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SemTest Pt.1: By JIM ROWE Check all those semiconductors in your collection with this easy-to-build test set! How many discrete semis have you got in your collection? Hundreds? Thousands? Are they all good? Don’t know? With our new Discrete Semiconductor Test Set you will be able to test a wide range of active components: LEDs, Diodes, Bipolar Junction Transistors, Mosfets, SCRs and Programmable Unijunction Transistors (PUTs), for gain (where applicable), voltage breakdown and leakage. You can even run tests on IGBTs and Triacs! O F COURSE, THERE are lots of semiconductor testers out there. These range from the handy pocketsized instruments produced by Peak Electronic Design Ltd in the UK to large laboratory bench instruments made by Agilent and costing many thousands of dollars. The former group are not able to test the range of semiconductors that we would like, while the latter instruments are beyond the 42  Silicon Chip reach of all but a few research labs. So Publisher Leo Simpson set me the task of producing a new design. It had to be easy to drive and would be somewhat similar in concept to the “Test Set For Transistors & Diodes” featured in Electronics Australia magazine way back in the July and August 1968 issues (yes, back in the olden days – and it was my design too!). It was pretty simple – using a bunch of rotary switches, a 50µA moving coil meter and olde-worlde point-to-point wiring – but it could perform most of the basic tests that were needed on the discrete semiconductor devices of the day. I took one look at that old 1968 design and shuddered: all that point-topoint wiring – all those switches – no PCB – an analog meter. Gaaakkk! What could Leo be thinking? Not only that, siliconchip.com.au it was designed long before Mosfets were even thought of and we would have to include them, of course. In the fullness of time (a silly expression glossing over the trials and tribulations – not to mention the blood, sweat and tears – of producing a completely new design), we came up with the SemTest. It’s otherwise known as a Discrete Semiconductor Test Set – which is too much of a mouthful. It’s around half the physical size of the 1968 design and it’s controlled by a microprocessor, with a 16x2 LCD panel used to display the device to be tested, the test to be run and the test results. There are a minimum of front panel controls: one rotary switch, one pot and five pushbuttons. And the curly problem of catering to all the different semiconductor sizes and pin-outs has been solved by employing an 18-pin ZIF (zero insertion force) socket. These sockets are normally used for programming microprocessors but they are ideal for this application. All the parts inside the case are accommodated on two medium-sized PCBs which are connected together by three IDC cables. However, before we jump into describing the circuitry of the SemTest in detail, we need to discuss the tests it can perform on each type of the most commonly used discrete semiconductors. After all, if you are contemplating building the SemTest, you will want to understand all the tests that it can run. TESTS AVAILABLE ON THE DISCRETE SEMICONDUCTOR TEST SET Device Type Diodes, including zener & schottky (also Diacs) LEDs Bipolar Junction Transistors (NPN or PNP) Mosfets (N-channel or P-channel) SCRs & PUTs (also Triacs) Test Parameter Extended description IR (BV) Reverse avalanche current with BV (600V) applied* IR (OPV) Reverse leakage current with OPV (10/25/50/100V) applied* VF (OPV) Forward voltage drop with OPV (10/25/50/100V) applied* VR (BV) Zener/avalanche voltage with BV (600V) applied* IR (OPV) Reverse leakage current with OPV (10V) applied* VF (OPV) Forward voltage drop with OPV (10/25/50/100V) applied* V(BR)CBO (BV) Breakdown voltage with e o/c, BV (600V) applied* V(BR)CEO (BV) Breakdown voltage with b o/c, BV (600V) applied* ICBO (OPV) Leakage current with e o/c, OPV (10/25/50/100V) applied* ICEO (OPV) Leakage current with b o/c, OPV (10/25/50/100V) applied* hFE with IB = 50A (OPV) Forward current gain with IB = 50A, OPV applied* hFE with IB = 200A (OPV) Forward current gain with IB = 200A, OPV applied* hFE with IB = 1mA (OPV) Forward current gain with IB = 1mA, OPV applied* V(BR)DSS (BV) Breakdown voltage with g-s short, BV (600V) applied* IDSS (OPV) Leakage current with g-s short, OPV (10/25/50/100V) applied* IDS vs VGS (OPV) (gfs) d-s current vs VGS (0-12V), OPV (10/25/50/100V) applied* V(BR)AKS (BV) Breakdown voltage with g-k or g-a short, BV (600V) applied* IAKS (OPV) a-k current with g-k or g-a