#290: Vintage Tech: Tektronix 576 Curve Tracer

in today's video we're going to take a look at a piece of vintage tech in this case vintage Tektronix type 576 cough tracer but before we take a closer look at the curve tracer itself let's talk about what curve traces are a curve tracers job is to essentially plot the voltage versus current characteristic of various devices for two terminal devices like diodes a Zener diodes tunnel diodes and other types of two terminal devices we're talking about a curve that kind of looks like this where we have voltage along the x axis current along the y axis and for like an ordinary diode as you apply forward voltage eventually the diode starts conducting and you get current that flows a Zener diode will also conduct in the reverse direction we covered that in a recent video other types of two terminal devices might have different shapes but a basic job of the curve tracer is to apply a varying voltage and measure the resulting current the same thing can also be done for three terminal devices the three terminal devices such as bipolar Junction transistors or B JT's Junction field effect transistors or J FETs MOSFETs Triax e'en Junction transistors are all examples of devices that we can test with the curve tracer now we're still generating essentially an IV characteristic but also now we're going to be doing it over a family of curves and here's how that family is generated let's say we want to look at the collector current versus collector to emitter voltage of a bipolar Junction transistor we can essentially sweep those curves at various settings of base current so the third terminal for the devices is also being driven by a step generator that step generator can generate current in either direction for testing PNP or NPN bjts or can generate step voltages either positive or negative for testing devices like J Fetzer MOSFETs so depending on what devices we're going to be hooking up we can set up various steps to generate sweeps of IV characteristics through the device so why would we want to do this well some common apple Asians are to actually characterize a device and actually see how a device performs under certain conditions also maybe to do failure analysis to see if a device is breaking down or something like that or something has failed to see possibly why it's failed or what under what voltage and current conditions it fails another very common application is device matching many curve tracers like the 576 include a means to very easily switch between two devices under tests so you can flip back and forth very easily to look at the IV characteristics of a pair of devices to see how closely they match other applications might be to look at things like leakage like collector to emitter leakage for example of a transistor with the base turned off either shorted to emitter or ground it we're also looking at breakdown characteristics when does the Vice actually break down so all of these things can be done and more with a curve tracer now there are a lot of controls on a curve tracer and basically what we're going to use them to do is just say the current measurement what display scale we're going to have how many micro amps or milliamps per did it per division we're going to have vertically as well as the polarity that we're measuring whether it's positive or negative current in terms of the applied voltage we're going to control whether we're applying an AC voltage or a varying DC voltage and of what polarity and a what amplitude also control or horizontal scale how many volts per division we're going to have horizontally versus our current Berlocq vertically curve tracer typically does this by sweeping a voltage through a resistor and also you got controls to control what that resistor value is and that will provide a current limit when you're actually testing devices the vise connections are typically made down on a test fixture I'll take a look at that and how we're going to connect up the base or emitter where the things are going to be shorted to measure two terminals or we're going to do a three terminal connection with a step generator and also we've got controls for the step generator itself whether we're applying a current or voltage per step and what and how much voltage or current change we're going to have on our step by step basis to generate our family of curves what polarity what number of steps are going to apply and any offset we're going to apply to that step generator we want to start at zero or do want to start at something positive or negative and then sweep in one direction or another and then of course how fast we want it to step through the various family of curves so let's take a closer look at this beautiful 576 now before we take a close look at the curve tracer let's get an overall view our variable control down here controls the amount of sweeping supply voltage that's applied to our device under test a device is connected on this test fixture down here there are banana jacks where you can hook up various test fixtures or leads to hook up to your various devices under test switch down here allows me to switch left or right to control which of these device pairs were actually testing so we can again make it easy to compare two different devices to do matching the switch here controls while for the three terminal devices whether we're going to be applying the step generator to the third terminal or to leave it open or to short it out to make things like leakage measurements this set of controls right here controls the maximum voltage that could be applied across the two terminals that could ranges from 15 volts