If the load is constant or has a limited range, I recommend that you contact Mehdi Samdi of Amatherm to see what he has to offer in the way of thermistors--they are supposed to run hot--that is how they work--otherwise the resistance does not drop. They are also indestructible if applied correctly. He can get you samples right away.

 

Well I've tried thermistors too. Problem there is that we are overmolding this circuit to fit on the output cable of the power supply, so being buried in overmold the thermistor doesn't have the ability to dissipate any heat and becomes extremely hot.

I think the best option here is a damn switch on the output cable to press once the cable it plugged in.

 

Another potential solution is to use a pre-charge resistor that is shorted by a relay or MOSFET after a short time delay--this technique is used all the time in getting inverter capacitor banks charged. Quite simple!

 

Hmm this is a good idea. How exactly does this work? I have tried using a relay before but the problem is as soon as the power supply is plugged into the wall the relay closes since the coil is in parallel with the output..

 

Select a resistor that is just high enough in resistance to limit the initial surge, then turn on a transistor a fraction of a second later that shunts the resistor. Use a 555 circuit, mosfet and small power resistor (perhaps ohmite OX series).

 

Ok I like this idea. How does this 555 timer chip actually work? I'm not sure if I've ever designed a circuit with one and a transistor before... Do you have a sample schematic where I could adjust the transistor/resistor to meet my current limiting needs?

 

OK, will get back to you

 

Ok thanks. I'm confused how it will work without me manually turning the resistor off. I'm missing something

 

When power is removed, the 555 timer will reset as its capacitor self-discharges

 

Well the issue is that the power supply is always connected to the wall, so theres always 24 volts coming out on the DC output end. Wouldn't the timer never discharge? There is a spark when the power supply is connected to the equipment. Maybe once I see the schematic for this circuit I will see how it could work.

 

Send me a sketch of how it is wired--I don't get it

 

Is this what you mean? Its just a PSU with 24DC out. So what I'm saying is the PSU will already being plugged into a wall before its plugged into the equipment, which makes it spark because of the inrush current. If we put the 555 timer on the DC output cord won't it turn on as soon as we plug the PSU into the wall? This is what happened when I tried using a relay and a resistor, the relay closed as soon as I plugged the PSU into the wall, not when I plugged it into the equipment.

 

If the timer is connected to the power supply, yes it would never reset.
If connected to the equipment, it would reset OK.
So it sounds like you have no control over modifying the equipment--right?

If so, there is potentially another way to make it work--this would involve a comparator monitoring di/dt--when it sees a high rate of change of current, it resets the timer fast enough to extinguish the arc. This will work provided the load is subsequently constant.

 

Yes thats the issue.. We can't modify the equipment at all only the output of the power supply... We can't even modify the power supply itself so it makes this project very difficult.

How does the comparator circuit look?

 

A comparator is simply a fast threshold detection device--an op amp often can perform the same function. Will sketch a circuit--have some good ideas

 

Cool. Thanks alot man I owe you a beer..or few lol

 

Don't drink--what a bummer!
Used to get big bucks for this stuff, but big bucks are gone forever it seems.
Now I'm retired and make out with bits ad drabs here and there.

 

Well I really appreciate it. I recently graduated with a degree in EE but like I said, it was mainly power systems...Newton Raphson theory, fault analysis, fun stuff like that. Of course, my job has to do with circuits and not power systems(figures but its only my first job out of school). I've had basic circuits but it was a couple years ago and I didn't use it alot. I'm learning a ton again just from this project and its extremely helpful.

 

Check out new circuit--this one samples voltage across the shunt resistor--when the current exceeds 8A (something far above normal operation) the 80mV signal is amplified by a factor of 100 to 8V. This level "thresholds" the 555 so that it flips states and starts discharging the capacitor--the output goes to zero and turns off the MOSFET--this all happens within about 20uS so there is minimal arcing. At this point, the load must be charged via the 4.7Ω resistor (5A short circuit). After a 0.5Sec timeout, the 555 changes states again and turns on the MOSFET--total voltage drop is now only 0.06V @ 2.2A (P = 0.13W) COOL!

This will all easily fit on a 3 sq. inch circuit board using the rule of thumb: (6 components per sq inch for thru-hole)--smaller if smt--no adjustments required

 

Wow that's a very clever design I like it a lot. I will work on putting it together and let you know how it goes. I have a good feeling about this one