Projects

Bob Harper

Issue 17, November 2018

When prototyping a new circuit, re-calibrating an old circuit, or perhaps even reverse-engineering a circuit, a resistor decade board can be a handy tool to establish the ideal resistor value.

Resistors are the most common component you will see on most PCBs. In previous issues, we explained the main purpose and uses of resistors as; limiting current flow, developing a voltage from a current flow, forming a voltage divider, or together with a capacitor or inductor as a timing network.

We also spoke of Resistance as being a result of four physical measures; length, cross-section area, the material used, and temperature. R = ρ × L/A, but the temperature formula is a little more complex.

We have also shown you a collection of circuits and methods of calculating what is happening within the circuits mainly using Ohm’s Law (V=IR etc.) and the Power Rule (P=VI etc.)

An experimenter might wish to determine the effects of replacing a single resistor with another value. This might be in an attempt to increase the gain of an amplifier, or voltage from a power supply, or to calibrate an instrument, or many other questions a good experimenter might think of. It may be simply to see what that resistor does!

Of course, the risk is that the change in resistor value might destroy the expensive electronic device being played with, but then, when has that stopped us?

CAUTION: The intensity and volume of the expletive emanating from the experimenter is directly proportional to the experimenter’s estimated value of the item destroyed; and inversely proportional to the time taken for the failure to occur. (Many experimenters are expert in more than one common language.)

The suggestion is that for every modification the experimenter should attempt to estimate the extent of any possible damage including the possible energy released, the components that will be affected, the relative cost of all jeopardised components, their replacement cost and difficulty to replace, or even source, and the cleanup cost to repair, if possible, the charred remains of the PCB and enclosure. Having been precautioned, please proceed.

Typically, a home experimenter will use a handful of available values, often second hand from past projects, or removed from various “Organ Donor” circuits. Values are read from the resistor colour code, or measured with an Ohmmeter when the experimenter hasn’t bothered to learn the simple colour codes (Previously explained in issue 3).

Another common method is to use a potentiometer and adjust the setting to the original resistor’s value using a multimeter, before replacing the original resistor. When only a lower value of resistance is needed, the ‘pot’ can be wired across the resistor, and adjusted until the correct effect is established.

Then the resistance of the pot is measured, remembering that it was in parallel, so any replacement will need to be soldered across the original resistor. Otherwise, the original resistor must be removed and replaced by the new value calculated from the parallel equivalent of the original resistor and the pot. It all gets a bit messy, and it is difficult to avoid changing the setting of the pot before it is measured.

There are two methods which have been regularly used to create a “Dial-Up” Resistance. The first uses a ten position rotary switch for every decade of resistance, with nine resistors of the same value soldered between the contacts of the rotary switch.

If a resistance from 1 Ohm to almost 10M Ohms is wanted, then 7 switches would be used, wired with 9 × 1MΩ + 9 × 100kΩ + 9 × 10kΩ + 9 × 1kΩ + 9 × 100Ω + 9 × 10Ω + 9 × 1Ω. The maker might like another decade with 9 x 0.1Ω as well, but the issue is the contact resistance of the switch and wiring will be close to that value.

While convenient, it is large and clumsy, but reasonably reliable, as long as it is not mistreated.

A smaller and cheaper version can be made using a PCB, a set of pin headers, one for each band, and 9 sets of resistors, one set per band. This circuit is exactly the same as for the rotary switch circuit, but instead of the switches, it uses pin headers and jumpers.

Either version requires some bending of the resistors legs and soldering to the PCB or switches. The PCB version doesn’t need a case, and the resulting PCB becomes a part of your toolkit.

For electronics or science students, and their teachers, it is a great soldering exercise, providing useful practice at hand skills, identifying resistance values by colour codes, bending and fitting components, a lot of soldering practice, including prep and after-cleaning, and visual inspection of work. (4 x 9 x 7 + 2 = 254 soldered joints for a 7-band PCB).

For the lab room, the board leads into circuit building and is particularly related to the Wheatstone Bridge which is a great learning aid for an introduction to complex circuits. A Wheatstone bridge circuit and explanation will be included at the end of this article.

# HOW IT WORKS

For each band, there are 9 resistors in series. Taking the ‘Ohms’ band, 9 × 1Ω resistors in series making a total of 9Ω, unless my maths has failed me. The joints are all connected to the pins on one side of a 2 row × 10 pin, pin header.

