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SIMPLE DIGITAL for low-voltage me Here’s one of those really handy little projects that will cost very little but make life a whole lot easier when you want to measure voltage and current at the same time. E veryone would have a digital multimeter these days. Even the quite cheap ones have a huge range of measurements. All do the usual voltage, current and resistance but many throw in continuity (often with a buzzer), capacitance, 74  Silicon Chip transistor and diode checking, inductance, battery checking and so on. And when we say cheap, we mean it. You regularly see DMMs for less than ten dollars; indeed one retailer, Altronics (who happen to have a catalog in this issue) has even given DMMs away to customers when opening a new store! So why would anyone want to build a project such as this which simply measures one range of voltage and one range of current? And just as importantly, probably costs as much (if not more than) one of those many-function multimeters? The idea for this project arose when we were “playing around” with batteries and chargers (SILICON CHIP, December 2006 and January 2007). Two of the things you must know, and know instantly, when designing chargers and charging batteries are, of course, current and voltage. Even with several multimeters available (and used) I was always swapping leads around, trying to work out which leads belonged to which meter (Murphy’s law variation 1.3.3: multiple test leads, especially of the same colour, will automatically tangle and lead to errors). It occurred to me that what was really required was a simple meter capable of reading volts and amps at the same time. Of course, those two are mutually exclusive. Voltage is measured in parallel with a circuit, current is measured in series (see the panel “Meter Shunts and Multipliers”). But what if we had one device capable of doing both? This is it: SILICON CHIP’s simple answer to an oft-occuring problem.      A n d w h e n w e s a y siliconchip.com.au L PANEL METER easurements simple, we mean it: two digital panel meters in one small case, one set up to measure 20V DC and the other set up to read 20A DC. And if those ranges don’t suit your application, they can be easily changed. However it seemed to us that charging a variety of batteries up to 12V, a 20V maximum was about right. 20A might seem a bit excessive but if you’re charging car batteries, you could need that sort of reading. Again, if you want to change it, you can! The digital panel meters automatically scale down to show milliamps anyway, if that’s what you need. About this time that my attention was drawn to another Oatley Electronics project designed to work with these specific digital panel meters. It’s an add-on isolation board with either shunt or divider for different voltages and current. It also has a built-in DCDC isolated power supply to power the digital panel meter at a very economical 3-5mA. siliconchip.com.au Like most digital panel meters and digital multimeters, these meters do not have a common ground between the input and the battery. As a result they cannot even measure the voltage of the battery that is powering them. If it is desirable to have a common ground between the input and the battery it is necessary to derive a “floating” power supply to + SHUNT (0.0125 Ω) A CURRENT MEASUREMENT (20A) + – IN LETTERS REFER TO PC BOARD C TERMINATIONS E + 12.34 + IN VOLTAGE MEASUREMENT (20V) Which way to go? As I just mentioned, it’s based on a couple of panel meters. I toyed with the idea of using analog meters for a millisecond or two but digital meters are much better for reading relatively constant voltages and currents – one glance and you’ve got it. Analog meters come into their own when looking for changes in values – you can get a pretty good idea of the way a circuit is behaving by looking at the speed of change. Of course, a ’scope is usually even better for that purpose, so if I wanted to I could hook up old trusty and look at pretty pictures. But that’s further complicating the issue. OK, so we were going to go with panel meters. As luck would have it, just at that time I was looking at an Oatley Electronics advert and out popped some quite cheap digital panel meters (Cat DPM1) – at just $9.00. And even better, out of the box, they are wired for 20V DC full scale. So I picked up two of them along with a Jaycar sloping handheld enclosure (Cat HB6090) which looked just about the right size. by ROSS TESTER 10k B + IN 15k D F 1.234 – IN IFT1 1 100nF ON 4 5 3 100nF Q1 BC548 B 9V 2 A 1N4148 3 1nF 1k 100nF 5 4 1 1N4148 + K 1 2007 BC548 A ZENER SC  13V ZENER 4 E 15k + 100nF 2 C + 1N4148 K 15k POWER K A IFT2 SIMPLE AMMETER & VOLTMETER 5 2 3 4 C B E IF TRANSFORMER (BASE UP) Fig.1: It could have been as simple as two digital panel meters (DPMs) and a 9V battery but the isolating power supply and shunt board only adds a few dollars to the price. It consists mainly of the oscillator based on Q1 and IF transformer IFT1, which is coupled to IFT2 and the voltage-doubler rectifier which follows. The 13V zener diode protects against over-voltage. March 2007  75 Here’s the panel meter we used, with the rear view at right showing the chip which does all the work (the black blob in the middle). This one is from Oatley Electronics but is similar to many on the market. Note the labels on the side near the input (left) and power (right) pins – you can just see these at the bottom edge of the right-hand photo. power the panel meter. The lone transistor and its associated components form an oscillator with a frequency determined by the 455kHz IF transformer IFT1. The 1nF capacitor applies a feedback voltage from the transformer’s secondary to the base of the transistor to maintain oscillation. The output from IFT1 is applied to the input of transformer IFT2. IFT2’s output is applied to a voltage doubler made up of two capacitors and two diodes. The panel meter supply can be anywhere from 7 to 11V DC. The output of this simple supply is nominally 9V but it is possible that it could go higher, especially if a higher input voltage is applied to the oscillator. The 13V zener diode protects the panel meter in this case. require trial and error in cutting the shunt length to get the meter reading the exact current. To make life a lot easier, the shunt is instead wired to the add-on PC board which has provision to adjust the current reading via a voltage divider and preset pot. The board is the same size as the panel meter and is designed to solder to and stack on the back. Like the panel meters, it’s priced at $9.00 (Cat No K212). One of these was added to the order (I figured that only one would be needed, that to set up the currentmeasuring meter. The voltage-measuring meter could be used “as is”). The only other things that were required were four heavy-duty terminals, a 9V battery holder and an on-off switch. There’s not much to this project – either in terms of complexity or cost! In fact, because of its low cost it would make a great project for a school electronics class; something they would find really useful once completed (especially as school electronics, by and large, is limited to battery-powered projects). The voltmeter As we mentioned before, the voltmeter is already configured to measure 20VDC. The only things we need to is provide connections between the case terminals and the appropriate pads on the PC board and supply power. We’ll look at power shortly. As a voltmeter is connected in parallel with the circuit under test, very little current flows. And because we are measuring only low voltage, heavy insulation isn’t required. Therefore the connecting wires can be as thin as you like – we used two strands from a ribbon cable but just about any insulated hookup wire is The shunt fine. Of course, it would be possible to Just make sure it is routed out of simply add a shunt resistor across the the way of the battery case and power panel meter terminals so that it measswitch (especially when the case is ures current. However, this would assembled!). Power could be INPUT/ INPUT/ supplied direct from SHUNT SHUNT + the 9V battery, via the on/off switch to appropriate pins on the PC board. But 1nF 100nF BC548 part of the ammeter NEW PIC TO COME VR1 (following next) is 1 3 5 4 10k 2 a DC-to-DC isolated IFT1 2 IFT2 3 4 5 1 15k power supply which 100nF 15k can power the digital – – + + 15k panel meter at a very OUTPUT TO DPM economical 3-5mA. POWER TO DPM + – We checked: this can 9V just as easily supply Fig.2: assembly of the Oatley K212 Shunt Board is pretty simple – only the diodes, transistor and both DPMs. the two IF transformers have any polarity issues. This board sits on top of the header pins on the So to keep everyAmmeter DPM with the pins soldered to its underside. This same PC board can also be used as a thing simple we will DPM multiplier (hence vacant holes) but we used the voltmeter DPM “as it came” with 20V FSD. E D C 76  Silicon Chip B ZD1 F 2.2k 4148 4148 100nF A – siliconchip.com.au The heating-wire shunt shown fitted to the add-on shunt/ power supply board. Note that this should be done after the board is soldered in place, not as shown here (just to show where it goes!) Similarly, the photo at right shows both panel meters in position but the shunt board has to be soldered in position to the top (ammeter) DPM. Parts List – Simple Digital Ammeter/Voltmeter 2 LCD digital panel meters (Oatley Electronics DPM1) 1 Sloping front instrument case (Jaycar Electronics HB-6090) 2 red heavy duty terminals 2 black heavy duty terminals 1 mini toggle switch, SPST 1 9V battery holder, PC board mounting 1 50mm length 2-strand ribbon cable (or hookup wire) 1 200mm length extra heavy duty red hookup wire (20A) 1 200mm length extra heavy duty black hookup wire (20A) 6 solder lugs REMOVABLE PANEL (78 x 45mm) NEW AMMETER CUTOUT (68 x 30mm) CL EXISTING CUTOUT (45 x 18mm) CL 3mm VOLTMETER CUTOUT (68 X 30mm) * * 23mm Drilling details for the Jaycar HB6090 sloping front instrument case. siliconchip.com.au 7mm 12mm HOLE SIZES TO SUIT SWITCH AND TERMINALS USED * * 20mm * * 20mm 12mm Oatley K212 Shunt Kit (contains the following components) 1 PC board, 67 x 43mm, originallly coded K116 but now K212 2 miniature IF transformers 1 BC548 NPN transistor 2 1N4148 silicon diodes 1 13V 400mW zener diode 3 100nF polyester capacitors 1 1nF ceramic capacitor 3 15kW 1/4W resistors 1 2.2kW 1/4W resistor 1 10kW preset potentiometer 1 length heating wire, (0.05W