Circuit VR: The Wheatstone Bridge Analog Computer

We are normally amazed with a thing so straightforward can actually be so elaborate. For case in point, what would you believe goes into an analog computer system? Of class, a “real” analog computer system has opamps that can do logarithms, sq. roots, multiply, and divide. But would it shock you that you can make an analog unit like a slide rule making use of a Wheatstone bridge — essentially two voltage dividers. You never even require any active gadgets at all. It is an previous thought and a single that used to demonstrate up in digital publications now and once again. I’ll exhibit you how they work and simulate the unit so you really do not have to establish it unless you just want to.

A voltage divider is 1 of the least complicated circuits in the world to evaluate. Take into account two resistors Ra and Rb in series. Voltage arrives in at the top rated of Ra and the bottom of Rb is grounded. The node connecting Ra and Rb — let us get in touch with it Z — is what we’ll contemplate the output.

Let’s say we have a 10 V battery feeding A and a best voltmeter that does not load the circuit linked to Z. By Kirchoff’s existing legislation we know the present by way of Ra and Rb should be the exact same. After all, there is nowhere else for it to go. We also know the voltage drop across Ra plus the voltage drop throughout Rb should equivalent to 10 V. Kirchoff, conservation of vitality, whichever you want to connect with it.  Let’s contact these portions I, Va, and Vb.

There are quite a few strategies to go from in this article, but let us settle for that the recent by two collection resistors will be the similar as if it were a single resistor of equal value. That is, a 1 KΩ and a 2 KΩ resistor in series will draw as significantly present-day as a 3 KΩ resistor. That signifies Ohm’s legislation tells us:

I = 10/(Ra+Rb)

Now you can resolve for each individual voltage fall:

Va = I Ra

Vb = I Rb

In fact, our voltmeter at Z will evaluate Vb because it is grounded.

Large Furry Offer

Of class, you possibly know about voltage dividers. But we ended up heading to talk about Wheatstone bridges. The truth of the matter is these are just two voltage dividers in parallel and you evaluate the voltage between the two outputs (call them Z1 and Z2). You generally see this circuit drawn like a diamond, but do not permit that idiot you. It is however just two voltage dividers.

With out applying any math, you can see that if the voltage dividers are the very same then Z1 and Z2 will be the same and, consequently, no present-day will flow since the voltage involving the two points is zero. What comes about when the divider is not the very same? There will be more voltage on one particular Z place than the other.

Historically, this was used to evaluate resistance. You could use two matched resistors in part of the bridge, have an not known resistance in a person of the remaining legs and a variable resistor with a dial calibrated to browse ohms. You’d transform the dial until eventually a meter browse zero and read the resistance price from the dial. If the energy source is AC, you can also evaluate reactance making use of a identical circuit.

But the Slide Rule?

So how do you get from a piece of antique check tools to a slide rule? Let us alter the bridge so the still left-hand divider has resistors Ra and Rb though the other one has Rc and Rd. We can search at the algebra:

Z1=V (Rb/(Ra + Rb))

Z2=V (Rd/(Rc + Rd))

We want Z1 to equal Z2 so:

V (Rb/(Ra + Rb)) = V (Rd/(Rc + Rd))

We can divide both equally sides by V and get rid of that term:

(Rb/(Ra + Rb)) = (Rd/(Rc + Rd))

So to balance the bridge we have to have:

(Ra + Rb)/Rb  = (Rc + Rd)/Rd    reciprocal equally sides

(Ra Rd + Rb Rd) = (Rc Rb + Rb Rd)    multiply equally sides by Rb Rd

Ra Rd = Rc Rb      subtract Rb Rd from both of those sides

Ra = (Rb Rc)/Rd    Remedy for Ra

As a simple thought experiment, then, consider that Rd=1. If you set Rb and Rc then you can regulate Ra to balance and the price of Ra will be the answer. Or you can set Rb to 1 and enter numbers in Rc and Rd. Once you harmony Ra, you will know the outcome of the division.

In observe, although, you may want to scale the final result, in particular for division. For instance, if Rb=1, Rc=2, and Rd=1000 you would have to have to established A to .002 ohms which is really hard to do. In that circumstance, nevertheless, you could set Rb to a scale issue. If it were being, say, 10K, then Ra can be set to 20 ohms.


You could break out a couple of potentiometers and have a go at this. We’d recommend linear kinds until you are very useful at earning logarithmic scale dials. But due to the fact this is Circuit VR, we’d relatively do a simulation. Falstad suits the invoice, but any simulator is well up to the undertaking.

Analog slide rule simulation

There are two switches in the simulation. The prime “C” change allows you swap in the major resistor or a 10X, 100X, or 1000X selection resistor for C. The bottom “D” change lets you pick a 1 ohm resistor or a variable resistor for D. The ammeter in the center reveals the bridge equilibrium and will go through 0A when you are in balance.

Talking of variable resistors, I did place sliders for just about every of the resistors on the proper sidebar of the simulator. However, working with them typically places values in like 10.002K which reads 10K on the monitor and is a supply of error. Of program, you’d have the identical difficulty with genuine pots, so maybe that is a excellent simulation. Nevertheless, it is superior to double click the resistor you want to adjust and enter its price right. Naturally, you should not improve the three set C resistors or the mounted D resistor.

Upcoming Actions

A equivalent system in a 1960 Radio Electronics

If you want to see what this circuit appeared like in the flesh, examine out internet pages 48 and 49 of the June 1960 Radio Electronics. It may well have even been the pretty short article that spawned [Bil Herd’s] initially laptop or computer package. A related kit from Edmund Scientific employed three potentiometers to type the bridge in a frequent configuration. We’ve even observed a variation from GE that utilized an audio oscillator so you could listen to the null point using headphones. You can see both of those of all those on the posting commencing on page 65 of the December 1961 Well known Electronics. Or verify out a newer establish more than on

It would be an quick sufficient rainy working day venture. If you have an old-fashioned mirror scale meter from an aged multimeter, it would really glow in this software. Creating the dials in a CAD method and printing them out would be easy, as well.

If you want a obstacle, why not use an AC source alongside with variable capacitors and inductors to make a complicated quantity calculator? That’d be something and if you pull it off, we’d include it.

Meanwhile, we’d like to place out that real analog pcs ended up not this simple. On the other hand, by definition, this is an analog computer just like a serious slide rule. If you read the Radio Electronics article, you are going to see it can even chain an reply into the following difficulty just like you would do on a slipstick.