The top pin is connected to the first resistor, then the 8 joints between the 9 resistors are connected to the respective 8 pins between the two ends, and finally, the loose end of the last resistor is connected to the last pin.

The pins on the other side of the pin header are all connected together to form one terminal of the resistor bank. The top terminal of the resistors is the other terminal of the resistor bank. The resistor banks are connected in series by these two terminals.

Repeating these steps, you will eventually have the seven banks that I would usually use, or more if you need more banks, perhaps including 9 x 10MΩ.

# VOLTAGE DIVIDER

While not made for this purpose, it seems to me that the PCB could easily be used as a voltage divider. At the very least any two resistors in any bank could be used by placing jumper wires across the two and a third between them to make an equal split, or by using two resistors on one side make a 1/3 split, and so on.

# WHEATSTONE BRIDGE

A Wheatstone Bridge is a very simple circuit with an often overly complicated explanation. I will try to keep it simple.

Inspect the simple Wheatstone Bridge shown here with just four resistors. RM is the Multiplier Resistor; sometimes called the Range Resistor. Its value is compared with RS the Standard Resistor. The bottom two are RV the Variable Resistor, and RU the Unknown Resistor.

Each pair of resistors have a ratio found mathematically, and if all resistors are equal, all 1:1, then no current will flow across the bridge in the middle. That means that the Wheatstone Bridge is balanced.

It also means that because no current flows across the bridge, then the ratio between RM and RV is the same as the ratio between RS and RU. If RM is equal to RS then RV is equal to RU.

The Wheatstone Bridge was at one time a very common instrument in Science labs for measuring unknown resistances, even very low value resistors, quite accurately.

Because the current can flow either way across the bridge, the circuit is a common example of a ‘simple’ or beginners complex circuit.

A potentiometer added in the series with the ammeter will reduce sensitivity, and protect the meter. Back-to-back diodes may also be used, as drawn.

Also RM now has a number of Range or Multiplier Resistors. If RM is ten times RS, then RV must be ten times RU to keep the ratios the same.

Using our resistor board for RV allows us to determine the value of RU to the same accuracy as our resistor selection, which at one time was much better than the multimeters of the day.

# The Build:

Parts Required:JaycarAltronicsCore Electronics
9 x 1Ω 1/4W Resistors*RR1502R7686FIT0119^
9 x 10Ω 1/4W Resistors*RR0524R7510CE05092^
9 x 100Ω 1/4W Resistors*RR0548R7534CE05092^
9 x 1KΩ 1/4W Resistors*RR0572R7558CE05092^
9 x 10KΩ 1/4W Resistors*RR0596R7582CE05092^
9 x 100K Ohm 1/4W Resistors*RR0620R7606CE05092^
1 x 2 Pin Terminal Block*HM3172P2032BPOLOLU-2440
2 x Pin Jumpers*HM3240P5450FIT0140

## Parts Required:

* Quantity shown, may be sold in packs.

^ Core Electronics provide the values we need in large multi-value resistor packs.

# MAKING UP AND SOLDERING THE PCB

Let’s guide you through making up and soldering the PCB.

There is nothing new here, but as I’m suggesting it be used for training would-be engineers, I’m going to take a little time and a few more words for the description.

There is nothing static sensitive on this board, although lightning would no doubt chew its corners off given the chance. Therefore, making up this PCB is a good practice for learners.

# PREPARING TOOLS

Get the soldering iron out, and make sure it is clean, particularly the tip, but also the barrel. Make sure the tip is suitable to do the work. Don’t use a huge tip on small components, or a small tip on huge components. Some of you will only have one tip, and therefore have no choice, but do make an attempt at having a suitable iron for the work you want to do.

Check that you have the right solder for the job. I still use 60/40 Tin/Lead, but lead-free solder (99.3% Tin, 0.7% Copper) is recommended today. It takes a little more care but there is less chance of lead poisoning. I use 0.7mmø solder, but this job is fine for 1mmø.

# PREPARE THE PCB

Clean the board with some Isopropyl Alcohol, or even a little methylated spirits, but be aware that it will leave tiny crystals all over the PCB. The correct cleaner for the job is much better.

Check the board out for copper or solder bridges, missing or cracked tracks. They are very uncommon today with a commercially produced PCB, but it is always good practice to inspect the PCB before you begin.