per metre) – see text 2 10mm M3 bolts each with 2 nuts and washers March 2007  77 connect to this supply when we have finished off the ammeter. The ammeter SHUNT: 0.0125  250mm HEATING WIRE (0.05 /m) AMMETER DPM (UNDERNEATH) * * OUTPUT – + * * * B POWER – + F D C E A SOLDERED TO DPM BOARD UNDERNEATH VOLTMETER DPM 9V BATTERY POWER SWITCH CURRENT MEASUREMENT VOLTAGE MEASUREMENT Here’s how it all goes together: the ammeter DPM is underneath the shunt board at the top (mounted on the sloping section of the case), with the tops of the four header pins on the DPM board (marked with an asterisk) soldered to the underside of the shunt board. Two wires also connect the “power” pins to the same pins on the voltmeter DPM board. Otherwise, it’s pretty plain sailing. Note that the wiring from the current measurement terminals to the PC board is extra heavy duty; the wiring between the voltmeter terminals and its PC board can be light duty (we used two strands from ribbon cable). 78  Silicon Chip The ammeter starts off being the same as the voltmeter – we change it by adding the Oatley K212 ammeter shunt board. So we might as well start off by assembling that project. It’s pretty simple – apart from the low component count, only the transistor, diodes and zener are polarised. The IF transformers also have to go in the right way around or they won’t work – follow the pinout on the circuit diagram. One of the main reasons for using the ammeter shunt board is that it makes adding the required shunt a lot easier. The shunt itself is a short length of resistance wire which is used for under-floor heating. This wire, which is included in the kit, has a resistance of 0.05W per metre. Therefore, half a metre will have a resistance of 0.025W and 250mm will be 0.0125W – exactly the resistance we want for the shunt. This wire is soldered to a pair of spade lugs and secured to the PC board by two small bolts in the top corners. The same bolts secure the cables from the ammeter input terminals. This means that heavy currents are kept off the PC board – the lion’s share passes from the terminal, up the heavy cable, through the shunt and back to the terminal again. This wire does need to be thick! It has to be able to carry up to 20A so ordinary hookup wire won’t do. We used two short lengths of extra-heavyduty automotive hookup wire, rated at 25A. These were also soldered to spade lugs. When assembling the PC board, start with the two bolts. While there are two nuts on the bolts (one holds the bolt in place, the other secures the spade terminals), we also soldered the head of the bolt to the copper track on the opposite side of the PC board. That improves conductivity as well making the bolt captive. To complete the shunt board assembly, solder a pair of thin, polarised hookup wire (or two strands from a ribbon cable) about 100mm long to the power connection pads on the PC board. Leave the opposite end for the moment. siliconchip.com.au in the flat section of the case. Mark the case according to Fig.x and then drill a row of very close holes – almost touching each other - along the inside of line with a fine (eg, 1mm or so) drill. If you have access to a drill press, this makes life so much easier. When the row of holes is finished, elongate them so they form a slot. Break out the panel and smooth the cutout out with a fine file up to the line. While it’s best to make the cutouts nice and neat, any small “oopses” should be hidden by the panel meter escutcheon. The case lid is effectively sandwiched by the panel meter. When drilling the holes for the terminals, make sure you allow for the case mounting pillars in the corners. Remember you have to get a solder lug and nut/washer onto the terminals – if they are too close to the pillars, you won’t be able to. We’ve shown measurements to help preclude problems. The only 9V battery holder we could get was one intended for PC board mounting – we merely bent the pins out horizontal with a pair of pliers and soldered straight to them. A dollop of super glue or other adhesive is all that’s necessary to hold the battery holder in place. Right alongside this (next to the battery holder connections) is the on-off switch. A nice small switch looks best here but just about anything will be fine if it will fit! Assembly This slightly-larger-than-life photo also shows where everything goes. In this shot we’ve taken the loop out of the shunt (thick blue wire) because it hid too much underneath. But it needs to be looped so that the case bottom can screw on. The case The Jaycar case has a front panel divided into two sections. Most of it is flat, like any other case but there is a sloping section at the top. For our purposes this was perfect because it allowed room for the current meter with the piggy-back shunt board. The voltage meter fitted immediately below this on the flat section, with the four terminals across the bottom. The battery holder fitted nicely into the area between the back of the terminals siliconchip.com.au and the voltage panel meter – along with the on-off switch. Some surgery is required on the case to fit the meters and mount the terminals and switch but this is quite easily accomplished (the case is ABS). Even better, the sloping section has a