Test Equipment

Test Equipment
When developing new circuitry, it may be convenient to try different values of resistor in some position in the circuit (the resistor value may be dependent on the gain of a transistor or the actual resistance of an ORP12, or some such other situation).   For this, it is very convenient to have a resistor-substitution box which allows you to select any standard resistor at the turn of a switch.

These are not readily available on the market.   In years gone by, it was possible to buy custom wafer switches, where the number of wafers could be built up to whatever switch size was required, but these do not seem to be available any more.   A slightly less convenient method of construction is to use four of these, selected by a second wafer switch:


In the above diagram, all of the resistors in one range (100 ohms to 820 ohms, 1K to 8K2, 10K to 82K or 100K to 820K) are wired to a single 12-way switch.   The output wires then have any of these standard resistors across them, depending on the setting of the switch.   A second switch can then be used to select several of these groups, while still using the same output wires.   When boxed, it might look like this:


It can also be useful to have a versatile signal generator.   You can easily construct your own with variable frequency, variable mark/space ratio and optional variable gating.   If you do, you might as well make it with a low output impedance so that it can drive devices under test directly rather than having to provide additional buffering.   It might look like this:


The really essential item of equipment is a multimeter.   These come in many shapes, sizes and varieties and the cost varies enormously.   The reliability also varies a great deal.   The most reliable and the cheapest is the analogue type which does not use a battery (other than for the occasional measurement of resistance).   Although these types are looked down upon nowadays, they are 100% reliable:


The meter shown above is rated at 2,000 ohms per volt, so connecting it to a circuit to make a measurement on the 10V range is the same as connecting a 20K resistor to the circuit.   The big brother of this style of equipment is about five times larger and has 30,000 ohms per volt performance, so connecting it on a 10V range is the same as connecting a 300K resistor to the circuit being measured.   This one is battery driven, so if you get one of these, may I suggest that you check its accuracy on a regular basis:


The really excellent non-battery (ex-professional) Avo meter multimeters are still available through eBay at affordable prices.   These have 30,000 ohms per volt performance and are robust and accurate, having been built to very high standards.

A multimeter uses a 1.5V battery to measure resistance.   Ohm’s Law is used as the working principle and the operation is:


The meter shown in the diagram has a small resistance of its own.   This has a small variable resistor added to it.   This variable resistor will have a small knob mounted on the face of the multimeter, or it will be a thumbwheel knob projecting slightly from the right hand side of the multimeter case.   The 1.5V battery will be positioned inside the multimeter case as is the 1K resistor.   To use the resistance ranges, the multimeter probes are touched firmly together to form a short-circuit and the variable resistor adjusted so that the meter points to zero.

For the purpose of this discussion, let us assume that the internal resistance of the meter, when correctly adjusted, is exactly 1K.   If the resistor under test is exactly 1K in value, then the current through the meter will be halved and the meter will show a needle deflection half way across the scale.   If the resistor under test is 2K, then the current will be one third and the scale marking will be at the 1/3 position from the left.   If the resistor is 4K, then there will be one fifth (1K+4K=5K) of the full-scale current and the 4K mark will be 20% from the left hand side of the scale.

Two things to notice: firstly, the scale has to read from right to left which can take some getting used to, and secondly, the scale is not linear, with the markings getting closer and closer together and consequently, more difficult to mark and read, the higher the value of the resistor being measured.   The bunching up of the scale markings is why the more expensive multimeters tend to have more than one range.

A mains-operated oscilloscope is an excellent piece of equipment to own but they are expensive when new.   It is possible to pick one up at a reasonable price second-hand via eBay.   An oscilloscope is by no means an essential item of equipment.   One of its most useful features is the ability to measure the frequency, and display the shape of a waveform.   Most waveforms are of known shape so the frequency is the major unknown.   The following meter is not expensive and it displays the frequency of a signal on a digital readout:


So, when you are deciding what multimeter to buy, consider the following points:

    1. How reliable is it?   If you are opting for a battery driven unit, what happens to the accuracy if the battery starts to run down.   Does it display a warning that the battery needs to be replaced?   Mains-operated digital multimeters are brilliant but are a problem if you want to make measurements away from the mains.
    1. What DC voltage ranges does it have?   If you are intending to work mainly with 12V circuits, it is inconvenient for the ranges to be 9V and 30V as successive ranges.   Digital meters do not have this problem but the question then is, how accurate are they going to be in day to day use?
    1. Transistor testing options you can ignore – you are better off making your own dedicated unit to check transistors if you think you will ever need to do this – you probably won’t.
    1. Measuring current can be very useful so see what ranges are offered.
    1. Measuring capacitance is very useful, especially since many capacitors are not well marked to indicate their value.
    1. Measuring the frequency of a waveform could be a significant bonus but the question is; are you every likely to need it?
  1. Measuring resistance is very useful.   Every meter can do it.   There is no need to be over fancy on measurement ranges as you usually only need to know the approximate answer – is it a 1K resistor or a 10K resistor?


