I was asked to investigate over-voltage protection for aftermarket gauges for older cars. Not sure if we have to protect against load dump, or maybe that protection happens elsewhere in the cars (circa 1960-2005).
Proposed design has 18V max DC MOVs (AMCV-0805-180-C101N-T) in front of a 50V ceramic 10uF capacitor, that goes into a 5V linear regulator, then another 50v cap and an 8v max DC varistor. The “data” lines, mostly analog resistive sensors have large resistances and caps, so seem less of a problem.
Any advice, specific or general, would be welcomed.
This depends on the gauge that is being used. If all of your gauges use 5V, then a 5V regulator makes sense. Not sure what the 50V cap or varistor would do in this case if it is used in tandem with a linear regulator. Data lines don’t typically carry a lot of current or voltage by default in most applications. Any more info that you could provide like a circuit setup would be fantastic reference.
We have an output pullup resistor connected to 5V regulator, out to the external resistive sensor that pulls it down. The voltage from that divider goes to a 10k resistor and then a 0.1 cap to an analog in on the microcontroller.
I think we are interested in making it bullet proof.
Again, a diagram would help or any kind of visual. An MOV or Varistor is good for high voltage spikes over a short time, not necessarily long term voltage protection. Let’s assume the car uses a typical 12VDC battery. The only area you may need an MOV is before the 5V regulator because of voltage spikes that may occur at turning on the car or something like that. Once the car settles down to 12VDC, there are regulators that can take that 12V and convert it to 5 consistently. If all the gauges use 5V, that would be perfectly fine, however, I doubt all the gauges would use 5V based on technology. You’d probably need to regulate each one if they use a different voltage (if specific to one voltage); granted I don’t know what system you are dealing with (besides a car) nor do I have a reference as to the design except for the info you provided. I also don’t have much expertise in Over Voltage Protection methods. Zener diodes would be a good example of controlling a over voltage for longer periods of time.
It’s like this circuit snippet. The 5V is used to drive a 7-segment LED display ,and often a pullup resistor for an external sensor.
The 20 ohm resistor is just to take some heat off of the 7805 regulator. The MOVs may be overkill. Not sure if we need them and if so, how to choose them.
Based on this circuit design, there are already a few redundancy designs for safety. First off it begins with a diode to limit the current in one direction along with a resistor to reduce current. The first capacitor and MOV are some circuit protection for the 5V regulator, but it seems like an worst case scenario design to have those components. The output side of the regulator has even more redundancy designs, typically if diodes or silicon based parts like that fail, a open circuit would be formed instead of a short circuit. So the values for the varistors make sense from what it seems because the voltage should be beneath 12 beyond the resistor, but in case the diode and resistor see some sort of huge transient spike: most if not all of that would be directed to ground instead of the 5V regulator. I believe the 22V min would be the varistor voltage: the point where the varistor conducts and shorts the connection to ground. So in that case any varistor with a 22V minimum Varistor voltage would be adequate, but there really isn’t a way to tell what the maximum Joule amount would be without knowing what kinds of spikes the circuit may see (measured values). Beyond the 5V regulator seems like overkill to me personally, the possibility of anything making past the 5V regulator is improbable due to the diode, resistor, capacitor, and MOV before the regulator, but automotive do require certain safety standards, so it may be included for extra redundancy.
