Monthly Archives: April 2012

Advantages and Disadvantages of Digital Multimeters

Posted by on April 25, 2012 1 comment


A digital multimeter,also known as DMM, is a electrical instrument which provides combined functionality of ammeter, voltmeter and ohmmeter. It is most widely used because of its small size, price and ease in operation. A digital multimeter has an analog/digital converter that provides a digital readout. This increases the precision in reading the values.We will discuss some pros and cons of digital multimeters in the following paragraph.

Pros of Digital Multimeters

First, we will talk about the pros or advantages of digital multimeters.

1、They are more accurate than analog multimeters.

2、They reduce reading and interpolation errors.

3、The ‘auto-polarity’ function can prevent problems from connecting the meter to a test circuit with the wrong polarity.

4、Parallax errors are eliminated. If the pointer of an analog multimeter is viewed from a different angle, you will see a different value. This is parallax error. A digital multimeter’s numerical display solves this problem

5、Digital multimeter displays have no moving parts. This makes them free from wear and shock failures.

6、The reading speed is increased as it is easier to read.

7、Unlike analog multimeters, zero adjustment is not required.

8、Digital output is suitable for further processing or recording and can be useful in a rapidly increasing range of computer controlled applications.

9、With the advent of Integrated circuits, the size, cost and power requirements of digital multimeters has been drastically reduced.

10、Accuracy is increased due to digital readout. You can make mistake in reading the scale in analog multimeter, but digital multimeters have a LCD display to show accurate reading.

11、DMMs can be used in testing continuity, capacitors, diodes and transistors. More advanced digital multimeters can also measure frequency.

12、The ‘auto-ranging’ feature of a digital multimeter helps in selecting different measurement ranges, which can prevent damage to the meter if the wrong range is selected.

13、Portable size makes it easy to carry anywhere.

14、They cause less meter loading effects on the circuits being tested.

15、Some advanced digital multimeters have microprocessors and can store the readings for further processing.

16、They have very high input impedance.

Cons of Digital Multimeters

1、The LCD display depends on a battery or external power source. When the battery is low, the display will be dim, making it difficult to read.

2、In case of fluctuations or transients, it can record an error.

3、Warming of the meter during its use can change its properties leading to errors in measured value.

4、The A/D converter has a limitation on word length which can cause quantization noise giving rise to error in measured value.

5、There is a voltage limitation. If it is increased beyond the limit, the meter will be damaged.

6、The digital nature makes it unsuitable for adjusting tuning circuits or peaking tunable responses.

7、They are expensive due to high manufacturing cost.

Although, the digitial multimeters have the above mentioned cons, they are still widely used. In conclution
the adavantage is over the disadvantage. They bring a lot of convenience to our life.

The Recommended Illumination of different areas

Posted by on April 20, 2012 No comments

The digital lux meter is used for checking the level of luminance, luminance is a measurement of the amount of light falling on a surface. Digital light meter can be widely used in various fields such as construction, inspection, photography and etc. Different areas have different illuminations. So before testing the illumination with a Digital Light Meter, we should have a good knowledge of the recommended illuminations for different areas. So please take a closer look at the following recommended illumination of different fields and make you get the accurate readout:

Office Conference, reception room 200～750Lux
Clerical work 700～1,500Lux
Typing drafting 1,000～2,000Lux
Factory Packing work, entrance passage 150～300Lux
Visual work at production line 300～750Lux
Inspection work 750～1,500Lux
Electronic parts assembly line 1,500～3,000Lux
Hotel Public room, cloakroom 100～200Lux
Reception, cashier 220～1,000Lux
Store Indoors stairs corridor 150～200Lux
Show window, packing table 750～1,500
Forefront of show window 1,500～3,000
Hospital Sickroom, warehouse 100～200Lux
Medical examination room 300～750Lux
Operation room, emergency treatment 750～1,500Lux
School Auditorium, indoor gymnasium 100～300Lux
Class room 200～750Lux
Laboratory, library, drafting room 500～1,500Lux

How to Use a Moisture Meter for Outdoor Plants

Posted by on April 16, 2012 No comments

It is usual that our outdoor plants die from overwatering or underwatering. Both the overwatering and underwatering have impact on plants. Therefore the correct amount of water at right time is essential for nuturing healthy plants. In order to test the containing of water,a mositure is required. Indoor and outdoor plants can benefit from precise amounts of moisture. This artical will instruct you how to use a moisture meter for outdoor plants:

1、Check the battery moisture meter to make certain that it is new. A low or dead battery will not help you assess the water needs of the plant. Replace the battery annually. Check to make certain the probe is not clogged with soil or debris.

2、Test soil in midmorning, when the night’s dew has evaporated but the sun’s hot rays have not cooked the soil dry. The conditions cannot be too extreme or you will not get an accurate reading.

