Voltage Spikes

Paul Sutton

Paul Sutton

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Has anyone had experience with TVS(Transient Voltage Suppression) Diodes for protecting circuitry from spikes? The following excerpt comes from :

https://www.digikey.co.uk/en/articl...-diode-protection-enhance-can-bus-reliability

A TVS diode is a p-n device specifically designed with a large junction cross-sectional area to absorb high electrical transient currents. While the voltage/current characteristic of a TVS diode is similar to that of a zener diode, the devices are designed for voltage suppression rather than voltage regulation. A key advantage of a TVS diode is its rapid response (typically within nanoseconds) to electrical transients—diverting the energy of the transient safely to ground while maintaining a constant clamping voltage—compared to other suppression devices.


Theoretically, the protection mechanism is straightforward. Under normal operating conditions, the TVS diode presents a high impedance to the protected circuit, but when the safe operating voltage of the protected circuit is exceeded, the TVS diode operates in an avalanche mode providing a low impedance path to ground for the transient current. The maximum voltage to which the protected circuit is subject is typically modest and limited to the diode’s clamping voltage. The TVS device returns to a high impedance state after the electrical transient current subsides (Figure 2).

article-2019may-design-in-tvs-diode-figure2.jpg

I have experienced failure of an op-amp for the low level fuel indicator on a Virago which has relays in the circuitry. It is my intention to put a 15V TVS Diode across the power supply as in the diagram above. I already have a diode across the circuit.

I would appreciate any views - Thank you.

Any feedback
 
Thank you 2M. I blew another op-amp last Thursday. I have heard similar comments about MOVs for spike suppression. I have forged ahead and fitted diodes across all the relays I included when rewiring the bike. One mystery still being what to do about the starter solenoid??

For the thermistor fuel level circuitry I will make a power supply using a 7812 (with 0.3 and 0.1 mfds fitted) protected by a diode in parallel with a TVS diode. I think this will give spike free power to the op-amp.

On the topic of TVS diodes, their stated voltage seems to be generally +/-1 Volt. If the bike supply runs at 14.5V then a 15V TVS (i.e. 14 - 16V) diode will probably burn out. In my application I will go for 19V and let the capacitors and 7812 regulator combination deal with the remainder of any spikes.

I shall see what happens....
 
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Xjwmx, I have no idea where the spikes are coming from. All I know is the op-amp in my fuel level controller dies after about 30 seconds. The usual culprit in these situations are spikes. I have now put diodes across all relays and added additional protection. Tomorrow I hope to test everything once I have replaced the brake pads on my daughters VW.
 
They don't like input voltages much above the rail voltage, as you know. TVS diodes should do fine if there's big noise coming in from somewhere or ESD. Some op amps are more fragile than others and they're all interchangeable unless you're depending on special features of one. If it doesn't work, put the circuit up and we'll figure it out. One thing you don't want for sure is sparks in your gas tank :) There are all kinds of special designs for safe fuel level sensors
 
Xjwmx, part of the idea for adding in the 7812 voltage regulator was because I felt 14.5V was getting a bit close to the limits for signal input voltage. I ran the bike for 10 minutes earlier and the electronics held up. Tomorrow I will go for about 30 minutes as a test run. If I run into trouble I will drop to an LM7809.

Regarding sparks in the tank, I spent a while measuring currents and changing resistors in the circuit until I got something working in a mild temperature range. The thermistor I chose runs at approx. 40 Celsius when open to air/vapour. The original Virago thermistors drove a small filament bulb directly and ran at a high temperature. By high I mean too hot to touch.

I have taken Signal's advice, via 2M, and put heavy diode across the starter solenoid as well so I am hoping all is well....

Thank you all.
 
I'd have to see the circuit to offer much advice. If you think the noise/spikes is on the power supply, the line regulation of the regulator should handle that. You could look into increasing the line regulation (as separate from the load regulation). You can add additional circuitry to your 3-terminal regulator to accomplsih that;

Is it a totaly analog thing where you're running a meter with a needle? If you're driving it with an op amp directly the problem might be burning it up from trying to drive that. Like I said, can't say much useful w/o seeing the circuit. Noise spikes, etc. often gets wrongly blamed...
 
Background Information: The Virago used the thermistor to visually indicate when the fuel level was getting low because it does not have a petcock. The thermistor was mounted inside the tank at a pre-determined height. The thermistor was wired in series with a small filament bulb. When covered by fuel the the current flowing through attempts to heat the thermistor up but the liquid fuel conducts the heat away so it stays cool and consequently the resistance stays relatively high e.g. 1000 Ohm. Then the fuel level drops below the thermistor the air/vapour is inefficient at removing the heat so the thermistor heats up. As it heats the resistance decreases so the current decreases and we have a run away situation until there is sufficient current flowing to light the filament bulb. I chose to "improve" the system by having the thermistor trigger an op-amp to drive an LED. Also, by using an LED I could have the thermistor run at a lower temperature which is probably safer than the original setup.

