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New here. Help with design of logic circuit wanted

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ElFishi

New Member
I am new to this forum and would appreciate your help on the following problem.
I am looking to build a little logic circuit that pulls an output to logic high (i.e. 5V in my intended use) if a floating input voltage is positive (50mV<U<30V).
Reversely, the output should be logic low (0V) when the input voltage is negative (-30V<U<-50mV).
The input voltage will be small most of the time, although it will float around +30V.

Ideally, changing from high to low should have a few seconds delay. Low to high should change immediately.

I can feed the circuit from +30V or +5V, both are available.

Thanks for your input

edit: corrected sign for negative condition
 
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crutschow

Well-Known Member
Most Helpful Member
What is the impedance of the source output?
 

ElFishi

New Member
low

it is a 0,39 Ohm resistor after a PSU prior to some caps in front of a load of 30V and 0.2A, but the load can go up to 6A.
I want to switch when the caps start to discharge via the PSU.
 

MikeMl

Well-Known Member
Most Helpful Member
You want a "High-Side Current Monitor" IC like a Zetex 1009. I say like because the 1009 wont stand 30V, but I know that Zetex makes ones that do.
 

ElFishi

New Member
interesting. seems difficult to source,though. plus it doesn't have the specs.

Can I substitue something via the attached schematic I nicked from the ZXCT1009-datasheet?
 

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MikeMl

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Most Helpful Member
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ElFishi

New Member
yeah, but we don't have digikey and mouser. I am in Germany.
Plus the IC lacks some 10V in voltage.
 

MikeMl

Well-Known Member
Most Helpful Member
Find a German distributor for Zetex and get a ZXCT1081
 

Grossel

Well-Known Member
What about using a schmittrigger and a basic voltage divider?
 

ElFishi

New Member
excuse me, I might be unskilled in electronics but I'm not completely retarded. I do know that wikipedia exists and the hint to find a German distributor if americans don't do is kinda useless.

so to make my question more explicit: how would I wire up what type of component to have a Schmitt-trigger do what I want?
 
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MikeMl

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excuse me, I might be unskilled in electronics but I'm not completely retarded. ...the hint to find a German distributor if americans don't do is kinda useless.
You are also arrogant. If you had put your country of origin in when you registered for the forum, we might have had a clue as how to direct you. Besides you are lazy as well; Zetex has a German distributor.
 

Grossel

Well-Known Member
Okay, where goes the limits being unskilled and being completely retarded?

Not satisfied? I just tried to help.

If you know wikipedia, why didn't you tried to search yourself then?

You can make the resistor values so that the schmittrigger takes the floating voltage window as the hysteresis voltage. That's the answer. It might work or it might not work.

And what's the talking about some germans? Oh, I kinda guess it. The word schmitt-trigger is german. Sure it sounds like, but any opamps from any vendors should be good enough to make a schmitt-trigger outa. Better blame all other people that couldn't give it another name?

And the voltage diviter should be put in front of the schmittrigger, dividing the signal. As the output goes flowing (your output), any input impedance would cause voltage being 0V.

No - fuzzy responce = fuzzy thinking. You need two opamps (really think we can forgot about that german thing now). One that compares voltages slightly above 0V and the other one slightly below 0V.

Then connect the two outputs from the opamps into an AND gate. That should work.
 

ElFishi

New Member
The resistor I want to use to do the current sensing is a small value in line with the positve supply rail that I have available (V=+30V). So if I try to hook up a schmitttrigger to sense the voltage after the resistor and feed the opamp from the voltage before the resistor the switching voltage for the schmitttrigger will be V with a small window of V+50mV and V-50mV.

Will it work to build a schmitttrigger from an opamp with a switching voltage being exactly the positive supply voltage of the opamp?
 
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Grossel

Well-Known Member
Will it work to build a schmitttrigger from an opamp with a switching voltage being exactly the positive supply voltage of the opamp?
? Please explain little better.

No it won't work. But that's not a problem because I don't beleive you're gonna feed the opamp with ±50mV. You shold feed the opamp with at least ±3V (minimum voltage differs between different types opamps).
 

ElFishi

New Member
attached a sketch:
I guess this won't work b/c, if I use the rail voltage of 30V to feed the OpAmp, the switching voltage will be at or above the supply voltage.
So how would I adapt to make it work?
 

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Grossel

Well-Known Member
Hi. No I doesn't think that gonna work. As I have understand the problem, you doesn't have only a lower and an upper hysteresis level, but you also have a zeero point between (or close to zeero) that shall turn off the output. That requires two schmitt triggers, and a simple logic gate after.
 

ElFishi

New Member
I don't know how you understood the problem, but I can rephrase it for you with the aid of the schematic I scribbled earlier:
If the voltage drop over Rsense is positive, U(PSU)>U(LOAD), then I want the output at 0V.
If the voltage over Rsense is negative, U(PSU)<U(LOAD), then I want the output at +5V.
 

Grossel

Well-Known Member
Ok, then I assume you can get along with a regulat comparator.
 
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