Look around and see what is available, how much it costs and what appeals to you.   It might not be a bad idea to buy a really cheap multimeter and use it for a while to see if it has any shortcomings which are a nuisance, and if so, what improvements you personally want from a more expensive meter.

It might be worth getting a fancy bench power supply which allows you to set any voltage you want and which displays the current being drawn by your development circuit:


However, there is no need to spend money on a fancy unit when you can build an excellent unit of your own with voltage stabilisation, adjustable output, metered current, etc. etc.   Personally, if developing a circuit to be used with a battery, I believe you are better off powering the development from a battery, that way the characteristics of the battery are included in any tests which you carry out.

Power Supply:   If you wish, you can construct a very convenient development test bed power supply system.   This has the advantage that you can make it in the most convenient style for your own use.   You can also make the protection ultra-sensitive and build in additional circuitry such as transistor tester and resistor substitution box to produce an integrated test bed.   You could perhaps use a circuit like this:


Here, the power is supplied by a pack of re-chargeable Ni-Cad batteries or possibly, a mains unit with voltage stabilisation.   As in all actual circuits, the next thing in the circuit is always an on/off switch so that the power source can be disconnected immediately should any problem arise.   Next, as always, comes a fuse or circuit breaker, so that should the problem be serious, it can disconnect the circuit faster than you can react.   If you wish, you can build your own super-accurate adjustable circuit breaker to use in this position.

The two transistors and three resistors form an adjustable, stabilised output.   The FET transistor has a high output power handling capacity and a very low input power requirement and so is good for controlling the output voltage.   Resistor ‘VR1’ is padded with the 4K7 resistor solely to reduce the voltage across the variable resistor.   VR1 is adjusted to control the output voltage.   If the current draw is increased and the output voltage is pulled down slightly, then the voltage on the base of the BC109 transistor is reduced.   This starts to turn the transistor off, raising the voltage at point ‘A’, which in turn, raises the output voltage, opposing the variation caused by the load.

The output is monitored, firstly by a large milliammeter to show the current draw and secondly, on the output side of the milliammeter, a voltmeter.   This allows very close monitoring of the power supplied to the prototype, especially if the milliammeter is placed alongside the prototype.   You can build this circuit into a wide flat box which provides a working surface beside the milliammeter.

At point ‘B’ in the above diagram, a method for altering the current range of the milliammeter by placing a ‘shunt’ resistor across it.   When the switch is closed, some current flows through the resistor and some through the milliammeter.   This resistor has a very low value, so you are better off making it yourself.   Let’s say we wish to double the range of the meter.   Solder the switch across the meter and for the resistor use a length of enamelled copper wire wound around a small former.   Put a load on the output so that the meter shows a full-scale deflection.   Close the switch.   If the current displayed is exactly half of what it was, if not, switch off, remove some wire to lower the reading or add some wire to raise the reading and repeat the test until exactly half the current is displayed.   The lower the value of the shunt resistor, the more current flows through it and the less through the meter, which then gives a lower reading.

Please note: it is very important to have a fuse or circuit breaker in the power being delivered to your test circuit.   Any error in building the prototype can cause a major current to be drawn from the supply and this can be dangerous.   Remember, you can’t see the current.   Even if you have a meter on the current being delivered, you may not notice the high reading.   The first sign of trouble may be smoke!   You can easily fry the circuit you are building if you do not have a safety cut-off, so use a fuse or other device which limits the current to twice what you are expecting the circuit to draw.

So, after all that, what equipment do you really need?   You need a small soldering iron and multicore solder, a pair of long-nosed pliers and a multimeter.   One other thing is some tool to cut wires and remove the insulation prior to soldering.   Personal preferences vary.   Some people prefer one of the many custom tools, some people use a knife, I personally use a pair of straight nail scissors.   You pick whatever you are comfortable with.

Not exactly a vast array of essential equipment.   The other items mentioned are not by any means essential so I suggest that you start by keeping things simple and use a minimum of gear.

If you are not familiar with electronics, I suggest that you get a copy of the Maplin catalogue, either from one of their shops or via the Maplin web site.   Go through it carefully as it will show you what components are available, how much they cost and often, how they are used.   The specifications of almost any semiconductor can be found free at AllDataSheet in the form of an Adobe Acrobat document.

Finally, because it is not important, all of the circuitry shown so far has indicated current flowing from the + of a battery to the – terminal.   The discovery of voltage was made by Volta but he had no way of knowing which way the current was flowing, so he guessed.   He had a 50 – 50 chance of getting it right but he was not lucky and got it wrong.   Electrical current is actually a flow of electrons, and these flow from the battery minus to the battery plus.   So, who cares?   Almost nobody, as it has no practical effect on any of the circuitry.

Electronics Tutorial

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