short, OPV (1/25/50/100V) applied* IAK with IG = 50A (OPV) a-k current with IG = 50A, OPV (1/25/50/100V) applied* IAK with IG = 200A (OPV) a-k current with IG = 200A, OPV (1/25/50/100V) applied* IAK with IG = 1mA (OPV) a-k current with IG = 1mA, OPV (1/25/50/100V) applied* VAK(ON) (OPV) a-k voltage drop when on, OPV (10/25/50/100V) applied* *Both BV and OPV are always applied via appropriate current limiting series resistors RSERIES A A DUT* VOLTAGE DIVIDER RELAY9 K siliconchip.com.au ADC0 (DEVICE VOLTAGE) K OFF = FWD ON = REV ADC1 (DEVICE CURRENT) Diodes & LEDs These sound simple enough but there are different sorts: standard silicon and germanium signal & rectifier diodes, zener/avalanche diodes, schottky barrier diodes, LEDs and Diacs (bipolar breakover diodes, which are actually a 2-terminal thyristor). The new tester can perform basic tests on all of these devices. A simplified version of the diode test circuitry used in the SemTest is shown in Fig.1. It’s very straightforward, yet can be used to measure any of four basic diode parameters: (1) VF – the voltage drop when conducting in the forward direction; (2) IR – the leakage current which flows when a reverse “operating” voltage (OPV) of 10V/25V/50V/100V is applied via an appropriate series current limiting resistance; (3) IR – the current which flows when +V (BV OR OPV) * DIODE, ZENER OR LED RSHUNT Fig.1: the basic diode test circuitry. It uses Relay9 to switch the polarity of the diode under test, a shunt resistor to allow current measurements and a voltage divider to interface with the microcontroller. a higher “breakdown” voltage (BV) of 600V is applied (again via a suitable series current limiting resistor); and (4) VR – the voltage drop when the diode is conducting in the reverse direction in “avalanche” breakdown mode. All four of these tests can be applied to test zener/avalanche diodes, signal & rectifier diodes, schottky diodes and even Diacs. The last two tests are not available for testing LEDs as these devices can be damaged if sufficient current flows during avalanche breakdown. In fact, before you do an IR test on a LED, the SemTest warns you of possible damage if the lowest operating voltage of 10V is not selected. The diode test circuit of Fig.1 uses RELAY9 to switch the polarity of the diode under test. When RELAY9 is off (not energised), the diode’s anode (A) is connected to the test voltage source (+V) via series current-limiting resistor RSERIES. Note that test voltage +V is switched between the operating voltage (OPV) and the breakdown voltage (BV) level by the microcontroller, which also changes the value of series resistor February 2012  43 RSERIES DUT* C +V (BV OR OPV) C B B E ADC0 (DEVICE VOLTAGE) VOLTAGE DIVIDER RELAY10 E OFF = NPN ON = PNP RELAY11 ADC1 (DEVICE CURRENT) OFF = BVceo, Iceo or hFE ON = BVcbo or Icbo RELAY6 RSHUNT RELAY5 +Ibias –Ibias OFF = BVcbo, BVceo, Icbo or Iceo ON = Hfe (PNP) OFF = BVcbo, BVceo, Icbo or Iceo ON = Hfe (NPN) NOTE: ±Ibias LEVELS ARE SET VIA RELAYS 3 & 4 * NPN OR PNP BIPOLAR TRANSISTOR Fig.2: the basic test configuration for bipolar junction transistors (BJTs). It uses four relays to perform all of the basic tests normally required on NPN or PNP devices. RSERIES to suit the various tests. In operation, the micro switches +V on only during the actual test and then off again at the end of the test. For the “reverse bias” tests, the micro energises RELAY9 which simply reverses the diode polarity so that the cathode (K) is connected to +V instead of the anode. The rest of the diode test circuit includes a voltage divider, used to allow the micro to measure the voltage across the diode under test, by means of the micro’s analog-to-digital (A/D) converter input ADC0. The micro also switches the voltage divider’s ratio to suit the voltage source used for each test. Finally, there’s a shunt resistor (R SHUNT ) connected between the cathode (or anode) of the diode and ground. The top of this resistor is connected to the ADC1 input of the micro so it can measure the voltage across RSHUNT and then calculate the device current. Again, the value of RSHUNT is switched by the micro, in this case to suit the current range required for the selected test. By the way, since the voltage drop across RSHUNT effectively adds to the device voltage as measured via the voltage divider and the microcontroller’s ADC0 input, this has 44  Silicon Chip the potential to introduce a small error in the device voltage measurement. This voltage drop across RSHUNT is quite small, with a maximum of 2.0V for a “full scale” current reading of 20mA (or 