When adjusting voltage levels for process computers…
Resilient attenuator on the cheap!
When adjusting voltage levels for process computers, accuracy is not always important: in many cases, good isolation from the incoming high voltage spikes is of more significance.
A primitive design for an attenuator usually follows the lines shown in figure 1 – a small diode and a “stock” resistor is used, which works while the incoming spikes are not particularly large.
…This simple design works, but is not particularly resilient.
As EMV demands increase, cascaded zener diode designs get attention – the first references to them can be found inside Seifart’s Classic from the GDR. In principle, these designs follow the ideas outlined in figure 2.
The second diode limits the voltage across the first one…
While this design works, it is not particularly efficient from size and cost points of view. Furthermore, the voltage of Z1 is usually selected to be significantly larger than that on Z2, thereby leading to the well known antipattern of “wagging the reference”.
The brutality dividend
A very interesting alternative comes from Micro Commercial Components, and has seen quite a bit of use in process computer designs by yours truly. The SMBJ5xx8 family contains a set of Zener diodes which can dissipate toasty five watts of power – let us look at the SMBJ5342B-TP, which is priced as following:
With that settled, a circuit similar to the one shown in figure three can be designed. Here, the main loss takes place in the filter resistor, with the series resistor limiting the current flowing into any clamping diodes inside of the process computer.
…A high-power zener makes life a lot easier
In practice, such designs can survive insanely high voltages if the resistor is beefy enough – a system designed by yours truly handles literally dozens of volts while transferring down to 3V3. Of course, this problem can be solved in a variety of ways-for example, multiple smaller resistors can be connected into a series circuit. But this, sadly, is a topic for a rainy day…
The datasheet of this part family contains an explicit warning about fakes. This practice, which happens relatively rarely, indicates that this part is especially “hot” – make sure to purchase the diode from approved distributors, which you can easily find at OEMsecrets.