3、Push the probe into the soil three-fourths of the way for outdoor potted plants. Read the meter. Pull the probe out and repeat the test two more times. Out of three readings take the average as the correct soil reading. Each time you test, the reading will be a little different.

4、Clean and sanitize the probe before moving to another plant. Use rubbing alcohol and a clean cotton swab to remove any bacteria or fungus that might pass from pot to pot. It is easy to infect other plants if you do not clean the probe.

5、Test the soil in garden beds by pushing the meter almost all the way in. Again, you should take several readings and average them to get a good estimate of the soil dampness.

What Is a Digital Illuminance Meter?

Posted by on April 12, 2012 3 comments

A digital illuminance meter is an instrument used to measure the amount of light in one area. Most digital illuminance meter units are capable of measuring up to 5,000 foot candles, and the measuring unit is typically separate from the meter itself for easy manipulation and for better measurements. Light measurements are required by photographers, farmers, gardeners and moviemakers to ensure the shot or plant with the best lighting . If a certain lamp is being used, such as a mercury lamp, the light measurement might need to be adjusted.

When it comes to needing an exact measurement of light, it can be very difficult to take an accurate measurement with sight alone. While a digital illuminance can reach the accurancy. Most meters consist of two parts: the meter and the measuring unit. The measuring unit is placed on the ground, with a sensor facing the light source. The meter itself displays the foot candles.

All digital illuminance meters record light in a measurement called foot candles. A foot candle is defined as how much one lumen of light would illuminate a surface that was 1 square foot (929 square centimeters). The common amount of foot candles that a digital light meter can measure is 5,000 foot candles, but some meters might have higher measurements.

Measuring light to the exact foot candle might seem unnecessary for most people, but in some industury areas and for some people, this measurement is needed.Photographers, who want to achieve a certain shadowing effect, need a specific amount of light. The same goes for moviemakers, because the amount of light can enhance or destroy a shot. Farmers or gardeners might need to measure light as well, because some plants will not grow if the light is too dim or intense. This is mostly if the farmer or gardener is using a lamp or artificial light source, because these sources can be changed and calibrated, but the sun cannot be.

If a lamp is being used, such as by a photographer or gardener, the light measurement might need to be altered, depending on the lamp type. The digital light meter’s manual will detail any lamps that need the measurement altered. For example, if a mercury lamp is being used, the foot candles typically will need to be adjusted by 0.1 to make the reading accurate. The range of the light source might also require that measurement be altered, depending on the meter, to make the light reading accurate.

Some Safety Guides When Using A Digital Multimeter

Posted by on April 9, 2012 1 comment


When you want to test electronic units such as voltage, ohms and others, the multimeters are required. There are two types of multimeters: analog multimeters and digital multimeters. Nowadays, digital multimeters are replacing the analog types due to their convenience and functionality. The advantages of owning digital multimeters are that they are convenient, handy and precise. Futhermore, you need not worry about not being able to read the measurements accurately. Digital multimeters display readings in digits thus you get precise values down to the last decimal point.

While just like any other electronic device, digital multimeters can also cause hazards especially when mishandled. Such risks as electric shock or other types of injuries, therefore it is crucial for people who usually do measures to know the safety tips when using a digital multimeter.

Firstly, you need to do is to wear a pair of insulated gloves while operating with a digital multimeter. Although the voltage that you usually need to test out may be small, it can still cause discomfort if you get yourself mildly electrocuted in the process. Check the test leads for any damages. If you see some damage, then do not use the meter and risk yourself from getting electrocuted. Do not touch the probes to the voltage source if the lead is plugged directly to a 10A input jack.

Secondly, make sure that the circuit that you are going to measure does not go beyond 4800 watts; otherwise this will also cause you slight shock. When working with voltages that are above 60 V DC, then you need to be extra careful since these voltages can cause the risk of shock hazards. However, in instances when you need to work with such voltages, make sure that you double your protection and wear high-quality insulated gloves.

On the other hand, make sure that all of your fingers are behind the finger guards that are placed to separate the test probe from the meter when taking your measurements. The finger guard is made with insulated material to protect against electrical shock.

Lastly, test your battery all the time. Most digital multimeters come with a battery indicator thus if the battery is near empty, make sure to replace it. Replacing the battery does not only save you from getting false readings but it also prevents fluctuation of energy in the unit which can cause electric shocks.

Digital multimeters will serve you through the years with proper care, regular maintenance and calibration.

