Posts Tagged ‘voltage measurement’

Voltage Divider Assembly

10 April 2017

voltage divider assembly inside
A few years ago I purchased a pair of differential voltmeters because I was looking for aluminum equipment cabinets and thought these might work. The front panels were actually a full 1/4-inch think, which was a little more than I bargained for. These were military-grade equipment, and I think both of these items had been in use by the Army.

I got them from Fair Radio Sales “as-is.” The shipping cost almost as much as the equipment did. These meters were made by John Fluke Mfg Co, Inc, of Seattle in the early 1960s. They used mostly vacuum tubes and various other technologies now considered Legacy. The idea behind a differential voltmeter is that you compare a known voltage with an unknown one, and use a meter to tell when they are equal. The settings you used to get the known voltage are then equal to the voltage you wanted to measure. Ponderous. Today’s digital voltmeters do the same thing, except they “turn the dials” for you and present the result on a readout screen.

voltage selection dials

Marked dials function as an old-school digital readout.

This assembly is just the voltage divider for the known voltage. It consists of a set of switches and precision resistors arranged so that when you put in a reference voltage, the output equals the voltage you dial in. For accuracy, the resistors used have to be high-precision. These ones have a tolerance of +/- .02% which these days is unheard of. I saw a refurbished working version of this equipment for sale for over $1,000. It’s considered an ultra-precise laboratory-grade device.

inside the voltage divider

From what I can tell this equipment was entirely hand-assembled. That was how it was done in the “old days” of electronics. The colors involved are kind of pretty but they also served to help the assembler be sure he or she had the right part or was sticking the right wire in the right place. All the resistors were made of lengths of fine wire wound onto forms then glued in place with clear paint. Fluke may have constructed the rectangular ones themselves. The yellow cylindrical ones were made by an outside firm.

voltage divider back side

I couldn’t get over the workmanship put into these components, so I kept one of them. But this one is now extra and is destined for the recycling center.

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Electronics Project – Technical

21 December 2015
V-I box front panel

Front panel for my “V-I Box” – reused Extron video equipment.

This is a technical article about a project I recently finished (for the most part). It doesn’t work that well, but it is quite complex so gives me a chance to cover several topics while talking about just one project.

V-I Box

V stands for voltage, named after Volta, an Italian.
I stands for current (French intensité de courant) as used by the French scientist Ampère.

In electricity and electronics, any component will have a characteristic “V-I Curve” showing the relationship between voltage across the component and current through the component. These days, we usually use “curve tracers” to get this graph, but you can also plot it a point at a time using a variable power supply.

Last year I had constructed such a variable supply for a little presentation I gave at work about transistors. Later I made another one to use to demonstrate the operation of vacuum tubes. I wanted to preserve these projects (and the parts used in them) in a more compact space, so I decided to squeeze them into an old Extron (video processing equipment maker headquartered in Anaheim CA) enclosure. The photos show what resulted.

The design is far from ideal but retains most of the features of the older designs, while making some changes to increase the current capacity of the lower voltage source and keep part of the Extron front panel.

Following is a discussion of some of the features of this project.

V-I box guts

Inside my project…

Creating a 1-250V variable supply

I was not prepared to create a supply that could ramp from 1 volt to 250 volts in one smooth transition, so had procured several power supplies, 4 50-volt supplies, a 25-volt supply, and a 1-25 variable supply in the form of a DC-DC converter.

I then had to create logic that would switch through the supplies in 25-volt steps. As part of this scheme, the supplies are put in series (or “stacked”) with each other. This is only possible because most power supplies (not some of the old tube ones, though) use isolation transformers so that the output and input can be at different DC levels. This isolation usually works up to at least 1,000 volts.

The high-voltage sub-system receives a binary code (from 0 to 255) and must decide how to connect the supplies to get a supply voltage about equal to the value of the code supplied.

To implement this I used 4 relays using 5-volt coils, so I could use logic signals to switch the voltages. The DC-DC converter was a cheap one using a potentiometer to set its output value. To make it variable by remote control, I had to set up a control loop using an op amp to drive an optical isolator. The output transistor of the isolator would server to replace the potentiometer.

Other voltage sources

My earlier designs had two other voltage sources, one to bias the tube grid or transistor base or gate, and another for the tube heater. I used a low-power op amp to supply the bias voltage, as hardly any current drive is needed, and I thought +/-10V would be a sufficient range.

I had used a variable linear voltage regulator for the heater voltage, but in this design decided to leave that out and just make my 5V control electronics supply available for that use.

I also needed a supply to run my DC-to-DC converter. It is a buck converter, so I needed greater than 25V. And I only had +/- 12V rails and +5V available from my control electronics power supply. So I used a boost converter driven by the +12V rail to get about 30V which I fed to the buck converter.

Measuring voltages

Providing panel meters for measuring instruments is always a challenge. Today the most common design uses a microcontroller with an A-to-D (analog-to-digital converter) driving an LCD (liquid crystal display). However, I had already purchased a bunch of little modules for the earlier boxes and wanted to use the. So I fit four of them into the new panel. All they do is display the input voltage when powered by at least 5V. These modules have about 370K input resistance and can display up to 99.9 volts. You can get all kinds of different ranges. As I wanted one display to show the entire range of output voltages, I had to divide the input by ten and settle for 25.0 maximum readout. The other module is used for the 1-50V segment of the output supply, and operates at full 3-digit precision. The bias voltage is displayed on the LCD that came with the original equipment.