removable “face plate” with a cut-out obviously designed for a panel meter – unfortunately, though, not quite the right size for the Oatley meters. We simply enlarged this cut-out to suit and then cut a similar-sized hole Assuming you have completed the shunt PC board, it’s time for final assembly. Start by mounting the voltage DPM on the flat of the case and then the current DPM on the sloping section. Both are mounted by removing their nuts, separating the front escutcheon from the display board proper and sandwiching the case between the two. Tighten up the nuts to lock in place. Soldering the ammeter shunt board to the DPM is a little tricky because you don’t have a lot of room to solder between the two boards. You’ll need a pretty fine soldering iron tip for this job. The power and output pads on the shunt board line up with the appropriate pins on the DPM. Note that this is done before attaching either the shunt or input March 2007  79 cabling, as it will just get in the way while you solder. The appropriate pads on the shunt board line up with their respective pins on the DPM. The soldered joins are the only thing which holds the shunt board in position. To complete the project you need to mount the four input terminals, the power switch and battery holder, run the heavy duty ammeter input cables and the light duty voltmeter input cables to their respective terminals and connect the power wires to the shunt board. The latter are the other ends of the two wires you previously soldered to the power input pads on the ammeter shunt board. Solder the black wire direct to the “–” pin of the battery socket and the red wire first to the power switch, thence to the “+” battery socket pin. Similarly, solder a pair of fine insulated wires (again, a pair from a ribbon cable is fine) between the two power supply pins on the ammeter board and the matching pins on the voltmeter board, as shown in the photograph. Finally, connect the two ammeter input wires between their input terminals and the bolts on the ammeter shunt board, then the shunt itself also between those bolts. You may notice we looped the shunt through 360° to keep it all neat. current (say 5A or 10A). This might also become a necessity if your multimeter only goes to 10A maximum – many do! While not perfect, this should result in an FSD reading close enough for the vast majority of applications. Calibrating the meters If you can find some different coloured heavy-duty input terminals, this would mean less chance of getting the current and voltage clip leads mixed up. We couldn’t – so both sets of input terminals are red and black. So we made up a couple of different coloured alligator clip leads (from heavy-duty figure-8 cable for current; ordinary figure-8 for voltage). If you stick to red and black for voltage, polarity is obvious. The current cable can be any heavy-duty cable you can lay your hands on (eg, auto cable) as long as it is polarised – either by colour or a stripe. The panel meters automatically show reverse polarity with a “–” sign. The voltmeter should not need any calibration – it comes ready for use. The ammeter, on the other hand, will probably need adjustment because we have added the shunt board. With the 250mm of heating wire specified, you should get pretty close to 20A FSD – in fact, you might decide that near enough is good enough! If it’s not, you may need to adjust the trimpot on the shunt board. Use another meter in series (eg, a multimeter on its high “20A DC” range) and adjust the pot so they both read the same current. Actually, providing 20A DC for calibration is not that easy to do, so you might have to do it with a lesser In use About Meters, Multipliers and Shunts We’ve been talking at length about meter shunts and multipliers. But if you’ve never come across the terms before, they can be confusing. Fear not! Help is at hand . . . Before we start, though, there are twofundamental and most important concepts which you must remember: to measure current, the meter is connected in series with the circuit. To measure voltage, the meter is connected in parallel with the circuit. This is shown below. (BREAK) X CIRCUIT UNDER TEST POWER SOURCE AMMETER – IN SERIES CIRCUIT UNDER TEST POWER SOURCE VOLTMETER – IN PARALLEL It may surprise you to learn that all meters, whether displaying current or voltage, are actually showing the current passing through them. When we are measuring current, all of the current has to flow through the meter. When measur80  Silicon Chip ing voltage, only a miniscule current flows through the meter (in fact, the smaller the better if we are not to get misleading readings caused by the meter “loading” the circuit under test). OK, with those to facts under your belts, here’s another one: with few exceptions, all meters, whether digital (as in our case here) or analog (ie, one with a moving pointer) can be made to read voltage or current. You do this, probably without realising, every time you use your multimeter. You can switch it to read voltage or current but the basic meter movement stays the same. When you switch to a different voltage or current range, the switch connects various resistors inside the multimeter into and out of circuit. If you’ve ever taken the back off