Training Feature
Part 2: Oscilloscopes - A silly scope?
During the previous article we discussed in outline the advantages of an oscilloscope over a digital voltmeter and how these advantages can be exploited to good effect during some diagnostic procedures. Let us now discuss how to set up the ‘scope correctly to maximise its effectiveness.
On an oscilloscope, the voltage value is displayed on the vertical scale (axis) and the time value is on the horizontal scale (axis). The voltage scale is selectable as it would be on a normal voltmeter and it can be adjusted to display as much or as little of the pattern as you wish. If the scale selected is too high the pattern will appear smaller in height and a low scale will only display part of the pattern with some of it overflowing the top of the screen. It does not normally matter if you select a voltage scale that is too low, as long as the signal voltage is not higher than that which the unit can safely handle.
On some oscilloscopes the voltage scale represents the total voltage that can be fully displayed, i.e. 20 Volts. Alternatively, the screen will be marked in grids or divisions and the scale will be measured in volts per division, which means that each vertical division represents a proportion of the total voltage. In the case quoted above where the total scale is 20 Volts, when 4 divisions are marked on the display, the scale selected should be 5 Volts per division, giving a total of 20 Volts as before. Some oscilloscopes display both scales as well as digital values for the peak and average voltages in addition to the trace itself.
Setting Timescales
It is also possible to adjust the time scale to suit specific needs. The time scale chosen e.g. 100ms on the diagram determines how much of the pattern you wish to see across the display, from left to right. The ranges available may be displayed in time per division or total time, similar to the voltage ranges. Typical time scales can range from microseconds (millionths of a second) to a number of complete seconds per division or total scale.
Traces/patterns
When no connections are made to a circuit, a trace should be displayed as a straight line at zero volts. Sometimes the line will not be straight, usually because of interference from room lighting etc. Simply joining the signal and earth test leads together will normally ensure a straight line at zero volts. The interference is not usually a problem when actually testing a circuit as the leads are effectively joined together by the circuit itself.
Some scopes can display more than one trace at the same time, each of which can be positioned on the screen where you wish. A different voltage range can be selected for each trace although the timescale will be the same for all traces displayed. More than one trace allows signals to be compared, not only for their relative timing but also for their relative voltage and pattern shape.
Triggering the trace
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Whilst on many occasions simply determining that a pattern actually exists is sufficient for a diagnosis, it is sometimes necessary to examine the pattern more closely. For this it would, of course be preferable if the pattern remains in the same place on the screen and a “Trigger” function is provided for this purpose. Without a trigger the pattern will scroll across the screen in a random fashion, making close examination impossible. To set a trigger it is necessary to tell the oscilloscope the voltage at which you wish to trigger, or start the display, when the pattern passes through this voltage, the trace will remain “still”. The trigger voltage level must be set somewhere between the minimum and maximum operating voltage of the trace; otherwise nothing at all will be displayed. Bear in mind that some traces will never reach the maximum or minimum voltage available and if you set a trigger level outside the normal operating range you may see nothing and assume a fault exists, even if the signal is ok. If you are unsure of the operating voltage of the circuit, it is usually preferable to switch off the trigger when first checking for a trace and select the trigger after you are confident that the trace exists.
Some oscilloscopes provide a function for the trigger point to be moved left and right across the screen - the pattern will follow the trigger allowing you to position the display exactly where you wish on the screen. It is also possible to select whether you wish the trigger point to be active when the voltage is rising or falling. This helps you to measure the on/off times or duty cycles of some signals accurately.
Connecting to a circuit
In order to check most signals accurately it is best to connect to a complete, live circuit by backprobing a plug or using a special breakout harness in the circuit under test. Only when confirming the actual power supply terminals to sensors and actuators does it sometimes become necessary to disconnect a plug. Note: Avoid testing a disconnected live circuit whenever possible as this may cause fault codes to be stored and/or warning lamps to be illuminated. In some cases default values may be set by the ECM, confusing your diagnosis. In addition to this, disconnection will prevent current from flowing in the relative circuits and any high resistance faults that may be present will not be seen.
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ProAuto Limited are an automotive technical training company based in Shrewsbury, Shropshire.
Their core business is the design and delivery of technical training to the automotive industry, which includes vehicle manufacturers, component manufacturers, diagnostic equipment manufacturers and independent garages. They run courses from numerous select venues nationally, so a course is never too far away. For further details you can visit their website atwww.proautotraining.com, email atinfo@proautotraining.com or telephone on 01743 762050.