200µA on the low range). To eliminate this problem, the firmware automatically corrects the reading. It does that by subtracting 100mV for each 1mA of device current on the higher range, or for each 10µA of current on the low range (ie, it automatically subtracts the voltage across the RSHUNT). Testing Diacs Before we move on, let’s look at how a Diac can be tested with the SemTest. It should connected to the diode A and K terminals (either way around) and first given the diode VF test with the lowest (10V) setting for OPV. This will show you whether the Diac is shorted (which will give a reading of no more than about 0.25V and a current of about 2.5mA) or “OK” (which will give a reading of close to 10V). If you do get a reading of very close to 10V, you can repeat the above test at 25V or 50V until the Diac breaks over into conduction. Typical Diacs break over at between 25V and 35V, with a current of less than 200µA. When the Diac does switch into conduction, the VF reading suddenly drops to a much lower level – probably around 5-10V – while the current jumps up into the 3-10mA region. If the Diac behaves as described, you then do the test in the other direction: ie, switch back to the 10V setting for OPV and then test it with the IR (OPV) test selected. This will let you check the Diac’s operation in the reverse direction. You should again see it drawing a current of less than 200µA with only 10V applied, with the current jumping up to between 5mA and 15mA when you select an operating voltage of 25V or 50V so that it “breaks over” again. If your Diac gives these expected results in both tests, it is working as it should. Testing transistors Testing bipolar junction transistors or “BJTs” is more complex than with diodes, because there are NPN and PNP types and they have three leads rather than two. Fig.2 shows the test configuration for BJTs. This uses four relays to perform all of the basic measurements normally required for NPN or PNP devices: (1) ICBO – the leakage current passed between collector and base, with a selected operating voltage (OPV) applied and the emitter open-circuit; (2) ICEO – the leakage current passed between collector and emitter, again with a selected operating voltage (OPV) applied but this time with the base open-circuit; (3) V(BR)CBO – the breakdown voltage measured between collector and base, with the emitter open-circuit but with a breakdown voltage (BV) source applied via a series current-limiting resistor; (4) V(BR)CEO – the breakdown voltage measured between collector and emitter, with the base open circuit but with a breakdown voltage (BV) source applied via a series current-limiting resistor; and (5) hFE – the common-emitter forward current gain, measured at any of three base current levels (IB = 50µA, 200µA or 1mA). The choice of base current levels is provided to cope with small and medium-power devices. As you can see from Fig.2, RELAY10 is used for setting up the BJT circuit for testing either NPN or PNP devices. RELAY11 is used to perform the base/ emitter switching for the various tests, siliconchip.com.au RSERIES DUT* D S +V (BV OR OPV) D 22 G G ADC0 (DEVICE VOLTAGE) VOLTAGE DIVIDER RELAY12 S 1M OFF = N–CH ON = P–CH ADC1 (DEVICE CURRENT) RELAY13 RSHUNT OFF = G–S SHORT ON = G CONNECTED TO Vgs 10k * N–CH OR P–CH ENHANCEMENT MODE MOSFET RELAY14 ADC2 (MEASURE Vgs) Vgs 10k ADJUST –Vgs +Vgs K ADJUST +Vgs OFF = +Vgs (N–CH) ON = –Vgs (P–CH) VR10a 10k ZD3 12V K VR10b 10k ZD4 12V A 10k 10k A –Vgs Fig.3: the Mosfet test circuit. Only three relays are used and these allow all the main tests normally required for both N-channel and P-channel Mosfets. The positive VGS (gate-source) voltage is derived from zener diode ZD3 and varied by VR10a, while the “negative” VGS voltage is derived from ZD4 and varied by VR10b. while RELAY5 is used to switch on positive base bias current (+IBIAS) for hFE testing of NPN devices. RELAY6 is used to switch on negative base bias current (-IBIAS) for hFE testing of PNP devices. Additional relays (RELAY3 and RELAY4, not shown in Fig.2) are used to switch both +IBIAS and -IBIAS between the various current levels. As with the diode testing circuit, either operating voltage (OPV) or breakdown voltage (BV) can be applied to the transistor being tested, via series current-limiting resistor RSERIES. Again the micro switches the OPV/ BV source on only for the actual test, and then off when the test is ended. It also changes the value of RSERIES to suit each kind of test. As before, there is a voltage divider across the device being tested, feeding the micro’s ADC0 input so that