High voltage display.

Sometimes it becomes necessary to have a display for high DC voltage. An analog meter is always a possible solution, but the resolution and dial accuracy are usually quite modest unless you are willing to pay tens of Euros for a high quality meter.

I recently ordered from Banggood a digital display for 70 to 500 VAC. I needed to have something to indicate the output voltage of my Variac transformer. I soon realized that the fact that it could display voltages up to 500 units, could be used to hack the meter for general use. In principle it could be used to indicate up to 500 volts, milliamperes, or even degrees of temperature just to name a few.

The meter is based on a microcontroller incorporating AD converter and three-digit 7 segment driver. No published specifications can be found. The AD conversion is most probably programmable to enable the same chip to be used for different display ranges. Apparently the ADC resolution is always fixed (10 bits?).

There are numerous versions of the same panel meter available. Some are even equipped with current shunt resistor to display DC currents. This particular model, has the type number DSN-DVM-568AC printed on the PCB and it seems to be the most versatile for its display range of 500 units and autoranging capability.

One interesting feature is the way the 7 segment display is multiplexed. Instead of the normal  strategy to address segments and digits with specific control signals, each individual segment and decimal point (24 in total) is addressed using a 5 by 5 line array.

Is It Safe to Use?

Of course the first thing with Chinese electronics is to study its safety and reverse engineer the schematics to see what is needed to hack it. This is something I have learned from the superb YouTube videos of DiodeGoneWild.

The schematic above is drawn to roughly present the components in the actual PCB layout.

As usual, the high voltage safety leaves lots to hope for. The operating voltage for the chip is created from the input AC through a capacitive dropper and a bridge rectifier. Although the meter can display AC up to 500 volts the dropper cap has only 630 VAC spec. The peak voltage of 500 VAC is over 700 volts 

The SMD resistors in the input chain cannot be considered safe for such high voltages. The input leads are marked L and N (signifying live and neutral, respectively). Yet it might be difficult to construct a device where neutral input can be always guaranteed when non-polarized power plugs are used. It is true that since there are only two input leads, the risk is small if the installation is properly isolated.

The measured voltage is also used to power up the meter as well. Therefore, the display does not wake up until the input voltage is about 60 to 70 volts AC. However, if the microcontroller chip power is supplied separately, the display starts from 0.00 volts. So this would be the strategy for this hack.

The DC Hack.

The following image shows the components which are not needed if using this meter for 500 VDC display:

Depending on the required voltage range it might be advisable to replace all the voltage divider components with good quality resistors. The input pin (#13) of the microcontroller chip has a maximum voltage of 2.0 volts, which produces the maximum reading of 500 units. Consequently, you need to calculate the voltage divider to output 2.00 volts from input of 500 volts. The existing 10 kOhm trimmer pot on the PCB can be useful to adjust the reading, so leave it in place.

To reuse the original components as much as possible you can connect the hot part of the voltage divider to the cathode pad of the original input diode (just left of the microcontroller chip in the image below).

For 500 VDC use the required additional resistance is 605 kOhms, which might be constructed using 560 and 47 kOhm resistors and compensating the 2 kOhm excess with the trimmer. You may need to select resistors which read a little low for their face value.

The existing capacitors are not required but leaving them in place is also OK. They do not cause any problems and may even help to reduce noise at the input. The PCB could look like this after the clean-up of the AC input components.

The voltage for the electronics cannot be tapped from the measured DC voltage. The heat dissipation would be just huge. Therefore a suitable DC source should be available. There are several components on the PCB to regulate the 5 volt input and all of them are not necessary for low voltage input. 

It is expected that the current consumption of the microcontroller and display will be about 15-20 mA. Therefore, the power dissipation of the 78L05 regulator IC is not too high. The  6.8 V Zener diode protecting the input of the regulator creates a current sink, we must get rid of it. If you have a supply of 7 to 12 volts DC, you can connect it directly to the input pin of the 78L05. 

Higher voltages may be used with the help of the voltage dropping resistor on the right. Its value is just too small now that the Zener is removed. You can replace it with another which can consume the better part of the voltage difference and let the 78L05 run cooler.  A suitable value for 12 VDC input supply would be 330 Ohm 1/2 W.