Measuring currents

In modern electronics, currents are always measured by converting them to voltages first. The old analog current meters responded directly to input current. To convert a current to a voltage, just pass the current through a known resistance, then measure the voltage drop across the resistor. This may then be amplified if needed. In my case, I needed to amplify the signal so that I could use my little voltmeter modules as current meters. (You can also get modules that have this capability built in.)

For the “heater” (+5V supply) current, I used a 0.2 ohm resistor. This would drop 1 volt at 5 amps, so I needed to amplify it to give my meter a range of up to about 3 amps (reading of 30.0). I used an op amp in “quasi differential” configuration to get this reading, so I could put the resistor in the high side of the 5V rail.

For the main supply I used a 4 ohm resistor, as I expected to draw only about 30ma maximum (30.0 reading) from this supply. 30mA through 4 ohms gives a voltage drop of 120mV, so I had to amplify this by a gain of 250. I used an op amp in non-inverting configuration for this purpose.

I used the 30V supply driving my DC-DC buck converter to power these op amps. This was close to their maximum supply rating of 32V!

Front panel controls

The front panel that came with the Extron equipment had an LCD, some pushbutton switches, and 4 little knobs. The knobs felt like potentiometers, but they turned out to be rotary encoders. Instead of replacing them (would have been a lot simpler) I decided to use an Arduino to make the rotary encoders function like digital potentiometers. This was handy for controlling the high voltage supply, but was overkill for the other variable supplies.

It took some fiddling and internet searching to get some workable code for the encoders, but once I got it, they worked satisfactorily. In order to convert the digital values back to control voltages, I had to send them out to a 32-bit shift register and then run R-2R ladders from those 4 8-bit outputs to get analog values. As the bias voltage needed to be bipolar, I wrote the code to display “0” on the screen when it was outputting 127. That made the control voltage for -10v about 0.5 volts, and the control voltage for +10V about 4.5 volts. So I had to provide my op amp with a gain of 5 and an offset of -2.5 volts.

One of the knobs controls the LCD backlight, which has to be pulse width modulated. I found a cute little voltage-to-duty-cycle circuit on the internet which I used for this purpose. I could have used the Arduino, but had run out of PWM outputs.

The LCD

The LCD is run in 4-bit mode using the standard LCD library for Arduino. This requires 6 control pins, not counting the backlight and contrast circuits. Fortunately, the front panel LCD was a totally standard model and interfacing it to the Arduino was no problem once I found its pinout on the internet. (It doesn’t use the more common single row of 16 pins, but rather the less common double row of 14 pins to one side.)

System noise and a mitigation strategy

Worst case, this system could attempt to switch 4 relays on at the same time. Relay coils are highly inductive loads and these coils draw about 70mA each. This can produce a lot of noise on the 5V line, and was causing my system to oscillate or reset under certain conditions. Though I haven’t taken all possible steps to reduce this problem (such as running the Arduino on an isolated rail), I did create a circuit that detects whenever there is a change in the signals that run the relays, and then applies them in sequence to the relays over several seconds, rather than all at the same time. This does give the system a more sedate personality, though I have not eliminated unwanted resets.

The relays with sequencing circuits are in the upper-left side of the enclosure, as it is pictured. The 4 50-volt power supplies are underneath them.

Making connections

In a complex project, connecting all the sub-assemblies together is a huge issue. I am trying to get better at this by standardizing on .1-inch spaced headers and connectors for most applications. I have a source of cables using these connectors which can carry quite a lot of current. Most such cables are extremely flimsy and only good for signals, not power.

For the main power connections between the supplies and the front panel, I used do-it-yourself high current connectors with locking plastic housings. I used to get these at Radio Shack, but making them myself from parts isn’t too bad.

I also use old-fashioned terminal blocks for higher voltage or higher power connections. These require the use of crimp-on lugs which are not cheap. However, if you know how to use the crimping tool, and fit the wire to the correct lug barrel size, they work very well. I used to use soldered lugs for this purpose, but the terminal block strategy keeps things more modular.

Ending cycle

I spent many hours over a number of weeks on this project, and all to preserve some hardware that I hardly ever use. So it’s time to move on to projects more along my main purpose of electronic art. I’m hoping this write-up will assist me to take my attention off this cycle of action and start some new ones.

Voltage indicator

4 April 2014

This post is an experiment for me.
I don’t usually describe how I do my design work.
The objects, generally speaking, are not that photogenic, and electronics is a bit of a dry subject. But some people just aren’t familiar with it, and they should be. This is the age of electronics on earth.

most of my patch bay

My patch bay – right side.

Design opportunity and goal

While some electronics designers work within the framework of marketable consumer or industrial products, I don’t. It’s strictly “for fun” you might say. But any designer requires opportunities to do his or her work. After all, you can always buy OTS (off the shelf) if the item you want has already been designed.