a multimeter you’ll see a whole swag of resistors connected to the switch contacts. These resistors are called shunts and multipliers and are, for the most part, simply very high precision resistors. In the case of shunts designed for high current, they have extremely low resistance (perhaps only a few milliohms or so). Ohm’s law in action! Every meter has a certain amount of internal resistance. Apply a certain volt- age across that “resistor”, then a certain amount of current will flow through it. The exact amount of current will be according to Ohm’s law (I=E/R) and the meter will indicate that current. At the meter’s designed maximum current, the pointer will indicate maximum, which is known as full scale deflection, or FSD. This term comes from analog meters where the pointer moves to the top end of the scale. While digital meters obviously don’t have a pointer or scale, the term has stuck. Multipliers What happens if the meter is reading full scale and you add a resistor, exactly the same resistance as the meter, in series? As the overall resistance is doubled, if the applied voltage stays the same, the current halves. Therefore the meter will read half. That also means the meter can read higher voltages without risking damage. Using that same series resistor, you would be able to apply twice the voltage and the meter would read full scale. Add a resistor that is ten times the meter’s resistance and you would have overall eleven times the original resistance (the meter resistance itself plus the 10x series resistor), so you could apply eleven times siliconchip.com.au TAKE YOUR PIC Picaxe.com.au DISTRIBUTOR: MicroZed.com.au Developed for students, & professional performance makes PICAXE the most easy-to-use micro ever: PICAXE “programmer" is two resistors and a 4.5V battery! ALL PICAXE ITEMS ON OUR SHELVES! STOCKISTS In AUSTRALIA: altronics.com.au (Retail and Mail Order) oatleyelectronics.com School Electronic Supplies VOLTMETER POWER SOURCE sicom.co.nz surplustronics.co.nz (School orders only – John - 03 8802 0628) the multiplier needs to be exact. CIRCUIT UNDER TEST “MULTIPLIER” the voltage and the meter would once again read full scale. This resistor is known as a multiplier and is found in all voltmeters – including your multimeter when it is switched to a voltage range. In the multimeter a known high-precision resistor is connected in series with the meter movement which makes the meter read a certain voltage “full scale” (as set by the switch). Change the setting on the multimeter to a different voltage range and a different multiplier is switched in. The multimeter manufacturer marks the scale so that it reads directly in volts. Resistors used for meter multipliers are much more accurate than normal resistors – it’s not unusual for a multiplier to be accurate to one or more decimal places (eg, 100.3W). A normal 100W resistor, as you would use in a project, even one accurate to 1%, could actually be anywhere from 99W to 101W. That’s not good enough for a meter multiplier. For the meter reading to be exact, siliconchip.com.au In NEW ZEALAND Shunts Most meter movements are designed to read full scale with very little current flowing through them. A typical analog multimeter movement might only need 50mA for FSD – obviously, far too low for most practical uses (we often want to read five or ten AMPS – 100,000 times as much or more!). How do we do it? We use a resistor in parallel with the meter movement. Some of the current will still pass through the meter but some will bypass the meter and flow through the parallel resistor. This resistor is usually very significantly lower in resistance than the meter movement. It’s called a shunt, because it “shunts” some (indeed, usually a lot!) of the current away from the meter. With a known value meter movement and a known resistance shunt, you can work out what proportion of current flows through each and therefore you will know what overall current makes the meter read full scale. AMMETER CIRCUIT UNDER TEST POWER SOURCE “SHUNT” The basic analog meter movement may only need, say, 1mA through it to read full scale. A typical resistance for this type of meter would be 200W. If you want it to read 2mA instead, you would add another 200W resistor in parallel with the meter – each would take half the current, or 1mA, therefore the meter would show full scale for 2mA. Say you wanted it to read 1A (1000mA)? You would need to make the shunt take 999mA and leave 1mA for the meter. From Ohm’s law, you can work out that the meter has .001 x 200 or 0.2V across it when it is reading full scale; therefore your shunt resistor needs to be or .2/.999 or 0.2002W. Maths time: what should the shunt resistor be if you wanted to have the meter read 10A? If you said 0.2/9.999 or 0.0200W, you’d be right. Before we finish, what about a multimeter that reads Ohms? Believe it or not, this is simply a voltmeter powered by the multimeter’s internal batteries. The resistance you are measuring becomes part of the multiplier and the meter reads its value direct. That’s also why you cannot read resistance in a powered circuit – the voltage across the resistor in the circuit will almost certainly cause the multimeter to give a wrong reading. SC March 2007  81