Just remember that there is common ground for both supply and measurement zeroes. This is necessary to observe if you e.g. make the DC supply from tube radio filament transformer and try to measure the B+ of the same equipment. Common ground is not usually tolerated in tube amplifiers.

Tube Tracer Build 2 - Added features

Auxiliary Heater Supply

Since the heater supply of uTracer has its limitations at low heater voltages, I decided to include an auxiliary supply for the few cases where low heater voltages are needed. This created a need for a switch between the internal and auxiliary supplies. The Chinese buck converter takes its input from the same laptop supply and has a small footprint on the face panel. 

The auxiliary heater supply has both voltage and current regulation.
The output voltage has in theory no lower limit.

One can justly argue that the design could be smarter, but I just did not want to have any complicated sidetracks in this project. The only actual drawback is that since the auxiliary supply uses the same 19 Volt supply, it shares a common ground with the uTracer and cannot be used with directly heated cathodes.

Case Modifications

The design required that the face plate needed something to get fixed to. Therefore, I attached aluminum L profile with rivets inside the original aluminum frame of the case. I needed to remove some of the foam rubber padding so that the aluminum frame was free of any padding.

I used 10*20 mm profile on three sides and 25*25 profile on the frame back, where the hinges create more stress. The two upper rivets of the original hinge hardware were removed and replaced with screws that were drilled also through the L profile.

The AC input was connected to the case in the right back corner. The case body is made of Masonite with ribbed red outer surface. Making the necessary holes for the IEC male connector in the wall was easy but the screws need to be secured with decent size washers and nuts

In the final steps it became evident that some of the foam padding on the lid frame was interfering with the closing of the case. It was necessary to remove 2-3 mm of rubber foam from the edges of the lid. I could have made a cleaner finish if I just took my time to fetch better set of tools for the task.

The Face Plate

I designed the uTracer face to be mostly empty. Only the auxiliary heater supply, fan intake and the indicator LEDs break the surface. The tube adapters cannot be completely hidden but I hoped that they would not be much visible under normal operation. The tube under test has a prominent central position.

My visual image was very neutral – white or grey. After many iterations I ended up using aluminum composite material called Dilite, which has two 200-micron sheets of aluminum and 1.6 mm of black polyethylene between them. There are many different surface colors available but mine has warm satin white finish on both sides.

I hoped to have access to a CNC router, which could cut the desired shape cleanly and accurately. Unfortunately, the size of the face plate, 390 x 240 mm was too big for any CNC machine in my neighborhood, so I had to cut the rough size using a table saw and the openings with a scroll saw. Dilite was very easy to work with. Cutting with a carpet knife and finishing with files and sanding pad resulted in totally clean edges.

Tube Tracer Build 5 - Final

Conclusions

The visual appearance of the uTracer case has many of the characteristics I hoped to achieve. I am particularly happy with the selection of materials. The results would have been even better, given a bit more time for planning and sourcing of parts which would better match with each other.

The case is almost complete. Texts and graphics for the face plate need to be added.
In the front exhaust holes for the cooling air are visible.
The fixed USB cable in the rear left corner will be coiled and stored inside the lid for storage and transport.

The concept of using tube adapters with screw terminals is in a way elegant but the result is not very portable if you need to retain the capability to test many different tube types. The uTracer itself can of course very easily be moved around in its case. However, since the adapters need quite a lot of space for storage, one may need a second case just for the tube adapters. It may still be possible to add small pockets inside the lid to store up to 8 spare adapters.

In home use this is not a problem as long as you can find the correct adapter when you need it. If you frequently have the need to test a limited set of tube types, it is even feasible to hard wire a dedicated adapter for each individual tube type or family.

Originally, one of the most important reasons for the choice of Dilite material for the face plate was the sandwich structure which naturally lends itself to engraving. The texts and graphics on the face plate would have been very exiting to design and produce. Unfortunately, as described above the CNC router had so small working space that this could not be accomplished.

Eventually, I decided to use water transfer decals and the results were great although only time will tell, how resilient the decal surface will be.

The case is here in its final form. Decals have dried and texts are readable, although not perfectly aligned.

Tube Tracer Build 4 - Solutions and Decisions

Thought process

The tube adapters and the docking bay were by far the biggest  part of my personal contribution in the case design. As soon as I decided to use tube adapters, I had the mechanical principle pretty much fixed.

