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.

 

After further reading I see that a pull-down resistor (using 10k between gate and ground) is needed (1900pF capacitance) to stop the FET from conducting after the control signal is removed. However, it still does not solve the problem of the FET not fully interrupting all the current if the the leads from the 555 are still physically connected to the FET.

 

How are we supposed to guess whether you added a grounding resistor to the gate of the transistor without you giving us a schematic drawing?

C'mon. You know we need that.

edit: simultaneous post, but we still need a drawing.

 

Ok, I have drawn (sorry it's not the best) the schematic and included a photo of my 555 circuit board.

Schematic
http://i1248.photobucket.com/albums/hh494/stephanjohnson83/Schematic.jpg

555

http://i1248.photobucket.com/albums/hh494/stephanjohnson83/555circuit.jpg

I spent my down time at work today reading more about MOSFET's and understand more about N-channel enhancement mode than I did when I started this. I will attempt to learn more when I can (work lots + baby at home) so I can understand what advice you people can provide. It seems there are lots of schematics using a single power source but none I could find using an internal battery to switch an exterior source.

If the schematic I provided is unclear in anyway let me know and I can post an additional photo once I can find a scanner to use.

By the way, how cool is this website? It didn't even occur to me that a resource like this was available or I would have asked advice before attempting the build.

Sorry I noticed the schematic appears upsidedown in the link but in photobucket it is shown as right side up. What the hell photobucket?! Editing didn't correct it either.

 

stephanjohnson,

From you're schematic, I can see at least one and possibly two serious problems with it. First, You're N-Channel Mosfet has the 555 timer 12V battery ground connected to the Drain when it must be connected to the source. The MOSFET has an internal body diode connected between the drain (cathode) and the source (anode) and that's probably why the current still flows. Re-connect the 12V battery ground to the FET source.

Next, you're schematic shows a separate 9V supply (I assume a battery) but I don't see it on your photo of the ckt. What I'm about to say only applies if the 9V source/battery is actually separate from the 12V supply and not generated from the 12V supply using a voltage regulator IC or a dropping resistor or a zener diode.
Now, the 9V supply is connected in proper polarity to the fet, but the FET source must have a common-ground with both the 9V & the 12V supplies. The ckt should work properly if you simply move the 9V ground connection to the FET from it's drain to its source.

In fact, I don't understand why you're using two supplies (or batteries) to begin with. Either the 12V or the 9V will work fine for both the 555 ckt and for the FET power circuit.

Also, get rid of the 1 ohm pot in series with the gate as it does no good. And if the 555 ckt is supposed to cycle, you are missing a wire between the 555-pin-2 and pin-6. And, pin 4 (RESET) should almost always go to pin 8 as it should in you're case. If you're load on the 9V supply is noisy, like a motor (or even if it's not), it's best practice to put a 0.1uF cap from the 555 Pin-5 to 555-Pin-1.

Good luck,
Kamran Kazem, V.P., CTO
Magnetic Design Labs, Inc.

 

Your circuit should look like this. BTW, don't expect longevity from a 9V bat. They're not workhorses. Also, R1 is used to suppress ringing.

 

-bump-

If you persist in doing this you'll be bumped right off the forum.

 

You absolutely got me straightened out. My drawing was even more incorrect than what I had built but you were totally right about me using two separate batteries and the ground needing to be common between them. I also made the changes to the 555 circuit you suggested.

The reason I was using the two batteries was that 9v was simply a test to make sure it was working. The actual goal of this project is to switch on and off a large rectifier used in cathodic protection (a DC current system protecting oil and gas pipelines from corrosion). Pipe-to-soil voltage potentials have to be obtained the instant the rectifier is off, hence the 5 sec on 2 sec off cycle. This interrupter will be inserted between the rectifier's positive output and the anode bed.

Which brings me to my next question, do you have any advice on using multiple MOSFETs in parallel to achieve the current handling capability of 50-75 amps? (Most rectifies are <75 amps and <50 volts, usually have a circuit resistance of 0.5 - 5. ohms depending on soil conditions)

The MOSFETs I have (http://pdf1.alldatasheet.com/datasheet-pdf/view/10007/NTE/NTE2395.html) state Id = 50amps max Pd = 150W max and my thoughts were to use three of them to switch no more than 75 amps to be safe. I would like to mount the FETs onto their own heatsink away from the 555 board but am not sure what the best way to physically connect them together would be. Solder wire large enough to handle the max 25 amp per FET directly to the leads?

 

Curious, why does the grounding have to be common between the two batteries? What if I wanted the FET to control an in depended circuit? Would I better off using an SCR?

 

What if I wanted the FET to control an in depended circuit?

That's OK, but you need some way to keep all voltages touching the MOSFET within specified ranges for that particular MOSFET. For instance, you may need to keep the drain-source voltage within ±30v and the gate-source voltage within ±15v. As long as these are maintained, the FET will be safe, and will conduct when the gate voltage exceeds the source voltage by a few volts. (Most MOSFETs have a body diode and will conduct from source to drain if source voltage exceeds drain voltage.)

The easiest way to make this all work is to use a common ground for multiple power supplies, but it's not the only way.