DC current interrupter, MOSFET controlled by 555 timer

[Stephan]

I am building a DC current interrupter used to momentarily interrupt rectifiers used in cathodic protection. My prototype is required to interrupt <25 amp DC at <50 volts. I have successfully built the 555 timer circuit to produce my 5 sec high, 2 sec low timing cycle I wish the interrupter to follow. I have connected the timer to the MOSFET transistor and attempted to interrupt a 2 amp DC current, but it is not working correctly. It functions correctly on the high, allowing current flow but during the low it continues to conduct roughly half the current. I think it may have something to do with my materials selection.

The 555 timer circuit is standard astable, roughly 5 sec high 2 sec low powered by 12 battery. I don't believe this is the problem, it cycles an indicator LED on/off in parallel with the positive to the transistor.

The power transistor I am using is NTE2395-KN in the TO220 package.

The test current I was attempting to interrupt was a 9 volt battery nearly shorted with the multimeter in series showing the current flow.


In my attempts to troubleshoot the issue I did notice that when the transistor is still conducting during the low, it will remaining conducting until I reverse bias the base or remove the load. If I instead use the ohm function on my meter as a test load, it very nicely alternates between 0.2 ohm high and OL low. I suspect there is some property to the 2395 that I do not understand correctly resulting in my confusion as to why this is happening. I selected the 2395 for it high current handling and fast switching properties as I had planned to use multiple 2395's in parallel to interrupt DC currents up to 75amp as some rectifiers in my area are running that high. If I have gone about this the wrong way and anyone has a suggestion as to how to build an efficient interrupted I would appreciate it. I did not go with a mechanical relay ( I know that would solve the issue) as it is very desirable to have the interrupter's internal battery last for several days of continual use during an extended cathodic protection survey. My understanding is that MOSFETs would require much less power than relays that can handle the same amount of current.

I can take picture or draw diagrams on what I have done if that will facilitate anyone helping me. I did take electronic engineering for a few years in college, but I am old and my brains are soft from being out of the field so long.

[Major_Problem]

Please post the schematics, so I can understand your problem better.

Greetz,
Wim