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Measuring the signal on a complete, operating circuit is effectively connecting your oscilloscope in parallel with the signal source. The signal probe (various colours) should be connected to the circuit and the earth probe (normally black) to a good earth point on the vehicle chassis or body.
There is no need to be nervous about connecting to a circuit in this way; it is normally very safe to do so with modern digital oscilloscopes, which present an extremely large impedance (resistance) to the circuit under test. The process can be compared to reading a book; you take the information in but the words themselves are left on the page for future readers. Any problems usually only arise through physical damage to the connectors caused by rough handling.
If you wish to display more than one trace it is normal for the earth connection to be “common” to all the signal probes.
Remit Training Ramps up its offer to Support MOT Businesses
Since the acquisition of Remit by the Retail Motor Industry Federation two years ago Remit has gone from strength to strength and positioned itself as a leading industry training provider offering the most business efficient and cost effective apprenticeship delivery model in the marketplace, linked with an innovative selection of adult funded and unfunded programmes.
With Remit’s primary focus over the past two years being the articulate delivery of the learning provision for its 3,000 apprentices, the business is now perfectly positioned to continue the expansion of its training provision into the adult sector.
Through the RMI’s interactions with the member base Remit is perfectly positioned to be able to interpret the needs of the industry and react as the first point of contact when a training request is received from a garage, and with this insight into the industry’s needs, over the past year Remit have been able to evaluate and plan a strategy of support in the technical arena.
Suite of Courses
Planned for the forthcoming year ahead Remit will introduce a suite of technical training courses ranging from basic electrics to advanced multiplexed system diagnostics and the new F Gas air conditioning qualification, all of these courses are based on a one day duration either in the workplace or at designated training centres.
As an organisation the RMI has the largest MOT support business in the UK and a national field based operations team dedicated to supporting test stations with MOT Quality Control programmes, MOT manager training courses, risk assessments, appeal and legal support and a dedicated office based support line, and making the most of Remit’s expertise in training we are now responding to demand and will shortly be in the position to enhance the product range by introducing a training product that will support technicians with progressing through the NTTA (nominated tester training assessment).
High Failure Rate
Current legislation dictates that to become a Nominated Tester (NT) an individual must have achieved a recognised qualification such as City & Guilds Craft studies level 2 or a recognised level 3 NVQ, and without these the individual must pass the VOSA NTTA assessment which currently has a failure rate of 60%.
The package Remit will be offering will give the technician the opportunity to be pre-evaluated and a development plan generated that will align their skill and ability to a recognised qualification that will provide eligibility for them to attend the VOSA Nominated Tester Training course.
All individual development plans will be different, based on an individual’s academic status, in some cases it could lead to a one day practical and online assessment, in other cases academic qualifications could need to be gained through classroom and distance learning studies.
For further information on this product, the locations and planned delivery dates at specific training facilities and the prices associated with the level of training required for your business or for any other enquiries please contact: Stuart James at stuart.james@remit.co.uk.
Retail Motor Industry Training (remit) is a learner-focussed automotive training provider providing structured apprenticeships across the UK, working in partnership with local colleges.
Air Conditioning Qualifications
Make sure you are ready for the change in legislation which comes into effect on 4th July 2010.
Preparation is everything!
From 4th July 2010, all Mobile Air Conditioning (MAC) technicians working with cars and car derived vans, must have achieved as a minimum requirement, a refrigerant handling qualification which fulfils the European Union F Gas Regulation (EC842/2006 and Annex to Regulation EC307/2008).
Below you will see the ways which IMI Awards offer solutions to the EC regulations.
Go to: www.imiawards.org.uk for further information
Level 3 Award in Automotive Refrigerant Handling (EC842-2006)
This VRQ, which is approved by DEFRA, offers a single unit solution that provides experienced automotive Mobile Air Conditioning (MAC) technicians with evidence that they meet the minimum F gas requirement (EC842/2006 and annex EC307/2008). Successful candidates will be issued with a certificate which shows they are compliant.
Level 3 Certificate in Automotive Air Conditioning and Climate Control
(EC842-2006)
This qualification provides automotive technicians the opportunity to develop new Mobile Air Conditioning (MAC) skills and enhance existing ones. Successful candidates will be issued with a certificate which shows they meet the minimum F gas requirements and have the essential knowledge required by air conditioning diagnostic technicians. This DEFRA approved VRQ, contains 3 mandatory units:
• AC1 Refrigerant Handling (EC842/2006 inc annex EC307/2008)
• AC2 Mobile Air Conditioning Principles
• AC3 Advanced Diagnostics for MAC and Climate Control Systems