Designing A Multimeter (PW-June-1965), by K. Berry, presented by Peter Vis

[Andreas Thaler]

Multimeters are essential measuring instruments whenever an SLR has a power supply on board.

Anyway not everyone knows exactly how a multimeter works.

This article shows the basics of designing and building a simple multimeter. It is one of those wonderful beginner and hobbyist projects, which allows one to make their own simple measuring instrument.

Designing A Multimeter PW-June-1965

This article illustrates the basics of designing and building a simple multimeter.

www.petervis.com

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Please note that this article is intended for informational purposes only. Unless you have a solid understanding of electronics, I advise against using a homemade multimeter to measure high voltages, as such a device is not certified for safety.

Therefore please keep this safety warning in mind:

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A word of caution

Dangerous high voltages can be present when handling electronics, especially in conjunction with electronic flash units and mains. Therefore, familiarize yourself with the safety regulations beforehand and ensure your safety.

All information provided without guarantee and use at your own risk.

 

[lumoStoria]

Ah, those were the times (before I was born) were it was OK to tell a beginner to built himself a voltmeter for up to 1000V .

Multimeters are essential measuring instruments whenever an SLR has a power supply on board.

Anyway not everyone knows exactly how a multimeter works.

To be precise, the article shows how a passive, analog multimeter works. I would suggest to not use this "ancient design" multimeters in SLR repair, maybe except for measuring the voltage of the battery. The problem is the "passive" nature of these multimeters, that is, the signal is measured directly without any amplification. This results in a low input impedance. For example, in the Design 2 multimeter described in the article the input impedance is 1kOhm/Volt. If you use its 10V range (should be sufficient for all voltages in an SLR except for flashes) the input impedance is 10kOhm. That means, whenever you measure a voltage, you put a 10kOhm resistor in parallel to the circuit. Especially in low-current circuits like in SLRs this will influence the measurement considerably, the voltage will drop and you will measure a value that is too low. This can result in the false assumption that something is wrong with the circuit. You could consider this as a macroscopic form of Heisenberg's Observer Effect.

Therefore, you should use an active multimeter for SLR repair that has at least 1MOhm (better: 10MOhm) of input impedance. The extra load put on the circuit by these multimeters is so small that it becomes irrelevant for practical use. Whether you use an analog or digital multimeter is up to your taste. Sometimes, analog multimeters are better in showing fast-changing voltages, often, digital multimeters are more convenient. I use both:

• A vintage active, analog multimeter I refurbished (Philips PM2505) with input impedance of 10MOhm.
• A modern digital multimeter (Brymen BM257s) with input impedance of 10MOhm.

Both can be used to measure up to 1000V DC without the danger of dying .

 

[Andreas Thaler]

As I said, this is just about how a multimeter works in principle, not about building one from scratch.

What you see here is analog technology that is now outdated. But the basic principles remain the same, and this is the easiest way to demonstrate them.

For actual work, I would only use a modern digital multimeter.

 

[koraks]

this is just about how a multimeter works in principle

Only this particular type, which is no longer being made or used. More modern solutions of course also involve Ohm's law at some point.

to built himself a voltmeter for up to 1000V

Then again, back in those days, you needed that voltage range!

 

[Andreas Thaler]

Only this particular type, which is no longer being made or used. More modern solutions of course also involve Ohm's law at some point.

When it comes to demonstrating, in general terms, how to convert electrical quantities such as voltage, current, and resistance into a readable display, this description at this component level is clear. However, this requires knowledge of analog electronics.

Due to its complexity, a modern digital multimeter can now only be represented at the block-level. But it can only function based on the fundamentals of analog electronics, which, after all, do not change. Digital electronics, too, is built upon these analog fundamentals.

Which approach works best for you depends on your level of knowledge. The good thing is that you can scale down your analysis of circuits. Otherwise, I’d have to try to understand in detail the interconnections of a huge number of transistors in a microprocessor. A ticket to madness

 

[Andreas Thaler]

[1,000 volts]

Then again, back in those days, you needed that voltage range!

Probably for measuring cathode-ray tubes in monitors, but these days I’d have to go to a substation with a multimeter to measure such voltages as AC

1,000 volts DC—where can I find that nowadays? 230 VAC is the standard household voltage in the EU.

 

[koraks]

Probably for measuring cathode-ray tubes in monitors

The article you cited dates to 1965. Transistors were in their infancy back then and tube circuits were very common. So not just CRT's, but vacuum tubes in general. This automatically meant in virtually all cases voltages >100V, but with most common/consumer/amateur equipment topping out at 700V or so most of the time. Typically DC, not AC, although the latter would of course also be relevant to measure e.g. the output of a power transformer or the ripple on a filtered DC line.

 

[lumoStoria]

When it comes to demonstrating, in general terms, how to convert electrical quantities such as voltage, current, and resistance into a readable display, this description at this component level is clear. However, this requires knowledge of analog electronics.

True, as a simple demonstration on how this could be (and has been) done the article is helpful. I appreciate that you provided the link to start a discussion on multimeters in SLR repair. Thanks for that. Still, the basic principle used in the article only applies 1:1 to passive, analog multimeters: The coil of a mA meter is used as one part of a voltage divider, resulting in a multimeter with low, range-dependent input impedance. Except for the use of voltage dividers this is both different from active, analog multimeters and digital multimeters.

Which approach works best for you depends on your level of knowledge. The good thing is that you can scale down your analysis of circuits. Otherwise, I’d have to try to understand in detail the interconnections of a huge number of transistors in a microprocessor. A ticket to madness

In my opinion, there is hardly any field, except microprocessor design, where you have to understand the functionality of ICs on transistor level. It's just not necessary. Similar for multimeters. More or less the only thing you need to know as a hobbyist is that you're putting a resistor in parallel to the circuit when using the multimeter as voltmeter and that you're putting a resistor in series to the circuit when using as current meter. In the former case, the resistor should be as high as possible, in the latter case as low as possible.

