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Things You Should Do Before Questioning Your Circuit Configuration

It Might Be an Open Collector Output...

Hello. I’m Masaki Miyata from Kikusui’s Solutions Development Section.. In this article, I’d like to talk about some of my experiences in product design. I hope you will enjoy reading them.

To begin with, when we first build a circuit and apply an electric current to it, a quite common experience is for us to find that it doesn’t work, and ask ourselves why not. We then set about checking and rethinking the circuit configuration and components, but can’t quite find a cause that seems to fit. What are the key points that we should be questioning in those kinds of situations? That is the main topic of this article.

Digital I/Os, timers and counters are a cause for suspicion.

The testsystem in question had a digital input/output (I/O) device embedded in it. I was using it as an open collector output. But in actual fact, this was what was causing the circuit to not work correctly.

At that time, I was not properly aware of the fact that, broadly speaking, there are essentially two types of devices that can be used as output circuits for devices such as digital I/Os, timers and counters: devices that are used as both sinks and sources (Figure 1), and (2) devices that must be used as open collector outputs (Figure 2), or current sinks.

In other words, I thought that an open collector output was a circuit in which both the emitter and the collector were outputs, and could be connected as in Figure 1.

The reason for this was that the specifications of certain devices that I had used in the past were such that the outputs could be used as both sinks and sources (Figure 1), but there were cases in which I had been using them as open collector outputs.

Figure 1. Transistor outputs that can be used as both sinks and sources
Figure 2. NPN transistor (open collector output type)

In the circuit shown in Figure 1, the outputs can be used as both sink and/or source outputs, and depending on the way in which the load is connected, they can act as either a sink or a source with respect to the load. For this reason, if we are using these kinds of outputs, there is a high level of flexibility (i.e. for correcting incorrect usage.)

On the other hand, the circuit shown in Figure 2 is the open collector output type. The flow of current is the same as for an NPN transistor, and it acts as a current sink. In the case of open collector outputs, the internal circuit consists of the emitter being grounded, and external connections can only be made on the collector side. Since it is only a current sink, we cannot drive other components with this NPN transistor.

An open collector cannot possibly function as a source...

The first drive circuit that I built was the one shown in Figure 3-1.

Figure 3-1. Example transistor output drive circuit (NPN)
Figure 3-2. Example transistor output drive circuit (PNP)

However, because the digital I/O that I was using was actually an open collector type (i.e. current sink type) digital I/O, the specifications were such that it should be used with the emitter side grounded. In this case, we cannot make it function as a relay unless we use the circuit shown in Figure 3-2. Because I had made this mistake, I ended up having to remake the circuit board over again. The circuit that I ended up with after making the necessary corrections was that shown in Figure 4.

Figure 4.

Thanks to this mistake, though, I was able to learn that─when it comes to drive circuits for transistor outputs that can be used as both sinks and sources and for open collector output types─as long as you learn the patterns shown in Figures 3-1 and 3-2, that is enough to handle most situations. (Although, depending on the circuit, there are some cases in which you need to make some more involved changes.)

Then all you need to do is determine which transistor to use based on the current value of the transistor you will be connecting and the current value that you actually wish to apply.

From a veteran engineer’s point of view, transistor outputs (open collector output types, sinks and sources, etc.) are a rudimentary issue, and many may think that they are something that is obvious, to be taken for granted. But for newly recruited employees and young engineers with limited circuit design experience, I think that this is an area where it is easy to make mistakes. I myself, too, actually made these kinds of mistakes...

When it comes to circuit design, mistakes do happen. When searching for such mistakes, one thing that is surprisingly easy to overlook is the issue of, "What are the original input and output specifications of the device to begin with?" If you think that something seems amiss, please do try investigating this, as well as your actual circuit configuration itself.

TEXT BY
Masaki Miyata
Solutions Development Section, Solutions Development Department

[Major product development achievements]
Custom AC ripple superimposed power supply systems
Custom power supply systems, custom electronic load systems

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