The thermistor lives inside a special metal canistor:
Thermistor.jpg
The canistor protects the thermistor from direct splashing with fuel and has small drain holes at the top and bottom.The canistor is securely mounted to a frame that fits in the tank in the same manner as a petcock. I opened my old canistor and replaced the old thermistor with a modern 1K Ohm at 20 Celsius NTC (Negative Temperature Coefficient i.e. the resistance falls with increasing temperatue).

I found the following circuit on the internet:
FuelGuage.jpg


The 200 Ohm resistor results in a very small current flow to the thermistor, approximately 20mA. This heats the thermistor so it is barely warm to the touch. I wanted a higher temperature so the system could not be fooled on a very hot day so replaced the resistor with 100 Ohms which give about 35mA. I also replaced the 430 Ohm with a 680 Ohm to tone the LED brightness down a little.

My bike power supply runs in the range 14.2 - 14.5 V which may be a bit high for the op-amp so I am now supplying the op-amp from a 12V regulator:
Regulator.jpg


Between the bike power supply and the 12V regulator I inserted a silicon diode and a 19V TVS diode. The silicon should remove any spikes originating from the relays in the bikes circuitry. The TVS is designed to remove spikes so in theory should be better than a silicon diode because it has a quicker response time and can handle much higher surge currents. I added both as backups to each other.

Note: Most of the thermistors I have seen come encapsulated in a resin and look somewhat like a Tantalum Capacitor. If you soak these overnight in petrol the resin softens and is easy to remove with your fingers leaving the the bare thermistor which is quicker at responding to temperature change. This eliminates the risk of any bits of resin getting into the petrol/filter. The original thermistor on the Virago was not encapsulated.
 
I haven't thought much about all the thermistor stuff, but I can give a few pointers on the rest of the circuit.
First, did you really use a 741 or is it just a leftover from the internet schematic? Use an LM358 - they're cheap, ubiquitous and about as robust as they come. LM358 inputs can be exposed to rail to rail supply voltage, 741s won't like it.
The 7812 won't be doing much good - with this input voltage it'll barely have any regulation properties and it'll just be acting like a 2.5V series zener.
Driving an LED directly is a bit too much current to be taking from an op-amp, even a 358. I'd put in a transistor to drive the LED. You can then choose to power the LED from the battery or a lower regulated supply.
I'd aim to make a lower regulated voltage like 9-ish which will be plenty for a 358.
The inputs to the op amp are the most vulnerable bits and they draw virtually no current so the best way to protect them is put a resistor in series with each input (value doesn't matter - say 10k but make each the same value). Put them at the op-amp end, not at the thermistor end. That should be enough, but for belt and braces you could put your favourite transient suppression devices (common or garden zener diodes will be as good as anything at this point) on the op-amp side of these series resistors or even small capacitors from each input to ground, say 10nF.
 
I think the above is good advice. I haven't pondered it too hard yet, because the thing I noticed first is how does it tell between an empty tank on a cold day and a full tank on a hot day?
 
There will be extremes in temperature that can fool the system. In the tank there is no airflow so the cooling is still reliant on convection which is not very efficient. When the fuel gets hot, say in the mid 30 Celsius range bubbles start to form and by 40 Celsius the fuel is boiling gently due to the high volatile components. This is why you want the led switching on when the thermistor heats to say 40Celsius or above. I tested thermistors in fuel at various temperatures to see what I would be up against. I placed a cup of fuel in a hot water bath rather than use a pot full on the gas stove. I do not foresee fuel getting above 30 Celsius in the UK except under exceptional condition. If I replaced the 200Ohm resistor with something about 80 - 90Ohm then the thermistor will heat up near 60 Celsius, but I will not go that far unless I encounter issues.
 
What you have is known as a comparator circuit. I'd suggest swap the positions of the 200 ohm and the thermistor. Then connect the minus of the diode (cathode) to the output, and its positive to Vcc through the resistor. That makes it sink current for the led which it does better than sourcing it. I'd suggest using a dedicated comparator chip instead of the op amp. You won't have these problems anymore. You can add hysteresis or a delay to keep it from flashing. Look at this page, especially the last illustration.
https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html

Useful comparator background
https://en.wikipedia.org/wiki/Comparator

P.S. typical comparator chips are TTL logic compatible, which means they use a single-sided 5V supply, which is handy (7805 chip).
 
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Xjwmx, thank you very much. Tomorrow I plan to study up a bit and will follow your links. Sunday morn should be fine so plan on a nice long ride. I have to empty the tank to test if the circuit is working. Bit of a bugger to drain the tank on the Virago with no petcock.

Thank you.
 
After many small trips the circuitry is still working and the light has just come on today in agreement with a low fuel level. The issue with op-amps blowing may have been due to spikes or supply voltage, but all the diodes I added + voltage regulator was a positive step forward.

I think this matter is closed.

Note: After riding I parked the bike and about 10 minutes later the fuel level LED came on. The heat from the enginewarmed the fuel in the tank and tricked the thermistor. After a couple of minutes riding the light went off and the tank was no longer warm to the touch.
 

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