Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Help find this Voltage comparator part

Status
Not open for further replies.

ThE_sPaCeCoWbOy

New Member
I tried digikey and National Semi. I'm guessing I'm not typing in the right things...i keep getting results where I have to have a pull down resistor with the output and I'm not looking for that.

I"m loking for a voltage comparator. Must be capatiable iwth CMOS, output of the comparator would like to have higher then 3 volts, DIP, quad. If quad isn't available, then oh well, i'll take what I can get.

Thanks in advance.

space
 
ThE_sPaCeCoWbOy said:
I tried digikey and National Semi. I'm guessing I'm not typing in the right things...i keep getting results where I have to have a pull down resistor with the output and I'm not looking for that.

I"m loking for a voltage comparator. Must be capatiable iwth CMOS, output of the comparator would like to have higher then 3 volts, DIP, quad. If quad isn't available, then oh well, i'll take what I can get.

It's common for voltage comparators to have an open collector output, these require a pull-up resistor (which is presumably what you meant).

This gives considerable advantages, including making it extremely easy to interface to your CMOS logic chips - so why don't you like them?.
 
oppps....pull-up...my bad....

most of the comparators datasheet state they are compatible with CMOS....I just can't find one that puts out a high enough voltage so I didn't have to us a pullup resistor...

thanks,
space
 
ThE_sPaCeCoWbOy said:
most of the comparators datasheet state they are compatible with CMOS....I just can't find one that puts out a high enough voltage so I didn't have to us a pullup resistor...

But WHY don't you want a pull-up resistor?, it's that that makes it perfect to connect to CMOS - you simply connect the top end of the pull-up resistor to the CMOS +ve supply - giving perfect CMOS compatibility regardless of the supply to the comparator!.
 
Generally, when a comparator "fires", that is, senses a voltage which is at (or above) the switching threshold voltage, the output goes low. Note in the diagram for the LM139,239,339 that Sebi suggested, that the output is open-collector. A pull-up resistor (or load) is required for it to operate. JB
 

Attachments

  • lm139.gif
    lm139.gif
    17.5 KB · Views: 600
I have the lm339

Th problem I was coming up with was the fact that I'm applying 16 V to this comparator and when I connect pullup resistor to this supply, I'm getting the ~16V output. I can't connect this to a mosfet transistor unless I make a voltage divider. I really just wanted to take the output of the comparator and hook it into the transistor and be done and avoid the divider, but I guess alittle more work won't hurt me.

Thanks,
space
 
ThE_sPaCeCoWbOy said:
... but I guess alittle more work won't hurt me.

Hey space, it won't hurt any of us. :lol: Anyway, what were you looking for in terms of output voltage? You could use the pullup resistor as part of the voltage divider. For instance, if you wanted 4 volts out, use a 3Kohm pullup and add a 1Kohm to ground from the output. Just use the voltage divider formula to get whatever output voltage you want: (ground resistor / (ground resistor + pullup)) X voltage. When the comparator "fires", voltage at the output is reduced to ~0v by the output transistor turning on. JB
 
I just wanted to beable to hook up the output of the comparator to a mosfet transistor to turn it on....maybe 2 V instead of 250mV that the lm 139 put out to turn the transistor on and I was hoping to find a comparator that put an output of that. but it didn't happen. I got it working with what I have and used a voltage divider. and it worked fine.

on the side note though, I never could figure this out, but how comes when using a pnp transistor, having a dc motor hook up to it, when the input to the transistor is high, it slows the motor down, but doesnt shut it off?

Thanks,
space
 
ThE_sPaCeCoWbOy said:
I just wanted to beable to hook up the output of the comparator to a mosfet transistor to turn it on....maybe 2 V instead of 250mV that the lm 139 put out to turn the transistor on and I was hoping to find a comparator that put an output of that. but it didn't happen. I got it working with what I have and used a voltage divider. and it worked fine.

There's no need to take the pullup resistor to the same HT supply as the comparator, that's one big advantage of an open collector design. Simply feed the pullup resistor from a more suitable supply for your purpose.

A conventional output would be far worse for you, it would simply switch to the positive HT rail, giving a fairly high current output.

on the side note though, I never could figure this out, but how comes when using a pnp transistor, having a dc motor hook up to it, when the input to the transistor is high, it slows the motor down, but doesnt shut it off?

You're not taking it high enough, a silicon transistor switches on at about 0.7V between base and emitter, so you need to get the base up closer than 0.7V to the emitter. Sebi's diagram with two resistors is the normal way to do it, if you only use one resistor leakage currents could well hold the transistor conducting.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top