Low-level to High-level signal

Question

Hi all.

I tried to find the magic component on all famous retaillers but no way.

I'm looking for a solution which able to switching between a 28Vcc (state 1) and an open output (state 0), by a command signal of 200mV

When U>200mV is applied, that energize a relay which permit to supplied a 28V or let a 28V goes through itself.

When U<200mv is applied, relay is OFF, output is open.

I think it's possible to amplify the source signal before be able to command a relay.

But i'm completly lost in datasheets.

Does someone is able to help me for finfing an efficient solution?

Many thanks.

Jimmy

dougw · Answer

You could do this with a 30V rail-to-rail op-amp like an OPA171 or an LT1637. If the current needs to be higher than the op-amp can supply, you can use a FET on the output.

dougw · Answer

Diagram:

jw0752 · Answer

Hi Jimmy,

I think a comparator circuit like I just used in my recent Interface Build might work. You will have to use a better comparator than the LM 393 or you could use a rail to rail op amp. Don't forget you will probably need some hysteresis or you will get chattering at or near the 200 mV level as the circuit tries to decide whether it should be high or low. This means that the circuit will go high at say 200 mV but will not go low again until the signal drops below 190 mV. Here is a link to my blog with the schematic. In my case the trigger point is set by the 2 10K resistors on the non-inverting + pin 3 of the comparator at 1/2 Vcc. You will have to use a resistor voltage divider to move the trigger voltage to 200 mV if that is the level you are looking to be the change over point from High to Low.

https://www.element14.com/community/people/jw0752/blog/2018/11/27/back-up-battery-failure-inspires-new-test-tools

If this isn't what you are looking for give the guys some more specifics and someone will come up with other ideas.

John

geralds · Answer

High Douglas,

may be it works down closed to 0,xxx V. But normally comparators like this are working down above 1V, or 1.2V with its hysteresis.

Here a negative supply voltage may help a bit more, so the V- supply pin would get to -1.2V supply voltage for this comparator.

We have do not to forget too the GND noise.

Gerald

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Low-level to High-level signal

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