Re: positive rail input comparator
HV Enthusiast, Thu Jul 06 2006, 12:04AM


Some sites to name a few you can check are the following. They have parametric searches.

http://www.ti.com

http://www.national.com

http://www.linear.com

Also remember the rules. We aren't going to do your homework for you.

Re: positive rail input comparator
Steve Conner, Thu Jul 06 2006, 03:03PM

*cough* LF412

It's an op-amp, not a comparator, but I know for a fact it works up on the positive rail, because I use it that way in my DRSSTC interrupter. That's all the homework you're getting.

Re: positive rail input comparator
kell, Fri Jul 07 2006, 07:08PM

Looks like no comparators work at the top rail. I'll work around it.

Re: positive rail input comparator
HV Enthusiast, Fri Jul 07 2006, 07:33PM

You shouldn't be using a comparator in that fashion in the first place - hence the reason you don't see comparators designed that way. Use a suitable reference, and divide down your voltage as necessary. I don't know why you would want to trip at the Vcc of the comparator anyways.

Re: positive rail input comparator
kell, Sun Jul 09 2006, 08:27PM

This is for current sensing in the positive rail. I can do it with discretes, or with an op amp like the 412 (see the second diagram).

. + -----+--+-- Rsense --+---->
.        |  '-----,      | 
.       Rc        |      | 
.        |        c\|    | 
.         \e        |----' 
.    PNP   |---+--e/|  NPN 
.         /c   | 
.        |     R 
.      Rload   | 
.        |     | 
. - -----+-----+--------------->


. + -----+--- Rsense ----+---->
.        |               | 
.       Rc               | 
.        +------------,  | 
.        |       __   |  | 
.        c\|    / +|--'  | 
.    NPN   |---<  |     | 
.        e/|    \_-|-----' 
.        |     
.      Rload   
.        |     
. - -----+-------------------->

Re: positive rail input comparator
HV Enthusiast, Mon Jul 10 2006, 01:51AM

Why not put the current sense resistor on the (-) side. This will make current measurements much easier.

Also, if you do go for high side sensing like you are doing, use a differential amp, not a comparator.

Re: positive rail input comparator
kell, Mon Jul 10 2006, 02:34AM

That one's a no go. Poke around a the wiring diagram for floating field dc generators of the type used in GM trucks and American bikes of the 1950's, you'll see why.