There are multiple circuit designs for outputting digital signals such as the rotation pulse signals of rotary encoders, the OK/NG signals of measuring instruments, and the BCD signals that represent measured values.
It's not simply a matter of connecting the cables and expecting it to work.
Voltage outputs (TTL, totem pole, collector), voltage-free outputs (relay, open collector, etc.), and line driver outputs are used.
This time, we will explain open collector and collector output formats, which require special attention.
Transistors come in NPN and PNP types, but this explanation will focus on the NPN type.
Open collector output
Features
Since it has a voltage-free output, the receiving end can freely set the voltage (V+). (Below the maximum load voltage)
To receive it as a voltage signal, a resistor is needed to connect it to the power supply. Pull-up It is necessary.
Current flows in a specific direction, and the voltage does not become 0 V; a residual voltage of approximately 1 V is generated.
When directly turning on lamps or relays with an open collector, particular attention must be paid to the voltage (V) and current (I) values.
The maximum load voltage and maximum load current (inflow current) must remain within the specifications of the open collector output.
And is necessary.
The current flowing through it is given by the pull-up voltage V and pull-up resistance R,
I = V ÷ R
You can calculate it like this.
The current value I must be less than or equal to the maximum load current of the open collector.
The following example shows how to directly operate lamps and relays.
Based on the maximum load voltage and load current specifications, only lamps and relays with a voltage of 30 V or less and a current of 25 mA or less can be operated. Exceeding these limits may cause malfunction.
Collector output
This is the output with the collector pulled up with a resistor.
If nothing is connected, the output voltage will be Vo. Depending on the input resistance of the receiving end,
The voltage value at the high level will change.
The low level will not become 0 V, but will have a residual voltage (around 1 V).
The important point to note is that the input voltage value Vi on the receiving end will change depending on the magnitude of the output resistance Ro on the measuring instrument side and the input resistance Ri on the receiving end.
You might expect a measuring instrument to output a high level 10V, but when you look at the waveform on an oscilloscope, you might find it's actually 7V.
In this case, if the threshold for determining whether the signal is ON or OFF is set to 8V, it will remain OFF indefinitely.
The following is an example where the pull-up resistor value Ro on the measuring instrument side is 470 Ω.
Even if the voltage Vo on the measuring instrument side is 10 V, if the input resistance Ri on the receiving side is 1 kΩ, the voltage Vi will drop to 6.8 V. If Ri is 10 kΩ, the voltage Vi will be 9.6 V.
Furthermore, a smaller resistance Ri results in a larger current flow, increasing the load on the measuring instrument.
In the worst-case scenario, the power supply will be cut off, and the signal will be lost.
If the measuring instrument is changed or the receiving circuit is changed, the signal may no longer be detectable.
There are cases where this is the case.
Ro = 470 Ω
Ri = 10 kΩ
Vo = 10 V
at that time
Vi = 10 * 10000 / ( 10470 )
= 9.6 V
Ro = 470 Ω
Ri = 1 kΩ
Vo = 10 V
at that time
Vi = 10 * 1000 / ( 1470 )
= 6.8 V
This is the result.
If the high-level recognition voltage is set to 7 V, the voltage will be insufficient with Ri = 1 kΩ.
Depending on the output circuit of the measuring instrument and the receiving circuit, connection may not be possible.
Please check carefully before purchasing.
(Excerpt from the email newsletter issued on September 26, 2018)
