I know little about electronic circuits so I joined this forum with the thinking someone could help me.
I have a laser that uses a battery pack with four C cell batteries in series. I wanted to be able to use the existing battery pack, install a jack for an external power supply to plug into the battery pack to supply power to the laser without charging the batteries.
I have a 6v transformer that puts out 6v 200ma or 7.5v 100ma.
If I install a in5817 or in4001 diode inline from the positive side of the battery pack, will transformer supply power to the laser without charging the battery?

 

The battery supplies DC. A transformer supplies AC which might destroy the laser.
Maybe you mean you have a 6V/200mA AC to DC adapter. Its output voltage is probably not regulated and might be 10V or more when the load current is less than 200mA. Then the laser will blow up.

If you install a diode in series with the battery then it will not charge but its voltage will be reduced.
Why not use a jack with a switch in it. When the adapter is plugged in then the battery is disconnected by the switch. It is a common jack.