The Review For Mastech HY1803D Variable DC Power Supply

Posted by on April 4, 2012 No comments

Looking for a power supply with variable voltage and amperage, I found  Mastech HY1803D Variable DC Power Supply at histest.com. This is a great value general purpose power supply which doesn’t take up a lot of space on your test bench. It has large, brightly back-lit, LCD indicators with a metal case and a good look to it. Mastech HY1803D is also equipped with separate controls for both voltage and amperage, allowing for a multitude of uses.

I use it to test small motors and fans, and just to play around with equipment. Finding a supply that won’t break the bank is a challenge in itself, and most voltage and amperage variable supplies are $200 and up. If you only need power within the 0-18v and 0-3amp range, then this should be a good match. Banana plugs fit right into the sockets, so you can use test leads or alligator clips. The positive and negative terminals also screw in and out, so you can attach wires via other means as well. The display is LED lit, and looks very nice when on. The only thing I would improve on the supply is the power cord connection, which doesn’t seem to want to insert all the way in the back. A hard push will allow it to work, however. I recommend getting some test leads and banana to alligator leads as well, since this doesn’t come with any.

Meanwhile, I tested it out by hooking up a couple of 500 watt lights, and it delivered 18.8V and 3.1A, so it meets the specs. I heard only one relay click at about 9V, which means that it uses only two taps from the transformer… Other power supplies click a few times or more; more taps would make it waste less power throughout the range and run cooler, but if you use it infrequently wasting a bit of AC is not a big deal. The Mastech HY1803D seems decently accurate; my fluke 189 said 18.64V when it said 18.8V. The fluke said 3.077A when the mastech said 3.11A.

I’m not sure about the ripple though. I put the probe of my new Instek 1062a digital storage oscilloscope onto the banana clip outputs, and it measured approx 2mVpp ripple (varied from about 1.6mVpp to 2.4mVpp, mostly staying above 2mVpp). This is higher than the rated 0.5mV But when I tried switching between the 1x and 10x settings of the probe the measured ripple didn’t change nor did waveform ripple that I saw on the screen, so there’s something weird about my measuring setup; the 2V test signal on the oscilloscope at 1X did change to a 200mV signal with the 10X setting. I’m a beginner at using my DSO so take this with a grain of salt and if anyone can explain how to get a more reliable ripple measurement please tell me.

Linear Versus Switch-Mode Power Supplies

Posted by on April 1, 2012 1 comment

Introduction

Linear power supplies were the mainstay of power conversion until the late 1970’s when the first commercial switch-mode became available. Now apart from very low power wall mount linear power supplies used for powering consumer items like cell phones and toys, switch-mode power supplies are dominant.

So what is the difference in how they work? – (Techy stuff)

Linear power supplies have a bulky steel or iron laminated transformer. This transformer has two purposes
- It provides a safety barrier for the low voltage output from the AC input and reduces and the input from typically 115V or 230VAC to a much lower voltage around say 30VAC.

The low voltage AC output from the transformer is then rectified by two or four diodes and smoothed into low voltage DC by large electrolytic capacitors.

That low voltage DC is then regulated into the output voltage of choice by a dropping the difference in voltage across transistor or IC (the shunt regulator).

Switch-mode supplies are a lot more complicated. The 115V or 230VAC voltage is rectified and smoothed by diodes and capacitors resulting in a high voltage DC. That DC is then converted into a safe, low voltage, high frequency (typically switching at 100kHz to 500kHz) voltage using a much smaller ferrite transformer and FETs or transistors. That voltage is then converted into the DC output voltage of choice by another set of diodes, capacitors and inductors. Corrections to the output voltage due to load or input changes are achieved by adjusting the pulse width of the high frequency waveform.

Sounds complicated? Yes, but the pay off is worth it!

The advantages and drawbacks of both technologies

Size: – A 50W linear power supply is typically 3 x 5 x 5.5”, whereas a 50W switch-mode can be as small as 3 x 5 x 1”. That’s a size reduction of 80%.

Weight: -A 50W linear weighs 4lbs, a corresponding switcher is 0.75lb. As the power level increases, so does the weight. I personally remember a two-man lift needed for a 1000W linear. Today I carry a 2000W in my carry on luggage when I fly!

Input Voltage Range: -
A linear has a very limited input range requiring that the transformer taps be changed between different countries. Normally on the specification you will see 100/120/220/230/240VAC. This is because when input voltage drops more than 10%, the DC voltage to the shunt regulator drops too low & the power supply cannot deliver the required output voltage. At input voltages greater than 10%, too much voltage is delivered to the regulator resulting in over heating.

If a piece of equipment is tested in the US and shipped to Europe, or even to Mexico in some cases, the transformer “taps” have to be manually changed. Forget to set the taps? The power supply will most certainly blow the fuse, or may well be damaged.

Most switch-mode supplies will operate anywhere in the world (85 to 264VAC), from industrial areas in Japan to the outback of Australia without any adjustment.