up to 1500 volts and then the second scale down here we'll take a close look at allows you to select a series resistor value this controls the polarity of the voltage being applied and how we're going to be sweeping and then the other controls here control the vertical scaling any display offset horizontally or vertically the horizontal scaling and then they step controls down here in the step generator controls the display is just a large CRT and then the readouts are available right here let's take a peek inside and then we'll take a closer look at the various controls okay on the left side of the instrument we've got the CRT and then behind this shield is the step controls to control the applied voltage and then the high voltage generator is underneath this can so most of the circuitry is covered up here because there's some pretty high voltages that can be generated now the right side is a bit of the the lower voltage side of the instrument so all of the circuit boards are all kind of exposed and you can see that it's all printed circuit board construction all solid-state a lot of the circuit boards are all kind of labeled this one says the vertical current per division board this is the display offset board right here this is the horizontal volts per division board and then the step generate or amplitude controls and some other step generated control boards down here some of the step generator logic and vertical amplifier board there and then some of the low voltage directive rectifier and regulator boards back here and then some of the power devices mounted here to the back panel so it's pretty serviceable and the the service manual even indicates that a lot of the wiring and notice that's all different color coded is actually a table in the service manual that tells you what color code is used for different types of wiring whether it's power supply wiring or single wiring and things like that so really nice a solid-state design that was produced from the late 60s up to through into the 1980s so let's go Metro some devices with this curve tracer I'll take a closer look at some of the controls as I mentioned this control down here controls how much voltage we're applying to the device you can actually put the device in the fixture and then just walk the voltage up slowly to look and see what the device will do this control here sets the maximum voltage that would be adjusted down on the variable control down here below we could see the scale from 15 to 1500 volts and then the lower scale here gives me the series resistor we can see a lower pointer and this is actually operated by pulling on the control and turning it to change what resistor then is in series with the voltage that's being applied and there's another little scale up here that tells you what the maximum power would be a that would be applied to the device under test with that given voltage and resistor combination this control tells you what pole air is going to be applied to essentially the collector emitter or the anode cathode terminals whether we're looking at positive for NPN devices we can actually go to AC which would apply a positive n negative voltage sweeps for doing things like dye acts or Zener diodes and things like that where you need to measure the characteristic verba both positive and negative applied voltages and then you've got negative voltages only just for like P and P devices the mood control allows you a sweep switch from normal which is when we're actually applying a sweep voltage or voltage that is varying from zero to the maximum voltage or just apply a DC so you can actually just move a dot across the screen and apply just a DC voltage as opposed to something that is varying so the switch down here on the test fixture kind of tells us how our terminals are going to be connected with the emitter grounded or we're going to apply the third term role to the step generator or we're going to leave it open or shorted to the emitter or if we're going to essentially apply a step generator to the emitter or leave it open and you choose that configuration based on the type of curve you want to generate whether you're looking at leakage or a family of curves to turn the device on the switch allows me to select which set of terminals on the fixture we're turning on again makes it easy to match devices by flipping back and forth between the left hand and the right hand terminals the devices under test are connected to the terminals right here and that could be done through you know kind of these fixtures that were available to connect up three terminal device and you just plug this in there are different styles of term test fixtures available because it's just specifically for two tonal devices like diodes and things like that or you could even just make up your own with banana leads and alligator clips to connect up to whatever you want this control here controls the how much deflection we get vertically on the screen essentially amount of current per division that we get vertically this control is kind of interesting it allows me to apply an offset either left and right or vertically that allows me to zoom in maybe on a particular area of the waveform that may not be at the center or whatever and if you watched my Zener diode video you saw how I use this offset control to essentially take my DC point or my zero voltage point from here over to here so like you get a closer look at the breakdown in the Zener voltage now this horizontal control is just the number of volts per division horizontally on the on the curve much like the horizontal control is on a scope that would give you an amount of