I had made a “patch bay” – a kind of interconnection panel where lots of signal connections come together at the same place – for signals used to control my displays. I had purchased a blank piece of aluminum from a local welding shop, cut a slot down the middle (don’t ask me how) and drilled a bunch of holes in it for switches and connectors, then started loading it up with circuit boards.

But on the far left end I had four holes where nothing really seemed to fit: A design opportunity! I decided I wanted a voltage indicator that used four LEDs that got brighter as the monitored voltage got closer to each one’s center setting. The usual bar graph, of which I had made many using a commonly available part, uses ten LEDs that just go on when the input voltage goes above their set point, and off when it goes below. There is also a “dot mode,” which I like to use because it uses less power, where the LED whose set point is closest to the input voltage is the only one that goes on.

I didn’t want to use ten LEDs in the usual way. I wanted to use only four and have them get brighter and dimmer, the way my light panels are designed to do. I wanted each one a different color, but I only had three colors on hand, so the middle two are green, the top is white and the bottom is red. The red LEDs I have are a good deal less bright than the other colors, so I had to try to compensate for that, too.

patch bay left side

My patch bay, left side.

Dot display IC (integrated circuit)

I decided to use the dot display IC as the central component in this project. The two bottom outputs would go to the red LED. The next three to the lower green. The next three to the upper green, and the top two outputs to the white LED. You can set up the IC for so much current per output. Old LEDs required 20mA (milliamperes) to be bright. But modern LEDs only need 2. I tried limiting the current through the LEDs using series resistors. The brightest LED (white) would get a big series resistor (7K – kilohms) the green ones would get smaller ones (I think I used 1.5K) and the red would get the smallest series resistor, something like 100 ohms. These values were arrived at experimentally, and weren’t perfect, but good enough for this project.

How do you get an LED to get brighter and dimmer? You can simply drive it with more or less current. But almost exactly the same effect can be achieved by turning it on and off rapidly using a technique called “pulse width modulation” (PWM). This works because the body only takes a picture of its environment about 100 times a second. So any light flashing at about that rate or faster will appear constantly on. In technology, this is most commonly experienced when watching video monitors (or films). The picture on them only changes 30 to 70 times a second, but the motion appears continuous.

To do this with my dot display IC, I would have to make the input move back and forth through the set points of each of the dot outputs. I decided to use a “triangle wave” for this, and here it is:

triangle wave

This image is from my USB oscilloscope. There is a grid with the vertical and horizontal scales shown by the knobs. The period of this waveform is about 10mSec (milliseconds) and the amplitude is about 800mV (millivolts) peak-to-peak. So it’s oscillating at about 100 Hz (Hertz, cycles per second) and it’s about a volt high. The entire scale of my meter is 2.5 volts, so this signal should activate about 4 outputs at one time, with the one at the center of the oscillation the brightest.

I coupled this signal to an amplifier through a capacitor, then made the DC (direct current – not oscillating, or changing very slowly) level of the amplifier equal to half the input signal (0 to 5 volts). This created an input signal to the dot display IC of a triangle waveform going up or down depending on the slowly-changing DC level being measured, centered at zero to 2.5 volts, the input range of the dot display IC.

Does it work?

With the input set lower (see knob) the red LED is brighter than the lower green (by a little).

low input display

With the input set higher, the white LED is brighter than the top green.

high input display

The effect is quite noticeable, particularly to the eye (less to a camera). I was pretty happy with the outcome.

Limitations of measuring instruments

The range of voltages I use for my analog projects is zero to five volts, so that’s the only range my indicator needed to have. Since the electronics run on +7V and -5V, they can’t put out much more than 5 volts anyway.

But I noticed a funny thing happening with my USB oscilloscope when I didn’t connect it to my signal through a capacitor (to block the DC component of the signal).

clipped triangle wave

It was “clipping” off the top of the waveform! My cheap little USB oscilloscope only has a display range of plus and minus 5 volts! (at least at the settings I was using). This is a very small DC range for a professional oscilloscope, where more like plus and minus 50 volts is what is expected. This little scope had no DC offset built into the front end (all good scopes do) so if the signal goes beyond certain limits, it just disappears.

Here is a trace of the 12volt square wave that creates the triangle wave. Notice that it stops at a little under 5 volts. This means I will have to build a more compliant front end onto my scope if I want to see the entire waveforms in my 8-12volt projects.

clipped square wave

Comments?

I hope this post gives some small insight into the electronic design process. I kept it conceptual. The hardware details are VERY dry. You just look up the data sheets of the parts you decide to use and figure out how to connect the correct pins together. There are certain real-world considerations to take into account, such as using bypass capacitors on the supply pins so signals don’t get into the circuit in unexpected ways. And you have to know how to solder if you want to make a permanent circuit board. Solder is hot metal and it has stuff inside that smells pretty bad when it burns, so a lot of people don’t like it and I don’t blame them. It’s a great technology for military equipment, but hobbyists could probably get by with conductive glue.

Anyway, I rarely get real comments on this blog and would appreciate some. They don’t appear immediately; I have to go through them and approve or disapprove them.

Bye for now.