example of using a resistor is where we want to measure the current – here we need to look, but not interrupt things too much(and we’d want to use a small resistor).

Assuming we have a power supply, this will have a specified voltage, and also a maximum current that it can provide. It won’t force it’s full current into a circuit, but you need to make sure that the circuit doesn’t try to take too much current. You need to make sure that the circuit expects the voltage that the power supply provides (as a minimum). You can connect one or more circuits to the same power supply, so long as they all expect the same voltage, and the current added together is not too much for the supply. What you can’t do is connect two circuits one after the other (in series) so that the voltage is split, and the current re-used. With components, you might see this approach though.

When there is both voltage and current being used, the result tends to be something heats up. You might get light, or a motor moving, but there will be some heat as well. Maybe a lot of heat… This is one reason that it’s important to use the right value components – otherwise things will break. An LED is an example of how things can break easily. With an LED, there is a lot of resistance until it starts to work (and it starts to light up). As soon as it reaches the point it’s designed to work at, it can’t really work any harder, but it can’t push back and will carry on working till it gets damaged. This is a really special case where you need to restrict it, so it can work, but not get carried away. There are three ways to do this:

  • Use a really feeble battery, which can only just power it.
  • Use a special circuit which behaves like this feeble battery.
  • Use a resistor to weaken a normal battery. You still need to pick the right size of resistor.

To make the special regulator circuit, you’ll also use resistors, but probably a transistor or two as well. The difference will be that you can achieve better control with less wasted energy.

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February 28, 2018

Assembling and Testing electronic circuits. Preparation.

Before you start assembling a circuit in a working environment, there are some general preparations which you need to make. Since this is a work environment, it is covered by health and safety legislation. There might also be specific working practices defined, such as not working alone (in case of accident), only eating/drinking at designated locations in the workplace, clear access routes, etc.

Many chemicals and products which you work with can be dangerous (if ingested, heated, mixed with other products, or even if they are physically damaged. Legislation called COSHH defines some of the things you and your employer must do to minimise the risks, and you should have access to an Material Safety Data Sheet (MSDS) for everything you work with. You also need to be aware of risks from hot or cold materials, moving equipment and electrocution.

There will be instructions to describe the work you have to do. These can be detailed instructions for every step, or you may need to work with various types of schematic and standard procedures to complete the work. Within this, each operation may require you to use defined techniques for each stage. These may be documented, or you might be expected to use an accepted approach which is learnt by experience. Some working environments (aerospace for example) may have stricter requirements for these operations, requiring documentation and formalised regular training in technique. Regardless of the detail, the standards which you are expected to work to will be defined and you should take responsibility for ensuring that your work meets the standard expected.

It is important to use the correct techniques since the way that components are mounted and fixed can affect the safety or operation of a circuit when it is being used, long after assembly and testing is completed.

You must check that the tools you are using are safe and in good condition. This can include calibration and electrical safety testing (which both need to be repeated at regular intervals). You should know any risks specific to your workplace, and know how to act in case of an accident to make the environment safe, to summon assistance, and when to try to provide aid yourself without putting yourself in danger. If the task requires you to use a tool which you are not familiar with, it is your responsibility to raise this as an issue and avoid putting yourself at risk.

In addition to these items which will apply to every task, you need to prepare for the specific task. This is described in the next post.

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