time per division this is just voltage per division horizontally and finally we have these step generator controls we can determine the number of steps that we're going to apply and then if we're stepping current how many amps or micro amps or milliamps per step we're going to be applying for each of those steps that we go through we can also apply an offset so that instead of starting at zero we can start above or below zero and then use this multiplier control to decide how much we want to offset that step generator positive or negative we could also do pulse steps for high-power devices we can actually just apply the IV and pulses rather than continuously to avoid overheating the device and then we can control how quickly those steps are applied and even invert the steps for certain types of devices like a that enhancement or a depletion and mode FET all right now that we've got an overview of the controls let's go make a couple of measurements well first take a look at a simple diode I've got that set up in the text test fixture right down here and we've got it set to 15 volt maximum applied a one milliamp vertical per division horizontally one volt per division set up right here and we're not using the step generator so as I turn the voltage up what we're doing is applying an AC voltage so below ground and above ground we can see we're blocking in the reverse direction and we're conducting in the forward direction after about six or seven tenths of a volt so exactly what you'd expect a diode to do okay next I've got an NPN transistor down in the fixture down here and we've got the fixture set up to apply the step generator to the so the same vertical and horizontal controls a milliamp vertically upper-division a volt horizontally per division we're gonna apply 10 micro amps per step to the step generator and going to apply 4 steps with the 2x speed so to reduce some of the flicker we see on the screen so as I turn the collector to emitter voltage up we can see when we have no current applied to the base we're just seeing DC down here 10 micro amps 20 micro grams 30 micrograms 40 micro amps applied to the base as I turn the collector emitter voltage up we can see that the collector voltage or assuming the collector current remains reasonably constant versus collector to emitter voltage at a given base current now when we're using the step generator we get two more readouts here one that tells me how many micro amps per step I'm applying in this case to a bipolar Junction transistor and since we know the vertical scale in collector current versus per division and we know how much we're applying per step that vertical scale can also be calibrated in beta or current gain so this tells me that I've got a current gain of 100 per division so I can see I'm actually going a little bit more than one division here so I know that my the beta of this transistor is a little over a hundred in this case about 120 130 this will allow us to also see if beta varies as a function of base current okay now I have a j-3 10 J fed attached to the test fixture and I made a change to the step generator so instead of stepping current or stepping voltage in this case one volt per step and because of the J FET which is essentially a depletion mode device you get the maximum current when the vgs is equal to zero and you start reducing the drain current as you make vgs negative so i had to push the invert button here so i apply essentially a zero volts then minus 1 minus 2 minus 3 etc and we generate our family of drain current curves drain current versus VDS at various apply Viji S's this is vgs of zero right here fact that we have reduced the number of steps we can see we're going down so that's essentially zero volt and minus 1 and then minus 2 minus 3 etc so generate a family of curves for j FETs and again what's really useful sometimes with generating these types of curves is if you're developing a circuit that requires matched devices you can now match devices over a complete operating range as opposed to just a particular set of bias conditions that a multimeter might apply all right one final device hooked up to the curve tracer here let's see what it is that's a two thermal device we can see we have no controls on our step generator here well looks like a diagonal line tells us it might be a small value resistor or something both keep turning the device voltage up and see what happens Oh what is going on here a little bit of a jump and a change back to another characteristic over here you might have guessed it this is a tunnel diode I did a video on tunnel diodes a while back I'll link that down below but we can see that a tunnel diode in the reverse direction looks just like a low value resistor in the forward direction it starts conducting but then once it reaches this new voltage here this new current I should say it snaps back and actually draws a very little current and then continuing to bring the voltage up it starts to conduct almost like a normal diode and this reversal of this IV characteristic is essentially a negative resistance and I talked a bit more about that in the tunnel diode video but so it'd be interesting to take a look at what a tunnel diode looks like on a curve tracer now if you like what you see please give me a thumbs up if you haven't subscribed to the channel already please do so and thanks again as always for watching

Channel: w2aew

Views: 22,550

Keywords: W2AEW, Tek, Tektronix, 576, curve tracer, BJT, transistor, JFET, MOSFET, Diode, Zener, diode, step generator, vintage, test equipment

Id: c-y8UmoHbtw

Length: 17min 27sec (1047 seconds)

Published: Tue Aug 21 2018