Use a PCB vice if you have one, but as this job is mostly flat, a piece of corrugated cardboard will protect the bench, or mum’s laminated table, from soldering damage.

Students should be able to use a soldering/PCB vice, if possible, as it prepares a student for industrial practices. Naturally, for anyone constructing PCBs, these "helping hands" simply make soldering PCBs easier. They'll help avoid heat and solder getting into places you don't want it.

# Basic Soldering

1. Wipe the iron, wet it with a little solder and give it another quick wipe.
2. Place the tip against both parts to be soldered so both are heated together. (On larger components with a thicker section, I heat the component for a little time before heating the PCB track.)
3. Place the solder between the three, PCB track, pin, and soldering iron, until the solder melts and apply more solder around the leg and pad so the whole pad is wet as well as the pin/leg/wire, and remove the solder.
4. Watch as the solder forms a “meniscus” or a fillet between the pad and the leg. The fillet should not bulge up but should sag slightly between the wire and the pad.
5. On plated-through hole PCBs, some solder will flow down the hole and may need to be filled again.
6. Remove the iron and immediately do the next joint, or wipe the iron, wet it, and put it back.

# HARDWARE FIRST

Most electronics magazines and websites recommend a process known as “Hardware, Passive, Active” which means we solder the hardware on first as it is the most robust and least sensitive to heat and static electricity.

Place one header into its position on the PCB, with all of its pins in the holes, and carefully turn the PCB over. If you are using a PCB vice, the header may simply fall out if turned completely over, so only turn vertical.

Solder one pin, on one end of the header, then, with an insulated finger, i.e. not touching the “about to be very hot” pin, or using a screwdriver, etc. remelt the solder while pressing the pin header against the PCB, and allow the solder to harden. The header should be hard up against the PCB.

Solder the opposite corner, making certain that the pin header is still against the PCB on that end as well. Then when you are sure the pin header is properly seated, solder all of the remaining pins. Repeat this process for all of the pin headers, and the two pin terminal.

Some people, particularly hobbyists, prefer to solder all of the joints and then snip off the excess length of each leg. Many industrial workshops use a special leg cutter that cuts and bends the exposed end of the leg so it doesn’t fall out, and then solder over the end of the exposed leg so the iron wire is not corroded. That’s right, doesn’t a magnet pick up your resistors!

## PASSIVES

Once all of the hardware is soldered onto the PCB, then the passives come next, i.e. the resistors, capacitors and inductors, as they are reasonably robust, and mostly insensitive to static electricity. Some capacitors can be damaged if there is enough static about, but this board is perfect for learning as there are only resistors, and a lot of them.

Each resistor should have the legs cleaned, especially if they are not brand new from the plastic bag. You can cut a slit in an ink rubber and drag the resistor leg through to clean it, or alternatively use 3M scourer pads to pull the resistor legs through for cleaning.

So the resistor leads should be cleanly bent, all exactly the same, preferably around a jig or round nose pliers. Jaycar, Altronics, Core etc. all sell appropriate tools and bending jigs, or the jigs can be downloaded and 3D printed.

So, bend the resistor legs so they line up with the holes in the PCB. You may as well sit and do all of the resistors in one go, leaving them in little piles of each value. Make sure you have 9 of each value and we’re ready to load the board; i.e. place the components onto the PCB.

As with the pin headers, the components are prone to falling out, but a little trick is to bend the legs apart as each resistor is loaded.

Remember what we mentioned earlier. The legs can be cut off leaving a millimetre or so to be soldered, or soldered first and then cut off. If they are cut off right, you shouldn’t get scratched or cut by the leg ends, but remember that the sharp ends of the wire can make a cut nastier than a paper cut.

## TESTING

At this point, it’s time to test the board, and if necessary investigate any issues. Of course, we should be 100% confident in all that we have done, but it’s nice to prove it.

With no jumpers on the PCB, assuming you used the 7 resistor banks of the PCB we provided, a multimeter should show a total of 9 × 1MΩ + 9 × 100kΩ + 9 × 10kΩ + 9 × 1kΩ + 9 × 100Ω + 9 × 10Ω + 9 × 1Ω = 9,999,999Ω. However, with 1% resistors, they might all be 1% higher (10,099,998.99Ω) or 1% lower, (9,900,989.1Ω) or anywhere in between.

So, it is still a handy tool but be aware of its potential limits. With a large enough number of samples, statistically it should balance out to almost the expected result but statistics isn’t engineering.