At first, I wanted to see the adapters inserted in docking rails not unlike in old C-cassette decks. The rails would have a pivot hinge in the rear and some kind of locking mechanism in the front. After a brief design phase I discarded the idea as too complicated.

After about a week of planning, my design finally was simplified to a fixed rail solution. There are two pairs of retaining clips riveted inside the aluminum rails. The adapter PCB is pushed under the retainers until it mates with the D-sub connector.

The PCB outline resembles a very fat letter H. The four protrusions are held in place by the retainers. This way the adapters will always be docked in the same exact position. There is enough area in the center of the adapter to mount tube sockets up to 80 mm in diameter.

The cover plate for the docking bay hole is very simple. However, the need to have it stay in place during transport required some fixing mechanism. I finally came up with a solution, where the rear edge is secured with a small strip of Dilite board superglued under the cover. It protrudes about 3 mm beyond the rear edge and prevents it from popping out from the opening.

The front edge latch was more difficult. Due to esthetical reasons I hoped it to be very invisible. Magnetic locking was my first idea. I was a bit apprehensive to introduce magnetic field so close to a vacuum tube under test. 

Even if this was somewhat unsubstantiated, I eventually decided to construct a mechanical latch from a short piece of strapping steel band. A white plastic piece was superglued on the visible part to act both as actuating trigger and a latch claw.

Cross section of the adapter cover plate as a part of the face plate.
Case front is to the right. Dimensions are not in scale.

The cooling fan may not exactly have been necessary. With hindsight I probably would have needed more research of its actual benefits. However, after having read several stories of the uTracer building projects I seemed to time and time again bump into observations of heat generation in the linear regulators of the PCB. Adding the fan was therefore not a hard decision.

I wanted to see symmetry in the face plate, so I placed the fan in the rear center, behind the tube docking bay. Therefore, the fan fixing screws had to penetrate also the aluminum frame reinforcements. When I planned the final assembly steps, I noted that the fan screws need something to bite into without the possibility to have nuts for the screws. I had a small selection of sheet metal nuts, but I soon found out that they need a relatively even surfaces on both sides. This is unfortunately not the case with fan bodies.

Eventually I filled the fan fixing holes with epoxy putty and drilled small pilot holes for the self-tapping screws. The result was very good and the fan fixing became a breeze.

Wiring the connections

The laptop power supply was stripped from its case to reduce heat build-up. Input and output were connected using screw terminals fixed on small pieces of perf board and glued on the foam rubber.

The 19-volt output was connected to the uTracer board, cooling fan and auxiliary heater supply. The auxiliary heater supply was connected to the heater switch on the face plate.

The uTracer output connections were connected to the D-sub female connector in the tube docking bay. The RFI suppression beads were inserted on each of the four leads close to the D-sub connector. The heater outputs were routed through the heater switch. No RFI suppression beads were used on the heater cables as many users have reported problems in heater control, which were attributed to the RFI beads.

The on-board uTracer indicator LEDs were cut off at the height of about 7 mm. The remaining wires are used as male headers to connect two substitute LEDs on the face plate.

The uTracer final setup without the face plate. The uTracer PCB sits on the left.
Unfortunately it is not quite square with the case. 
The self-adhering feet cannot be moved once they stick with the bottom.
The two indicator LEDs are shown on front left. Green connector for the auxiliary heater supply on front right.

Tube Tracer Build 3 - Connecting the Tubes

Tube docking bay

25 x 25 mm aluminum profile was used to construct the docking bay for the tube adapters. Each tube socket type will have a simple PCB, which acts as the base for the tube socket. The electrical connections to the main unit are made using a nine pin D-sub connector. There are of course numerous possibilities for connecting the six wires, but considering the easy availability and low price, the D-sub connector was chosen. 

The heater cables are connected using pins 1 and 2 for one cable and 4+5 for the other. The use of two pins per cable increases the current carrying capacity of the connector.

