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Old 7th April 2003, 07:46 AM   #1
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Question Transistor Comparator ?

Is there a simple method to use
a couple of transistors to form a comparator ?

Vref will be selectable via a switch.
The voltages are very small that needs comparing.

Vref =
1. 2.4 mV
2. 6 mV
3. 12mV
4. 18mV
5. 24mV

I'm making a simple pass/fail detector.

If I select Vref at 12mV and
Vin < Vref, the circuit triggers.

I'm trying to avoid using op-amps.
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Old 7th April 2003, 08:45 AM   #2
mirlo is offline mirlo  United States
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Default depends ...

If you have a negative supply available, then you can build a simple differential pair comparator. However, depending on the supply voltage you have available, which limits the value of the load resistance you can use in the collectors, you will probably need several stages to get enough gain ... and you will need to carefully match the Vbe's of the input pair or buy a matched pair device.

But why not just use a comparator IC? Already has all the matching and stages you need.
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Old 7th April 2003, 11:26 AM   #3
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LM293, 393, LM239, 339 can be a start, cheap and can work from ground. No precision device but maybe sufficient for you?
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Old 7th April 2003, 01:54 PM   #4
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Default at those levels

noise is going to swamp any conventional comparator action. So you're VRef has to be derived from a precision, low noise source. Remember, you are down to the levels of strain guages etc. and it takes specialized techniques to attain accuracy.

This being said, you can use an FET differential pair, constant current source etc., etc. for a front end which will measure into the microvolts (or better). At this level you need in excess of 10megohms of input impedance. Use this ahead of a conventional comparator. The differential inputs should be shieled, everything bypassed. Don't forget also that at these levels temperature differentials will play havoc with measurements.

Take a look at the PDF's on Nat Semi's website for the LM111 or any of the other comparators since they have schematics of the innards.
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Old 7th April 2003, 02:20 PM   #5
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Default Re: Transistor Comparator ?

Quote:
Originally posted by thylantyr
Is there a simple method to use
a couple of transistors to form a comparator ?

Vref will be selectable via a switch.
The voltages are very small that needs comparing.

Vref =
1. 2.4 mV
2. 6 mV
3. 12mV
4. 18mV
5. 24mV

I'm making a simple pass/fail detector.

If I select Vref at 12mV and
Vin &lt; Vref, the circuit triggers.

I'm trying to avoid using op-amps.
What are you trying to do. A schematics maybe? You could use a LM3914 together with an amp.
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Old 7th April 2003, 02:54 PM   #6
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Default Re: Re: Transistor Comparator ?

Quote:
Originally posted by peranders


What are you trying to do. A schematics maybe? You could use a LM3914 together with an amp.
I need a circuit that can differentiate between a
0.2 ohm resistor going to ground and a short across
the same resistor to ground. Also, if the load is > 0.2 ohm, it will not trigger.

Perhaps, there is a different method of doing this I can't see ?
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Old 7th April 2003, 03:20 PM   #7
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Here's an untested circuit.

You set the max detection level using R1
for whatever load resistance you want to
test. I don't need precision, I need a simple pass/fail
checker.

Adjust R1 to trigger if load = less than .2 ohms,
but not trigger if it's greater than .2 ohms.

Output = collector of Q2 = ground when triggered.

I have not idea if this works in reality - hehe

Plus, R1 would consume alot of heat.
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Old 7th April 2003, 03:43 PM   #8
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Default Re: Re: Re: Transistor Comparator ?

Quote:
Originally posted by thylantyr
I need a circuit that can differentiate between a
0.2 ohm resistor going to ground and a short across
the same resistor to ground. Also, if the load is &gt; 0.2 ohm, it will not trigger.
Perhaps, there is a different method of doing this I can't see ?
Is the resistor connected to something? Can you tell us a little bit more about the whole application? Is it short circuit protection for an amp? If yes, Elektor had a such circuit some time ago.
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Old 7th April 2003, 03:57 PM   #9
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I want a user selectable short detector for an amplifier.
Set the senstivity using switches to select the reference, ie,

switch 1 = .2 ohms
switch 2 = .5 ohms
switch 3 = 1 ohm
switch 4 = 1.5 ohm
switch 5 = 2 ohm

This is the general idea.

The circuit will not be connected to the amplifier
circuitry, rather it will check external shorts outside the
amplifier before the output stage is connected
to the speakers using a relay.. ie,

amplifier output --> N.O. relay contact --> speaker (Com relay contact)

Short detector ---> N.C. relay contact

When powering up the amplifier, the relay is off and
the speaker is not connected to the amplifier. The
short detector will be connected to the same relay
on the NC contact to "sense" the load for faults.

I want the user to select the sensitivity because
the load is variable. You may have 1 ohm - 8 ohm
speaker loads.
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Old 9th April 2003, 03:29 AM   #10
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Tried the circuit, seems to work
but it has loose toleranaces plus
when testing for short, there is
unacceptable levels of dc at
the speaker, probably 1v worse
case @ 8 ohms.

I tried the LM339 version of
the circuit, seems to work better,
better precision and 10x less
dc offset. The comparator is the
winner so far.

The last problem to solve is
how to check for bridged
amplifier external short across
both output leads ? The circuit
only checks output to ground, but not
across another amplifier's output.

(amp modules are seperate PCB's)

more electronic puzzles......
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