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Sugestion for comparator

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AttO

New Member
Hello every body,

I want to make a voltage comparator , It should compare between 5 different voltages (about 15, 13, 11, 10, and 8.5) the pulse duration is about 15-30 ms,
what op-amp do you suggest for me to use as a comparator??
 

ccurtis

Well-Known Member
Not enough information to answer.

Supply voltage(s) and current available, regulated or unregulated?
Comparator output voltage and current drive required?
Internal or external reference voltage?
 

AttO

New Member
regulated +/- 15V, 100mA Supply for each comparator
output on display (LED or 7 segment driver or maybe microcontroller) not decided yet :)
reference will be zener diode for standard voltages, or using voltage divider (separate supply on the same board)

Please note that the input which will be compared with the reference will have sort duration (15-30 ms)
 
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Mikebits

Well-Known Member
So you want to have a circuit that detects 5 different voltage levels with a duration of 15 msec? This would require 5 compare circuits and some way to sample each to detect for a true condition, then latch the true condition.
I am not getting what your trying to do, can you paint the big picture?
 

AttO

New Member
i'm making a tester that will receive a discharge of capacitor (thats why the duration is in msec) i want to determine the capacitor charge level.
The capacitor is (42uf 6 KV), discharging it will be on voltage divider the divider output is the input of the comparator. I need a suggestion for this comparator , I've already tried the 741 but, as you probably know, was a big faliure
 

Sceadwian

Banned
What does your voltage divider look like?
 

ccurtis

Well-Known Member
As far as comparators go, the LM339 (quad, 16 mA output), or the LP339 (quad, 30 mA output) are standard and can be used for the comparison function.

I've already tried the 741 but, as you probably know, was a big faliure.

The 741 should work, as far the part itself goes, except it cannot drive an LED (output current is only a couple of mA). You should post that circuit, and then we can know.
 

Hero999

Banned
A 741 should be able to power an LED.

The real limitation is that the inputs must be kept 2V away for the supply rails to function properly.
 

AttO

New Member
What does your voltage divider look like?
It's just two resistors.

the 741 in my circuit triggers a monostable 555 circuit, so output current was not my problem, I think it's because the input voltage many times so close to supply voltage
 

Sceadwian

Banned
You're using 2 resistor to isolate 6 THOUSAND volts? Are they rated for that??
 
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Krumlink

New Member
I think it is just rated to 6kv but not literally 6kv voltage
 

Ayaskanta

New Member
How can a comparator compare 5 diff values? Two analog signals must be compared first and then the higher valued signal with the next and so on...

A comparator just indicates if the input signal is higher or lower when compared to a trip point. i.e. the actual value of the input signal is not reproduced at the output... Am I right?
 

AttO

New Member
You're using 2 resistor to isolate 6 THOUSAND volts? Are they rated for that??
yes i'm using 50W

I think it is just rated to 6kv but not literally 6kv voltage
and yes i'll use only about 2Kv

How can a comparator compare 5 diff values? Two analog signals must be compared first and then the higher valued signal with the next and so on...

A comparator just indicates if the input signal is higher or lower when compared to a trip point. i.e. the actual value of the input signal is not reproduced at the output... Am I right?
of course u r right maybe didn't explain my cct enough. I'm using 5 comparators the reference for each one is different but the input is common:)

Thanks ccurtis for the tip, and thank you all for participation
 
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Sceadwian

Banned
No not wattage, are they rated for the voltage.... You can't use a normal resistors on even 2 thousand volts, common resistors are only rated for 200 volts or so. You have to use long strings of smaller resistors to balance the voltage to bellow their limits or they don't act like resistors properly when high voltage is applied, it'll arc right through or around the dielectric.

Basics of High Voltage Probe Design
This website explains the construction of 50kv resistor chains but is equally valid for 2000 volts. 2KV can arc 2 centimeters in dry air, I'm sure it's well past the dielectric breakdown voltage of a common resistor. If your resistors aren't marked SPECIFICALLY saying they're safe to use at 2kv's figure they're only good for 200, and derate 100% for saftey. 42u at 2kv will discharge through a 100ohm load in 10ms's with a peak power of 40kw and 84 joules.
 

AttO

New Member
Thanks Sceadwian, I've totally missed this point, It's my first HV project.
I'll study this issue and read the link you send and be back... thanks again
 

Sceadwian

Banned
Think about it this way Atto. The 'starting' energy of defibrillators is around 100 joules. Not to mention more than enough to cause small resistors and IC's to explode. If you don't know what you're doing with high voltage one day you won't log on, and I won't wonder where you went =O
I accidentally set a board with a 80u 300volt charged cap on my pair of jeans once and even through the pants it was enough to cause my muscle to contract. I've brush my dry hand up against one as well and it felt like a slap to the face, and that' 300 volts. You do that with 2kv and you better hope there's someone around to call the peramedics.
 

Sceadwian

Banned
be8 You use 2 resistors to voltage dived 2kv?
 

be80be

Well-Known Member
They do it in test probes all the time but I would sure watch what your doing with 2000 volts and hook up you're test eq before powering up and never touch any thing with both hands
 
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ccurtis

Well-Known Member
It's just two resistors.

the 741 in my circuit triggers a monostable 555 circuit, so output current was not my problem, I think it's because the input voltage many times so close to supply voltage
The divider should be designed so that the voltage taps don't come near the power supply rails of the comparators.
 
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