Opamp vs Comparator

[cjdelphi]

I just built mysf a little charger circuit for a super cap, even a 2w 22ohm resistor is getting hot delivering under 300ma (taking forever for a 400f cap lol)

but, the question is.. i used a comparator vs an opamp, and when the voltage is reached the output pin drops to gnd voltage, most opamps sources current at a specified voltage (eg when it hits x 5v is spit out) so i had to use pnp's no problem

Without using an opamp can anyone name a voltage comparator which i can use with npn's where it sources and not sinks ?

Thanks (i have more npn than pnp)

 

[(*steve*)]

The answer is to use a pull up resistor.

 

[cjdelphi]

Good idea, i'll use that for this circuit, thanks!

 

[AnalogKid]

As you can see on the schematic ... uh ...

 

[Arouse1973]

You can also get comparators with push-pull outputs.
Adam

 

[dorke]

Most comparators(like the LM339) have an open collector output.
i.e they can sink current in the output low state.
If you need the "inverse logic operation" simply replace the input comparison(+) and (-) pins. you will need a pull up resistor on that open collector (10K ohm or so).
If you need 300mA a "current amp" is needed.
You can use a single NPN with the above "inverse logic" operation.
You can use the cap itself as the "collector load" of that NPN.

 

[cjdelphi]

Schematic?

What's a comparator that sources instead of sinks got to do with a schematic? (Not needed)

The pull up i used on the pnp to keep it at 5v from the opamp, not that i had an issue with the circuit, but a pull up helps the snap on/off side of things...

 

[cjdelphi]

Dorke, yup thanks for tip all is good though as is...

 

[(*steve*)]

there are also comparators with floating transistor outputs which can allegedly be wired to source or sink current.