The docking bay made of aluminum profile. Case front is on the right.
Small pieces of thinner profile are fixed inside to retain the adapter PCB's

The docking bay size is approximately 100*120 mm, but its body spans the whole depth of the case (front to back, appr. 240 mm) The body is fixed to the back profile with screws. There is a space of approximately 90*100 mm behind the docking bay, which was just suitable for a 80*80 mm cooling fan. I purchased the fan quite early and planned it to run from the 19 V supply. Therefore a 24 V fan was chosen although a 230 V AC fan would have been equally suitable now that AC input is available inside the case.

Tube adapters

As already mentioned, the tube adapters were constructed of single side PCB material. There is not much in terms of PCB design, since there are only the D-sub connector and two three-pin screw terminals soldered on the PCB. The tube sockets are mounted with screws and spacer blocks on the PCB and the necessary pin connections made with suitable lengths of connecting cable.

My first tube adapter PCB  for Noval socket tubes

If the tube uses top contacts (plate or grid), they can be connected to the screw terminals with top contact leads. The same leads can be used with any tube adapter.

Each adapter costs only about 2 € plus the price of the tube socket, so it is easy to make as many as needed. I made the original design using Eagle software. The CAM process used the Roland CNC router in the Oulu University FabLab. The input data uses simply PNG-images of the traces and outline of the PCB.

After my first prototype I noticed that it is easier to tweak the traces with an image manipulation software. Consequently, I made the final design using GIMP. I needed to add more insulation space around the high voltage traces (plate and screen grid) and broaden the heater traces to increase the current capacity.

The PCBs are essentially identical regardless of the tube socket type. I included a small pilot hole in the design to indicate the center of the tube socket, which helps to ensure that all tubes will be standing in the same center location. The tube sockets will be standing above the base PCB. The exact height is not critical since the adapter PCB sits about 39 mm below the face plate. The idea is that none of the sockets will protrude the face plate.

If I want to store tube adapters in the lid part of the case, the free height of the lid limits the total thickness of the adapter assembly.

The face plate has a rectangular opening for the installation of tube adapters. The opening is 100 * 120 mm with rounded corners. For cosmetic reasons I have docking bay cover plates cut from the Dilite stock. I plan to have several cover plates having different size holes for the tubes. The future will show how many I end up making.

The cover plates are used to cover the tube adapter opening when operating the uTracer. They are held in position by small strips of Dilite glued under the corners of the opening. Consequently, the covers are neatly flush with the face plate.

Tube Tracer Build 1 - Intro

Introduction

It was time to look back and see what really has happened. Yes, I have managed to complete the assembly of the electronics kit for the uTracer 3+. This was by far the most complex electronics project I ever endeavored, and the results were phenomenal at least in my quite subjective view.

Printed Circuit Board of uTracer 3. This image is not my own creation,
rather than taken from the cover of the instruction manual

The time it took was only three days and thanks to the thorough and systematic instructions that came with the kit I had no major problems in any steps of the assembly. The only time I needed to revisit an assembly step was when I accidentally dropped the PCB on the floor and one of the buck converter inductors broke loose.

The uTracer was quickly taken in test use in its skeleton form without any RFI suppression beads and having just a single noval tube socket with flying leads attached to the terminals. I was so anxious to verify the operation of the kit that in a few days I had tested all suitable tubes in my rather limited inventory.

I had been dreaming of many possible ways of finishing the project with a beautiful case and clever switching hardware for the different tube socket scenarios. As days went by, I had time to evaluate the options and I grew more and more fond of those with minimalistic wiring and switching solutions.

I have anticipated that I need maximum versatility, so that any possible tube configurations could be tested without any major rebuild. Therefore, I wanted to have a docking input connector, where I could plug in practically any kind of tube socket. I do not need to have more than one set on input leads, because the wiring changes can be made on the tube socket adapter. I little bit of manual labor never hurts one.

I had envisioned a box with a clean exterior, a docking bay for the tube sockets and the necessary cables, input power and USB. First, I thought to have the laptop supply external, but since there was no shortage of space, I ended up incorporating the 19 V supply in the case.

My eyes were soon tuned to spot any promising case candidates and when I was on one of my regular rounds in the hardware stores, I found a nice tool case in the local Bauhaus store. It looked to be just the right size, so it quickly became the base of my project design. That particular case was available in red or blue color and I decided to use the red one to have kind of surprise effect due to the contrast between the colorful exterior and clean business façade.

Additional parts of this project are described on other pages of this blog. See the menu on the right.