PS: Even if I don't measure 1000V I rather use a multimeter certified for 1000V to measure 230V houshold voltage than using one only certified for 300V .

 

[ic-racer]

Well, I think the take home message is that galvanometers measures both current or voltage, depending on if it’s wired in parallel or series with an electromotive force. This is a good concept to understand in light of the fact that many many many repairable mechanical cameras have galvanometers.

 

[Andreas Thaler]

I’ve always assumed that computer scientists, network technicians, programmers, and the like have a basic understanding of analog electronics. But I’m learning that this is often not the case and that it plays a minor role in today’s education.

A while back, the electricity meter in our apartment was switched to a digital one. The young technician saw an open SLR on my table and was impressed by the discrete components and multimeter. I asked him if he had dealt with analog electronics during his training. Yes, briefly and only in passing. Which means he knows little about the foundation upon which his highly integrated circuits and systems are built. Nor does he need to know.

I also understand that, when it comes to electromechanical SLRs, electronics is something you often have to learn from scratch. At least that was the case for me. But digital technology isn't exactly common knowledge either.

 

[Andreas Thaler]

In my opinion, there is hardly any field, except microprocessor design, where you have to understand the functionality of ICs on transistor level. It's just not necessary. Similar for multimeters. More or less the only thing you need to know as a hobbyist is that you're putting a resistor in parallel to the circuit when using the multimeter as voltmeter and that you're putting a resistor in series to the circuit when using as current meter. In the former case, the resistor should be as high as possible, in the latter case as low as possible.

When it comes to understanding a circuit as thoroughly as possible, I need to dig deeper.

If a microprocessor is involved, I should understand how it works, how logic circuits function, what a flip-flop is, and so on. Even that isn’t enough, but it’s more than just looking at components. Even a simple circuit designed to generate a logical 1 or 0 requires a basic understanding of analog electronics, such as how a transistor can be switched accordingly.

None of this is trivial.

 

[lumoStoria]

I’ve always assumed that computer scientists, network technicians, programmers, and the like have a basic understanding of analog electronics. But I’m learning that this is often not the case and that it plays a minor role in today’s education.

Do you refer to any of the participants of this discussion?

I may only only speak for myself (East-German education system) but we had analog and (basic) digital electronics as topic in physics lessons from grade 8 or 9 and also (I think) two semesters on electronics for computer scientists. However, besides being a useful part of one's general education I don't see a need for analog electronics in the curricula of computer scientists or programmers.

 

[Andreas Thaler]

Do you refer to any of the participants of this discussion?

If that were the case, I would have spoken to him directly.

 

[Andreas Thaler]

However, besides being a useful part of one's general education I don't see a need for analog electronics in the curricula of computer scientists or programmers.

But—and I’m speaking directly to you —if you want to repair electromechanical SLRs, you’ll have no choice but to gain a basic understanding of analog electronics.

This applies even to your Canon A-1, which, while digital at its core, receives analog signals as input and outputs digital signals as analog signals. Consider the aperture control in automatic mode, the control of the solenoids (including the aperture), or the light metering via an SPD. Nothing runs purely digitally.

 

[lumoStoria]

When it comes to understanding a circuit as thoroughly as possible, I need to dig deeper.

If a microprocessor is involved, I should understand how it works, how logic circuits function, what a flip-flop is, and so on. Even that isn’t enough, but it’s more than just looking at components. Even a simple circuit designed to generate a logical 1 or 0 requires a basic understanding of analog electronics, such as how a transistor can be switched accordingly.

None of this is trivial.

I think this is just the difference between "understanding as thoroughly as possible" and "understanding as thorougly as necessary". Everyone can decide for himself how deep he dives into any subject, especially if it's fun for him. However, I would not agree that for repair topics it is necessary to know everything to any level of detail. It is just necessary to know what is needed for the repair. This is just a question of economy of time.

Although I know how a flip-flop works (and most of the shabang) I can assure you that this knowledge is not necessary for people writing computer code. It's always a question of abstraction and find the right level of it for the task at hand. Even the machine code or the instruction set (much more higher-leven than a transistor or a flip-flop) is not necessary to know for almost all programming tasks.

 

[Andreas Thaler]

But I don't want to go off-topic.

I'll wrap up by saying that a basic understanding of analog and digital electronics provides a solid foundation. And for electromechanical SLRs, it's a prerequisite for being able to work on them.

And the more you know, the easier it is to make connections.

This comes in handy when troubleshooting.

 

[lumoStoria]

But I don't want to go off-topic.

I'll wrap up by saying that a basic understanding of analog and digital electronics provides a solid foundation. And for electromechanical SLRs, it's a prerequisite for being able to work on them.

And the more you know, the easier it is to make connections.

This comes in handy when troubleshooting.

Fully agree. Sorry for following on the off-topic route. Will go back to the bench .

 

[Andreas Thaler]

I may only only speak for myself (East-German education system) but we had analog and (basic) digital electronics as topic in physics lessons from grade 8 or 9 and also (I think) two semesters on electronics for computer scientists.

I consider the technical books on electrical engineering, electronics, digital technology, and precision mechanics in my collection from the GDR to be among the most authoritative. As far as I can tell, as an amateur.

 

[Andreas Thaler]

Fully agree. Sorry for following on the off-topic route. Will go back to the bench .

 

[Chan Tran]

For the AC volt the article is oversimplified. The problem is that the rectifier bridge drops the forward bias voltage and causes inaccuracy. In fact the AC voltage scale has to be drawn non linear and many meters do not use the full wave bridge rectifier to prevent too much voltage drop.