The switch-mode supply will also be able to withstand small losses of AC power in the range of 10-20ms without affecting the outputs. A linear will not. No one will care if the AC goes missing for 1/100th of a second when charging your phone, it will take 100 of these interruptions to delay the charge by one second! Having a piece of equipment reboot 100 times a day will cause some heartbreak!

Efficiency: -
A linear power supply because of its design will normally operate at around 60% efficiency for 24V outputs, whereas a switch-mode is normally 80% or more. Efficiency is a measure of how much energy the power supply wastes. This has to be removed with fans or heatsinks from the system.
For a 100W output linear, that waste would be 67W. A 100W switch-mode would be just 25W.

67W – 25W = 42W is the extra power lost

Doesn’t sound much, but don’t try touching a 40W light bulb! If the equipment were running 24 hours a day, then the extra losses would be 367kW hours, even at $0.1 per kW hour, that’s an extra $37 a year for a power supply that costs around $80.

As a quick note, in Europe, they are trying to limit those losses of all power supplies used by consumers particularly when operating off load (as many products are left plugged in 24 hours a day). Imagine 250 million power supplies eating up a couple Watts. That equates to the output of a whole power station!

Reliability: -
If reliability is calculated using a part count method, then the linear power supply will win. With the design & quality improvements made in the last few years with switch-mode parts & technology, in reality this advantage has been negated. I have demonstrated life testing data showing no failures after over 1,000,000 hours on some Lambda products.

Electrical Ripple and Noise: -
This is where the linear really scores!
Electrical Ripple and Noise

The linear obviously is a lot “quieter”, by up to a 10,000 times. The topology of the switch-mode supply with its high frequency switching technology had to have a downside right? So if the noise is 10,000 times worse, how can anyone use it? Sounds so bad.

In truth, there are some applications (studio mixers and very sensitive test equipment) where low electrical noise is critical. The others? One of my first sales calls in the USA was to a manufacturer who built semiconductor fabrication equipment. They used 8 really big linear units in a large box measuring 2x3x4 feet, it was heavy & actually was dictating the size of their end equipment. I told the engineer that I could replace all eight units with two modular products measuring 5x5x10”. He laughed and said the noise would be too great. I sent him samples and went to visit three weeks later. He was delighted with the performance and has been a long term Lambda customer ever since.

 

Transient Response:
Transient response is how a power supply reacts to a (fast) change in load.

If the output load quickly changes from say full load to half load, the output voltage of the power supply will rise (overshoot) before the internal control circuit has time to compensate, and then undershoot a little less as the circuit over compensates. The length of time is takes from the instant of the load change to the time the output voltage settles back into the load regulation limits can be critical to some loads. Here the linear again outperforms the switch-mode.

For a 50% change in load the switch-mode will often take 3000us to recover. A linear supply will recover in 50us.

Is this critical for all applications? There are a few specialized technologies where this is important and most engineers will advise you if this is critical. For the other instances on board capacitors at the end load & the inductance of cables is enough to reduce overshoot down ten-fold to where it no longer is a concern.

Low leakage currents and Conducted EMI: -

A widely used technique in the design of switch-mode power supplies is to connect special capacitors from the AC input terminals to Earth. This is an cost effective method to reduce noise from being fed back through the input wires and potentially affecting other equipment.

These capacitors have a side effect of allowing a “leakage current” to be passed through the Earth or ground cable. Many safety specifications have limits on the amount of this current that is allowed. UL1950 allows 3mA, medical industrials less than a tenth of that. The gaming industry is even tighter.

As linear power supplies are “quieter” and do not need these capacitors, they simplify the system filtering, and allow more of the system leakage current “budget” to be used for other parts like monitors. The overall size of the system filter can also be reduced. How much that impacts cost & performance varies from customer to customer.

Some switch-mode power supplies (like Lambda’s Vega series) are now available with increased internal filtering that allows for low leakage versions to be offered to meet medical specifications.

In Summary:

Linear

Switch-mode

 Comments
Size

_

_

 Typically 80% smaller
Weight

_

_

 Typically 80% lighter
Input Voltage Range

_

_

 10% vs. up to 300% range
Efficiency

_

_

 Calculate it long term!
Reliability

_

_/_

 Component count method, demonstrated probably equal
Ripple & Noise

_

_/_

 Up to 10,000 times – often possible to overcome though
Transient Response

_

_

 Up to 100 times -necessary in specialized areas
Low leakage Current

_

_/_

 Often used in medical systems, switch-mode gaining share

As you can see, depending upon what is critical to the Customer will influence the decision to go with either a switch-mode or a linear power supply.  It is often worth challenging the use of a linear.  Sales of linear supplies (>10W) fall every year as technology adapts and improves.