Resettable Fuses

[Atom3892]

I'm looking for some tips on sizing Resettable Fuses or PTCs.

I'm looking through some datasheets and I see that they are dependant on the ambient temperature.

I'm trying to protect a 24V off board supply line. The line connects to a DC Air Valve that draws 6W or 250mA steady state. There is an inrush current of 3.2A for 72ms upon turn on. The max ambient temperature is approximately 40oC, I'm using 50oC for board level calculations due to power dissipation from other components.

I'm looking at some PTCs with a steady state current of 650mA and a trip current of 1.2A. Looking at the curves it takes about 1 second for the PTC to trip with a constant current of 3.2A on it at 23oC.

The manufacture gives at 50oC the hold current is 450mA and the trip current is 650mA. Using the trip current value, I'm sort of using a curve in the PTC datasheet that matches the 650mA trip current. It shows that at 3.2A it take 200ms for the PTC to trip.

I'm asking if my logic is right in choosing a PTC based on my known inrush and steady state conditions along with maximum ambient temperature?

Any suggestions. I haven't dug into this using white papers or App notes yet. If someone can point me to some that would be helpful.

Thanks,

Adam!

 

[itsmoked]

Yes you seem to be on the correct track.
Go to Digikey and find your way to the supplier data sheets.

My on question would be why you think you'll have an inrush to a DC solenoid? AC has a large pull-in current that drops dramatically but a DC one doesn't.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Atom3892]

I'm looking at the Bourns MF-R050 component.

The datasheet for this particular 24Vdc air solenoid states a 3.2A current inrush for approximately 72ms.

I forgot to mention the solenoid is being turned on and off by a low side n-ch MOSFET.

Thanks,

Adam!

http://files.engineering.com/getfile.aspx?folder=c5bfabde-7b5b-440d-9112-b88ccbfa1aa2&file=mfr.pdf

 

[waross]

Hi Keith.
If the currents are correct, the valve may have a pull-in coil.
That is similar to the starter solenoid on your truck. When the moving core bottoms it cuts out the high current coil with a set of NC contacts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill; Strange. Never seen that in a solenoid!

Got a part number for that solenoid Atom3892?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

I like PTTCs, but using them is like nailing jelly to a tree. They basically prevent catastrophic failures like wires burning up. Sizing them on the edge can be problematic. Putting a finger on one can reverse a transition. If the power supply can't take almost a dead short for a period of time, I would think about using using the in an active role. Look into using a circuit like a solenoid driver National had one that allowed high initial current for a period and then drop to a hold value. This can easily be implemented with a little extra code in a micro. I have also done it with a transistor and a few gates.

 

[itsmoked]

You have a good point OperaHouse.

Use a current monitor and a real fuse. You only loose the real fuse if the rest of your hardware is already choked.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sreid]

Along the same theme as above, consider using an Automotive High Side Driver. These are internally protected from over current and shorts.

 

[Atom3892]

Thank you for your help with this. Going to the mfgs website and reading through the application notes it states this:

"DC valves have no inrush current. The amp rating
can be determined by dividing the voltage into
the DC watt rating."

 

[itsmoked]

Ahhh. The world is in its place again.

Keith Cress
kcress -

http://www.flaminsystems.com