the micro can measure the device voltage VDEV. Again, the micro changes the divider ratio to suit each kind of test. The device current is also measured in exactly the same way as for diodes, with shunt resistor RSHUNT used to effectively convert the device current into a small voltage for measurement via the micro’s ADC1 input. The micro siliconchip.com.au can also switch the value of RSHUNT to provide two current ranges: 20mA and 200µA. We should point out here that, as before, the small voltage drop across RSHUNT will effectively add to the device voltage measurement, introducing a small measurement error for V(BR) CBO and V(BR)CEO. Again the software corrects for this error by subtracting 100mV for each 1mA of device current on the higher range, or for each 10µA of current on the low range. Testing Mosfets Testing metal-oxide-semiconductor field effect transistors or “Mosfets” is not significantly more complicated than with BJTs, even though Mosfets are a voltage-controlled transconductance device rather than a currentcontrolled transadmittance device. As with BJTs there are again two types, in this case N-channel and Pchannel devices, with different polarity requirements for both drain-source voltage and gate bias voltage. There’s also a difference in terms of breakdown voltage and leakage current measurement, of course. Note, however, that the SemTest is only capable of testing junction FET or “JFET” devices in a limited sense, as these operate in depletion mode rather than in enhancement mode as used by modern Mosfets. Whereas Mosfets pass virtually zero drain-source current with zero gate bias and need gate bias in order to pass significant drain-source current, JFETs work the other way around; they pass a significant drain-source current with zero gate bias and need gate bias to be applied in order to “throttle back” the drain-source current. This means they require “negative” gate bias, in contrast with the “positive” bias needed by Mosfets. Despite this limitation, the SemTest is capable of testing JFETs for one quite important parameter: IDSS – the drainsource gate current with the gate tied to the source (ie, the zero-bias channel current). This is done via the same Idss test used for Mosfets (see below), the difference being that with Mosfets the reading should be very low (usually well below 200µA), while for JFETs the reading will be relatively high (probably 10-20mA). The Mosfet test circuit is shown in simplified form in Fig.3 and it’s relatively straightforward. Only three relays are used but these allow the February 2012  45 RSERIES A +V (BV OR OPV) DUT* A (AG) G K ADC0 (DEVICE VOLTAGE) VOLTAGE DIVIDER (KG) K RELAY15 RELAY16 OFF = SCR ON = PUT ADC1 (DEVICE CURRENT) RSHUNT OFF = G shorted to K (SCR) or A (PUT) ON = G connected to ±Ibias +Ibias (VIA RLY5) OR –Ibias (VIA RLY6) * SCR OR PUT Fig.4: the test circuit for SCRs and PUTs uses two relays for switching and is similar to that used to test bipolar junction transistors (BJTs). It carries out five basic tests. SemTest to perform all three of the main tests normally needed for either N-channel or P-channel Mosfets: (1) IDSS – the drain-source current with zero gate bias (ie, gate tied to source). This can be measured with any selected operating voltage (OPV) applied between drain and source, via a series current-limiting resistor; (2) V(BR)DSS – the drain-source breakdown voltage, again measured with gate tied to source but in this case with the higher voltage source (BV) applied between drain and source, via a highervalue current limiting resistor; and (3) ID – the drain-source current which flows at any gate bias voltage VGS (variable between 0V and approximately 12V), with any selected operating voltage (OPV) applied between drain and source. This allows the transfer characteristic of a device to be measured, and its transconductance worked out. As you can see from Fig.3, the Mosfet drain-source voltage and drain current are measured in exactly the same way as for BJTs and diodes, us- ing a voltage divider feeding ADC0 for the voltage measurement and shunt resistor RSHUNT feeding ADC1 for the current measurement. The OPV/BV switching and RSERIES switching are managed by the micro as before, as is the voltage divider ratio and the value of RSHUNT. The main differences between Fig.3 and the earlier test circuits are in the gate switching circuitry, involving RELAY13 and RELAY14. The first of these relays carries out the primary gate switching, shorting the Mosfet’s gate to the source for the IDSS and V(BR) DSS tests when it is not energised or connecting