Im after a few ideas,

I'm looking to be able to detect when a pin has been connected to a voltage,,,
sort of like a power on reset circuit,

BUT

I need it to be low power
I need to give a "logic" output ( 3v3 or 1v8 )
The voltage in, I need to detect when its above say 5v , but the input can go up to 30 volts,
The circuit needs to be safe when the say 30 volts is connected, but the circuit is not powered.

Things I have considered,

opto isolator, but the resistor needed on the led is a problem,
resistor / zenar, into a logic gate. Resistor power gets high when 30 volts in,
comparator, but cant find one thats safe with no power and 30 volts in,

Any thoughts ?

 

An opto isolator is a good approach. As for the resistor on the led, use a current limiter circuit rather than just a passive resistor.

The circuit below is one that I have used many times and will work anywhere from 5V to 40V keeping the LED current at about 10mA over the full voltage range. The transistors can be general purpose 2n3904 and 2n3906. The LED1 is optional and used as an indicator... since this circuit regulates current if you simply omit the LED1 and effectively "short" it, the LED in the opto still sees about 10mA.

 

The transistors can be general purpose 2n3904 and 2n3906.

Nice little circuit but using that circuit the transistors can not be 2N3906 which is the PNP close compliment to the NPN 2N3904.

Ron

 

"Nice little circuit but using that circuit the transistors can not be 2N3906 which is the PNP close compliment to the NPN 2N3904 " - and why is that?

EDIT - never mind ... The circuit uses only NPN's ... I have a mirror version that uses only PNP's

 

Otay, again pretty nice little circuit.

Ron

 

Below is the LTspice simulation of a voltage detect circuit that uses a TLV431 low-power programmable reference as a comparator to give an accurate and stable trip point.
The output goes high when Vref goes above 1.24V nominal, as determined by the voltage divider R2-R3 from Vin.
D1 protects the Vref input from going above 5V.
R4 provides a small amount of hysteresis to avoid oscillations at the trip point.
For the values shown, the trip point is at 5.01V for both a Vs of 1.8V (blue trace) and 3.6V (yellow trace).

 

Look at the LT6700 and LT6700HV parts: https://www.analog.com/media/en/technical-documentation/data-sheets/6700123fh.pdf

 

An opto isolator is a good approach. As for the resistor on the led, use a current limiter circuit rather than just a passive resistor.

The circuit below is one that I have used many times and will work anywhere from 5V to 40V keeping the LED current at about 10mA over the full voltage range. The transistors can be general purpose 2n3904 and 2n3906. The LED1 is optional and used as an indicator... since this circuit regulates current if you simply omit the LED1 and effectively "short" it, the LED in the opto still sees about 10mA.

Must admit, i had not thought of a two pin constant current circuit,
that looks good,

Might have look at using a dual transistor package to keep them thermaly bound, and for space.

 

Below is the LTspice simulation of a voltage detect circuit that uses a TLV431 low-power programmable reference as a comparator to give an accurate and stable trip point.
The output goes high when Vref goes above 1.24V nominal, as determined by the voltage divider R2-R3 from Vin.
D1 protects the Vref input from going above 5V.
R4 provides a small amount of hysteresis to avoid oscillations at the trip point.
For the values shown, the trip point is at 5.01V for both a Vs of 1.8V (blue trace) and 3.6V (yellow trace).

View attachment 197464

That looks interesting, but is U1 happy with volts on its pin with no volts on Vs ?

 

Look at the LT6700 and LT6700HV parts: https://www.analog.com/media/en/technical-documentation/data-sheets/6700123fh.pdf

They look interesting, but i am un certain as to are they happy with 30 volts in the input with no power on their Vcc ?

 

That looks interesting, but is U1 happy with volts on its pin with no volts on Vs ?

Perhaps not.
When would that occur?

 

First pass, using a FET's threshold voltage as a level detector.

Q1 can withstand 20 V on its gate, so R1 and R2 allow for a 40 V peak input voltage. If you are worried about transients, a zener diode (5.1 V, 6.2 V, 12 V, whatever) can be connected across R2, from the Q1 gate to GND.

Q1 begins to conduct at approx 2.1 V (typ), and this translates to the output changing state at around 4.5V to 5 V at the input.

R6 adds a small amount of positive feedback, creating a Schmitt Trigger input. This causes the output voltage to have more snap, a shorter risetime.

R6 does provide a current path from the input to the downstream circuit input when Vcc is 0 V, but Vout across R5 will be only 1% of Vin. At 30 V in this is less than 0.15 V, not enough to turn on a substrate junction in an unpowered chip. Also, the current would be limited to 30 uA.

ak

 

They look interesting, but i am un certain as to are they happy with 30 volts in the input with no power on their Vcc ?

https://www.analog.com/media/en/technical-documentation/data-sheets/6700123fh.pdf#page=10

The HV version 36V. They don't like negative voltages though without protection of a series resistor.

 

Perhaps not.
When would that occur?

its a voltage detector

the unit could be un powered with 30 volts on the detect input

 

An update on this ,

After talking with an even older guy than myself, over a few warm beers , ( British you know !! )

The conclusion was, I needed a constant current source driving the led in the OPTO issolator.
As per one of the previous threads with the two transistors.


we went back to basics, a three pin LDO can be used as a constant current source.
and amazingly, Microchip do a LDO that reaches 400 V

https://www.microchip.com/wwwproducts/en/LR8


so the answer is.. ( not tested yet ) to use the TO-252 package of the LDO,
and wire it as per fig 3.4 in the data sheet as a constant current of 3 mA ,
which drives the Opto nicely.