EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: perdrix on April 22, 2022, 02:00:24 pm
-
An existing circuit is using 1SMA5918BT3G (5.1V, 1.5W) zener diodes to limit the voltage on a line from an optical encoder which has a maximum load capacitance of 100pF. The idea was that if someone happens to apply (say) 12V or 18V to that line, the zener limits the voltage applied to the encoder and a polyfuse limits the current through the zener (yes it is possible to force some polarised connectors to connect in the wrong orientation).
Unfortunately the original designer of the circuit didn't realise that the typical capacitance of those zeners is about 1500pF ::) Result the encoder signal is fine at low speed but as the motor speeds up steps are lost because of the excess capacitive load.
Can anyone suggest a suitable replacement with a much lower capacitance or do such things not exist (I suspect they may not)?
Thanks
David
-
Clamping a digital signal with a naked Zener is a bad idea due to its capacitance at low voltages as you noticed.
A better idea is to connect the Zener through a normal switching diode (1N4148, etc.) to the line, with a resistor giving a low bias current into the Zener to keep the switching diode reverse biased.
If there is a high current to be clamped, it flows through the switching diode into the Zener, whose voltage rises slightly.
A second diode can be used, if necessary, to clip negative voltages.
-
Power diodes have big dies and high capacitance. They can sometimes even be used as tuning diodes. Maybe try a lower power diode? I don't know if a TransZorb can be used as a zener, but Vishay has some advertised as low capacitance. The datasheet doesn't really give enough info, but the zero voltage value is about 50 pF.
-
Low capacity Transients Absorbing Zeners are made in all packages and voltages for this,specif application
Microsemi, Semtech, etc.
https://www.microsemi.com/product-directory/transient-voltage-suppressors/683-low-capacitance-tvs (https://www.microsemi.com/product-directory/transient-voltage-suppressors/683-low-capacitance-tvs)
Jon
-
Clamping a digital signal with a naked Zener is a bad idea due to its capacitance at low voltages as you noticed.
A better idea is to connect the Zener through a normal switching diode (1N4148, etc.) to the line, with a resistor giving a low bias current into the Zener to keep the switching diode reverse biased.
If there is a high current to be clamped, it flows through the switching diode into the Zener, whose voltage rises slightly.
A second diode can be used, if necessary, to clip negative voltages.
Trying a visualise that - can you provide a simple schematic?
Thanks, D.
-
[attachimg=1]
-
Note that you need quite a beefy diode to handle the fault current through a polyfuse. They open quite slowly, 10s of ms. You still need a current limiting resistor, and you're basically fusing to protect that resistor -- which can be feasible when the resistor is quite beefy, too. Like, wirewound resistor -- which suggests the usual application, the fuse protects the wire, not the circuitry.
For a simple optical encoder receiver, series input resistance ought to hardly matter, so, no need for the fuse even, just limit current low enough that it doesn't burn anything up.
Another option [when you need bigger voltage handling ratios] is to use depletion mode MOSFETs as current limiters, adding a modest resistance for small signals but limiting the current at higher levels.
Tim
-
Rebonjour, we used a combination of TAZ (SMD multiple) and Schottky (SMD) to achieve level 2,3 transient protection for broadcast equipment digital audio BAL and UNBAL AES/EBU, AES3, AES2id.
The TAZ are on the connector side, and Schottky clamps the IC input or output to ground and Vcc.
The result is a field proven transient protection.
The RX or TX may be similar to the shaft encoder mentioned.
Also attached is a 5V TAZ comparison table (circa 2017)
The material is (c) AES Audio Engineering Socient and the author.
Kind Regards,
Bon Weekend,
Jon