the gate to a bias voltage source VGS when it is energised (for the ID versus VGS test). RELAY14 then performs the job of selecting either a “positive” VGS source for N-channel devices, or a “negative” VGS source for P-channel devices. The positive VGS source is derived from the test voltage (OPV) via zener diode ZD3 and varied by potentiome- ter VR10a, while the “negative” VGS source is also derived from OPV but via ZD4 and varied by VR10b. The latter is only negative by comparison to the Mosfet’s source terminal, which in the case of a P-channel device is connected to OPV. This explains why VR10a is adjusted upwards from ground (0V) to increase +VGS (for N-channel devices), while conversely VR10b is adjusted downwards from the device source voltage (representing zero VGS) to increase -VGS for P-channel devices. Since VR10a and VR10b are the two sections of a dual-ganged 10kΩ+10kΩ pot, they are simply wired in converse fashion so that the effective gatesource voltage advances from zero as the pot is turned clockwise. The micro is able to work out the effective gate voltage for any setting of VR10a or VR10b via the connection from the VGS source, as selected by RELAY14, to a third ADC input of the micro (ADC2). But because this only allows the micro to measure the “raw” gate voltage VG, relative to ground, this means that for P-channel devices it also has to measure the source-drain voltage of the device and subtract the measured gate voltage from it, to calculate the effective gate-source bias (-VGS). With N-channel devices this isn’t necessary, although the small voltage developed across current measuring shunt resistor RSHUNT will reduce the effective gate-source bias for these devices, by the same factor of 100mV for each 1mA of current on the higher current range or 10µA of current on the lower range. As with the hFE measurements for BJTs, the firmware automatically makes this correction. What about IGBTs? Although they’re not widely used in general electronics, insulated-gate bipolar junction transistors or IGBTs Issues Getting Dog-Eared? Keep your copies of SILICON CHIP safe & secure with these handy binders REAL VALUE AT $14.95 PLUS P &P Available Aust, only. Price: $A14.95 plus $10 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. 46  Silicon Chip siliconchip.com.au The lower board in the SemTest carries the PIC microcontroller, the power supply components and the test voltage selector switch. are encountered in automotive ignition systems, fuel-injection controllers, high power inverters and AC induction motor drives. They can be regarded as very much like an N-channel Mosfet and an NPN BJT/PNPN silicon-controlled switch combined, with a collector as the main positive electrode and an emitter as the main negative electrode. However, they have a gate electrode for voltage control instead of a base electrode for current control. IGBTs are usually quite high-power devices, so the modest test currents available inside the SemTest mean that it isn’t really possible to use it to fully characterise the performance of an IGBT. However, you can perform basic tests on an IGBT by connecting it to the SemTest’s Mosfet testing terminals (C to the drain terminal, E to the source terminal and G to the gate terminal). You then test it as if it were an N-channel Mosfet, making a mental conversion of the test results into the equivalent parameters for an IGBT. For example, the voltage reading you get for V(BR)DSS will correspond to the IGBT’s V(BR)CES (collector-emitter breakdown voltage with the gate shorted to the emitter), while the readsiliconchip.com.au ing you get for IDSS will correspond to the IGBT’s ICES (collector-emitter leakage current with gate shorted to emitter). You’ll even be able to get an idea of the IGBT’s gate threshold voltage VGE(TH), by using the Mosfet ID vs VGS test and finding the gate voltage where ID (corresponding to the IGBT’s collector-emitter current ICE) begins rising from its ICES “off” level. Testing SCRs & PUTs The fourth main type of discrete semiconductor device that the SemTest is capable of testing is thyristors or silicon-controlled switches (SCSs) – in particular, SCRs (silicon-controlled rectifiers) and PUTs (programmable unijunction transistors). Note that another name for an SCR is a cathode-gate SCS, while a PUT is more accurately described as an anode-gate SCS. They are both PNPN devices, and similar apart from the different gate connections. So in that sense they are essentially just two different “flavours” of SCS devices, like NPN and PNP bipolars or N-channel and P-channel Mosfets. As