overvoltage protection

[April]

I saw how helpful you all have been and was hoping I could get some help myself. I am trying to design an overvoltage protection circuit but don't know where to start. I will be sending a voltage signal ranging from -10V to +10V dc to an aircraft system. The maximum voltage it can receive without causing damage is +/-12.5V. Can someone please point me in the right direction or give me any ideas?

Thank you in advance!

 

[BillJ]

A common (and cheap) method of over voltage control, which is used in the input circuits of oscilloscopes, is a simple diode to each of the power supplies. The diodes are place to conduct if the signal exceeds the power supply by the junction voltage of the diode (about 0.7 for silicon switching and 0.3 for Shottkey)

 

[jbartos]

Suggestions:
1. Very often the overvoltage protection is implemented by overvoltage relays (electromechanical or solid state or they are part of an integrated protection scheme). Essentially, the overvoltage relay would be set to trip at ±12.5VDC. Also, there may be some relay accuracy or manufacturing tolerance involved, e.g. 5%. This means that the overvoltage relay set point may be set at some lower set point than ±12.5VDC.
2. If a mere protection, e.g. protection against transients, is needed without any signal being sent to any circuit interrupter, then back-to-back connected zener diodes or similar nonlinear devices will suffice.

 

[1346]

Use a TVS (Transient voltage suppressor) or "Tranzorb". It acts like a zener but has sharper reverse characterisrics. You could have used e.g. PSOTxx, only that these come in voltages of 8V, 12V or 15V.

Take a look at

http://www.us.st.com/stonline/books/toc/ds/index.htm

and see if you can find anything there.
Yours truly,
1346

 

[hardwaredesigner]

you can visit

http://www.national.com

and download its articles on power,that may enlighten you .

 

[Trex77]

You don't state the ckt impedances and the answer does depend upon it.

With a med to lo input R, a bi-directional TVS (transzorb) that follows the input resistor and then goes to the ckt's common or chassis will work well.

With a hi input impedance the TVS may have too much capacitance and act as a filter (you may or may not desire this action), if the capacitance it too high then using the dual diode clamps to the supply rails is a good answer. An alternate config. is, the TVS can be in front of the ckt's R, if a series fuse is placed before the TVS and the input.


The relay ckt will work, but may be way to slow to prevent damage. However it can be used in conjuction with the others.

 

[jbartos]

Suggestion: Please, notice that solid state relays are relatively fast just like many other solid state devices and their combinations in protection schemes.

 

[Warpdrive]

April, et.al.,

Solid State Relays are essentially FETs sometimes combined with optoisolators. These methods of overvoltage protection are most useful in battery chargers, yet tend to be more complicated in their implementation. The Linear Technology data sheets on the LT1510 and its related Application Note (#68), are good references for this sort of circuitry. (

http://www.linear-tech.com;

site search the LT1510)

For interfaces, the simplest, most effective solution is BillJ's -- using diodes clamped to a fixed reference (usually your system's local DC power supply). Adding decoupling capacitors in parallel with the diodes also help, as they provide a path for power spikes and high frequency noise that might corrupt your input signal.

Hope this helps. Enjoy.

-- Warpdrive