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Forums
4hv.org :: Forums :: General Science and Electronics
« Previous topic | Next topic »   

positive rail input comparator
Move Thread LAN_403
kell
Wed Jul 05 2006, 09:11PM Print
kell Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
 		I'm looking for dual and quad comparators that will accept a common mode input at the positive rail and a supply voltage of 18 volts.
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HV Enthusiast
Thu Jul 06 2006, 12:04AM
HV Enthusiast Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
kell wrote ...

I'm looking for dual and quad comparators that will accept a common mode input at the positive rail and a supply voltage of 18 volts.

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. smile
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Steve Conner
Thu Jul 06 2006, 03:03PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		*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. wink
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kell
Fri Jul 07 2006, 07:08PM
kell Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
 		Looks like no comparators work at the top rail. I'll work around it.
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HV Enthusiast
Fri Jul 07 2006, 07:33PM
HV Enthusiast Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
 		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.
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kell
Sun Jul 09 2006, 08:27PM
kell Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
 		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   
.        |     
. - -----+-------------------->
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HV Enthusiast
Mon Jul 10 2006, 01:51AM
HV Enthusiast Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
 		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.
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kell
Mon Jul 10 2006, 02:34AM
kell Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
EastVoltResearch wrote ...

Why not put the current sense resistor on the (-) side.

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.
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