Having measured the total resistance with a multimeter, which also has an expectation of some error, record it, and if it’s between the values above it may be OK.

Next, measure and record each decade bank, which should be 9MΩ, 900kΩ, 90kΩ, 9kΩ, 900Ω, 90Ω, and 9Ω respectively, all +/- 1%. It is unlikely that you will have any big errors, and if you do, maybe you have a wrong resistor in that bank. Check the colour codes of all of the resistors in the bank.

If you are still finding an unexplained error, you can measure each resistor in the series circuit. The circuit is, after all, all series. If the ends are not connected, then placing the multimeter probes across any one resistor will find only one path, and the resistance read should be the resistance of the chosen resistor.

## CLEAN-UP

If you haven’t trimmed all of the legs to length already, then do so now. Inspect all of the soldered joints looking for solder tags, spikes, bridges, dry joints etc. Even experienced soldering professionals can make mistakes, and they’re never allowed to forget it!

Dry joints are joints that moved while the solder was in its plastic state, between solid and liquid. The result is a crystalline appearance often with obvious cracks in the solder.

You should re-melt any dry joint, but in high-reliability soldering, the joint would be sucked clean and re-soldered with fresh solder.

After all, soldering has been completed, a flux solvent can be used to wash off any flux residue, and sometimes a “Conformal” clear coating would be applied to protect the solder from corrosion or chemical attack.

# SETTING A VALUE

Hopefully, by this point you are confident in your resistance board, but how do you use it?

Let us suggest you need a resistance equal to this year, 2018. You need to have a pin jumper for each resister bank, and at least one spare.

For testing with some feedback, connect your multimeter across the two pins of the terminals for the PCB. Initially, you should get 9,999,999Ω +/- any error. Don’t worry about the error for now.

You won’t need any resistance for the 1MΩ, 100kΩ, nor the 10kΩ, so the pin jumper can be placed at the top of the pin header for those banks, thus shorting out each whole bank. The multimeter readout should be 9,999Ω +/- error.

For the 1kΩ bank, place the pin jumper two positions down, and the multimeter resistance should read 2999Ω +/- error. Again we’ll deal with any error at the end.

The 100Ω bank should be shorted so the reading now becomes 2099Ω +/- error.

Setting the 10Ω resistor bank to the second pair of pins and the 1Ω to the second last set of pins should, if all resistors were perfect, give a reading of 2018Ω.

Of course, we should be so lucky!

It is very likely that there will be an error as we used 1% resistors. 2018 +/- 1% = 1998Ω to 2038.18Ω. You should have a value somewhere between these limits, but if the first value appeared, 1998Ω, you are 20Ohms short of the required 2018Ω. Increase the 10Ω bank by 20, and read the value again, and adjust this way until you get the value you want.

Similarly. if the value were 2038, although we can’t do much about the 0.18Ω, we can reduce the 10Ω bank by 30Ω, right? Well no, because we only had 10Ω loaded in the first place. We need to change the 1000Ω bank from 2000Ω to 1000Ω and increase the 100Ω bank to 900Ω. Then change the 10Ω bank to 90Ω and re-check the reading which may still be in error because we are now dealing with different resistors.

It may sound bad, may be difficult, but we’re looking at what should be worst case scenarios. In reality, you should rarely see such extreme errors, and the process is easier to do than to explain.

# IN CIRCUIT

What if you need to change the value in a live, i.e. an operating circuit, not to be worked upon for a circuit using voltages above 50 Volts. The best procedure depends upon the circuit, and some should be turned off before making the adjustment.

Others could be adjusted but only one resistor value at a time. The best way to do that is to have a spare jumper, and place it on the new value before removing the old jumper. Preferably, you should have a few spare jumpers, and experience will teach you tricks that we at DIYODE may not have thought of telling you.

# WHERE TO FROM HERE?

One of the common uses of this kind of board was to find a value of a resistor that was perfect for setting a specific frequency in an oscillator. It was also commonly used in calibrating some instrumentation on industrial sites.

How about a similar PCB for standard value resistors or capacitors?

FOR EDUCATORS This project is a cost-effective and educational tool for your students. The decade board can indeed be constructed without the PCB, however it does make for a more reliable construction, and a better overall understanding of what's happening. The PCB files are included in the digital resources for download, to purchase a quantity of PCBs for your requirements from any vendor you may wish to use.