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Overvoltage protection

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ADWSystems

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I have a prefabricated temperature control unit that for some unknown reason the genius designers used the same connectors for the sensor inputs and the control outputs. Unfortunately if you connect the 5V sensor to the 24V output, the magic smoke comes out. I'm getting tired of replacing and wanting for replacement sensors. I am looking for a small circuit solution that I can add into the backshell of the sensor connector to prevent overvoltage. The circuit does not need to limit the voltage to 5V, but can actually outright prevent the sensor from working. The sensor does not need to work is connected to the 24V output control port.

Sensor wiring is:
1-5vDC
2-Output
3-Ground

Control output wiring is:
1-24VDC
3-GND

OK, guys. What do you have?
 
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add a suitable PTC and key the connectors
 
PTC will current limit ... see Polyfuse
then Key the connector by color pen markings.
 
What Current Does this circuit Draw?

How about installing an Inline Fuse and a 5 Watt, 6 Volt Zener to ground.
As the Voltage goes up to the 6 volts, the Zener conducts, and if the fuse is properly rated for the excess current, it will blow.
 
chemelec said:
What Current Does this circuit Draw?

How about installing an Inline Fuse and a 5 Watt, 6 Volt Zener to ground.
As the Voltage goes up to the 6 volts, the Zener conducts, and if the fuse is properly rated for the excess current, it will blow.
Click to expand...

It is just an active sensor. I think it is a DS18B20, so max V 5.5 and total of 3.5mA.

Placing a zener across Vin and GND of the sensor will only limit the voltage to the sensor. With a zener and fuse in series (in parallel to the chip power/ground) will limit the voltage to the zener voltage until the fuse blows, then the power supply voltage (24V) will be sent to the sensor. There is nothing in this line up to stop it.

Funny the things you think about when mowing the the lawn. Seems like I need an transistor (NPN, maybe) with the collector and emitter in series with the sensor. The base somehow connected to a current limiting resistor and 5.3V zener to the incoming power. When the zener breaks over, the base turns on and the transistor turns off. Need time to actually draw and simulate it. But the lawn beckons while the sun shines and it isn't raining...
 
chemelec said:
What Current Does this circuit Draw?

How about installing an Inline Fuse and a 5 Watt, 6 Volt Zener to ground.
As the Voltage goes up to the 6 volts, the Zener conducts, and if the fuse is properly rated for the excess current, it will blow.
Click to expand...

Nevermind. I get the orientation now.

Power -> Fuse -> Zener (to ground) -> Sensor.

When the zener conducts, it forms a "short" more shunts more current than the fuse can handle and thus the fuse pops cutting power to the zener and the sensor. I will have to find some tiny mA fuses and a 5.5V zener. Does it have to be 5W? It should only conduct for the time it takes for the fuse to blow.
 
A 5 watt Zener is fairly Common.
And Safer to handle the Current till the Fuse Blows.
A 1N5339B is rated at 5.6 Volts.
A 1N5340B is rated at 6.2 Volts.
Or a 1N5342 is rated at 6.8 Volts.
I would recommend using the 6.2 or 6.8 Volt one so it doesn't draw much Current from the normal 5 Volts.
1/4 Amp BUSS Fuses are Common.
 
The choice of best circuit depends on how much current the sensor takes, how easy replacing a fuse is, and how accurate the 5 V needs to be. If you have little current and a small voltage drop is acceptable, a resistor and zener will work fine.

If the current is larger or the acceptable voltage drop is smaller, you need a transistor that allows the 5 V to pass, and a circuit that detects a larger voltage an turns the transistor off. You will also need a capacitor on the output to stop the voltage rising too fast, so that the transistor has time to turn off. Alternatively, you could have a delay in the circuit that turns the transistor on, so that the overvoltage detection that turns the transistor off works quicker.

The basic design is here:-
https://electronicdesign.com/site-f...tronicdesign.com/files/29/13788/figure_01.gif

You would need to reduce D2 to a lower voltage, or just link it out, to make the circuit work at 5 V. D1 would need to be reduced to 5 V or less to make the cutoff at the right voltage.

Could you instead block up pin 2 on the power connector to prevent the sensor being plugged in?
 
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chemelec said:
A 4.7 volt Zener will be conducting at 5 volts.
Not just Over the 5 volts!
Click to expand...
That's correct
At 5V this zener in spec will conduct 400uA typ.
It is exactly the reason why I did not recommend 5.6V

and you should know by now my rule of thumb for ESR.
Unlike LED's Vz is rated at 50uA
But these are 500mW Zeners so ESR is 2 Ω @500mW or 25mA ( Tony's Theorem ESR*W=1)
Thus at 50uA or 235uW you can expect ESR to be 100x the ESR at 20mA.
So a rough estimate of 200 Ω with a 300mV rise to 5V you can estimate current to be 300mV/200 or 1.5mA max. But data sheet says 400uA typ.

upload_2015-8-15_19-28-48.png
 
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The Zener is After the Fuse, Going to Ground.
So when the input voltage Exceeds the Zener Voltage, it Shorts out the Excess Voltage and Blows the Fuse.

With a 4.7 Volt Zener it will be Conducting all the time.
And Zener's are not very accurate on Actual voltage ratings, so no guarantee on your 400ua. It Could be HIGHER.
Also I Do Not believe that a Voltage Somewhat in Excess of the 5 Volts will cause any Problems.
Even the 6.8 shouldn't do any damage.
 
I'm not so sure about the 6.8V zener not doing any damage. Typical max is 5.5V, absolute max of 6.0V. I don't recall seeing/specing a 5W zener. Then again I've not used many zeners. The first zener I found in the 5.2-6V range was 5.6V 1W. It will only conduct for the time it takes to blow the fuse.

The zener and pico fuse would be the easiest, though not easy to "reset" (ie., replace the soldered fuse). But the concept seems fairly foolproof and the circuit easy to fit inside a connector's shell.
 
Someone said it already but I will say it again. Used keyed connectors. Connector 1 has a connector that will not plug into connector 2 and is best way, or just pay more attention, use big markings on connectors to differentiate. Does not seem that big of a deal if you know to look out for it.
 
There is no way to add a key to the connectors. I can't change out the connectors. This is not my design only my problem. Getting the techs to pay more attention is like trying to get the horse not to drink once it is at the water.

Looking at the items that Tony mentioned, hoping to be able to add them with the back shell to protect the sensors.
 
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