a result, the circuitry needed for testing SCRs and PUTs is not all that different from that needed for BJTs, as can be seen from the simplified circuit shown in Fig.4. Despite its simplicity, this circuit allows the following measurements to be carried out on SCRs and PUTs: (1) V(BR)AKS – the breakdown voltage for an SCR, with its gate tied to the cathode and a source of high voltage (BV) applied between anode and cathode via the usual current-limiting resistor RSERIES; (2) V(BR)AKS – the breakdown voltage for a PUT, in this case with its gate tied to the anode and the high voltage (BV) applied between anode and cathode, again via RSERIES; (3) IAKS – the anode-cathode current for either an SCR or a PUT, with its gate tied to either the cathode (SCR) or anode (PUT), and with any selected operating voltage (OPV) applied between anode and cathode via a current-limiting resistor RSERIES. In other words, the “OFF” current of the device; (4) IAK – the anode-cathode current for either an SCR or a PUT, with any selected operating voltage (OPV) applied between anode and cathode, and its gate connected to any of three sources of bias current: +50µA, +200µA or +1mA in the case of an SCR, or -50µA, -200µA or -1mA in the case of a PUT. February 2012  47 This view shows the partially-completed top board. It carries the LCD, the ZIF socket (not yet mounted) and most of the relays. It’s connected to the bottom PCB via three IDC cables. These measurements allow you to gain a good idea of the device’s triggering sensitivity; and (5) VAK – the anode-cathode voltage for either an SCR or a PUT when it has switched ON and is conducting. In other words, Vak is the device voltage drop in its conducting state. These measurements are really all that are needed to test and roughly characterise most PUTs and low-tomedium-power SCRs in general use. But please note that because of current limitations, the SemTest is not really capable of testing high-power SCRs – except in a basic “shorted or open” sense. Apart from anything else, the maximum gate bias current provided by the SemTest is only 1mA, which may not be enough to trigger a high-power SCR. As shown in Fig.4, the device voltage and current measurement arrangements for SCRs and PUTs are exactly the same as for BJTs. The only real differences are with regard to gate switching, where RELAY15 controls the initial SCR/PUT switching and RELAY16 controls whether the gate is connected to the cathode (SCR) or anode (PUT), or to a bias current 48  Silicon Chip source (via RELAY5 or RELAY6, with the actual bias current level selected via RELAY3 and RELAY4). Triac testing Triacs are another common form of discrete thyristor device, more widely encountered than SCRs. They’re used to control mains AC in many electrical appliances. Because Triacs are essentially gatecontrolled AC switches, the only way to fully characterise their behaviour is in a tester which allows them to be tested under AC conditions. However, because a Triac is very much like a pair of SCRs connected in inverse parallel, it’s possible to use the SemTest’s SCR/ PUT tests to perform a full range of measurements on a Triac. For example, if you connect a Triac to the SemTest’s SCR terminals with its A1 electrode connected to the cathode terminal, its A2 electrode to the anode terminal and its gate to the gate terminal (where else?), you can do all the SCR tests described above, ie, V(BR)AKS, IAKS and IAK for any of the three levels of +IBIAS and even VAK(ON). So you can give it a fairly thorough “DC workout” in its main operating “quadrant”. If you then leave it connected in exactly the same way but this time check it as if it were a PUT, you can thoroughly test it in a second quadrant. Finally, if you swap the A1 and A2 electrode connections so that A2 goes to the cathode terminal and A1 to the anode terminal, you will be able to test it in the other two quadrants, ie, by testing it again as an SCR and then as a PUT. So for a quick and dirty test, you just run the SCR tests on the Triac for just one quadrant. If you want to test in the other three quadrants, you need to run the tests three more times, as just described. The only limitation to this procedure is that the maximum gate bias current which the SemTest can provide is ±1mA, which as with SCRs may simply not be enough to trigger high-power Triacs. Summary That should give you a good idea of the discrete semiconductor devices that our new SemTest is capable of testing and measuring. Next month, we will present the full circuit details SC and start the construction. siliconchip.com.au