I have been looking at understanding how polyfuses work.
I have an old ATX power supply that I wan to turn into a bench supply, but want to limit the available output to 3A on all the available voltage rails that support the current. I looked at Dangerous Prototype's board, but the outputs are rated only at 1.25A.
Therefore I am looking to make my own board that I can fit into a case and pull some of the digital volt/amp meters on it, but want to make sure I understand how polyfuses work before I go order a bunch and find out I am clueless.
From what I have read they are placed in series to the power source and that their "max amp" is basically the same as the "max amp" on a regular fuse/breaker. If they read the "max amp" then there is a "trip current" that is "above the max" current. So if a 3A max amp poly fuse with a .1A trip current is hit with 3.2A for some reason the fuse activates. This part I generally understand.
What I don't understand is how the polyfuse's resistance comes into play. This polyfuse that I have been looking at.
This has a 50 ohms resistance. How does this factor into the circuit? Is it once the current is > 3.1A that it's a factor or is it always part of the circuit and the > 3.1A current resistance a different resistance?
They have a conductive powder (ie metal dust) in a polymer matrix. The particles
touch each other so it conducts well normally, but above a certain current the various
resistances at the particle interfaces means it will start to heat up noticably. This
causes the polymer to expand, pulling the particles further apart - current still flows
but the resistance increases so the heating increases and you get a runaway reaction
and the expansion pulls the particles far enough apart to rapidly increase the resistance
(the fuse "opens")
There are two stable states, with enough voltage available it remains hot and only carrying a
small current (enough to keep it hot) and has high resistance - the current is limited,
the voltage across it is high.
With low current it stays cool and low resistance and thus low voltage drop.
Thus is automatically resets (slowly) after the power is removed.
Also the temperature rise is limited as too hot and no current can flow, so there
is a limit to how hot it gets - its not going to destrroy itself
theNetImp:
What I don't understand is how the polyfuse's resistance comes into play. This polyfuse that I have been looking at ... has a 50 ohms resistance. How does this factor into the circuit? Is it once the current is > 3.1A that it's a factor or is it always part of the circuit and the > 3.1A current resistance a different resistance?
You are reading the datasheet incorrectly. The "hold" current is specified at 50 mA and the "trip" current at 100 mA. Given a resistance of 50 ohms, at 50 mA it will drop 2.5V and dissipate 125 mW which is considered acceptable for its specified use in a 50V/ 50 mA telephone "loop". Its purpose is to protect equipment against accidental contact with 240VAC power mains where if the remainder of the equipment provided a 30 ohm resistance to ground, then such contact would cause 3A to flow in which case the polyfuse is specified to "open" within 150 ms as against opening at 100 mA which may take up to a minute or so.
The calculations would of course involve quite different values for a polyfuse such as on the Arduino.
Note particularly - all fuses necessarily have some resistance in order for the element to heat up and - fuse! For very low currents, this resistance is actually quite substantial.
Hold Current: 0.5 A
Trip Current: 1 A
Current Rating - Max: 100 A
The trip current is the current that the fuse is rated for, so anything greater than 1Amp trips the fuse. When the fuse is tripped a hold current of .5Amps at a very high resistance is what will flow until whatever cause the trip is cleared. The max current rating is what the device can handle before it starts smoking and needs to be replaced?
The max current rating is what the device can handle before it starts smoking and needs to be replaced?
Yes. But, you'll never get 100A out of a small power supply.
This has a 50 ohms resistance.
That's too high for a bench supply. A regulated power supply needs a low source impedance. Voltage regulation itself lowers the effective source impedance of a power supply. Series resistance (outside of the regulator's feedback loop) reduces the effectiveness of the regulation.
You'll get a voltage drop across the polyfuse (Ohm's Law). For example, 5V across a 50 Ohm load is 100mA. If you power a circuit that normally requires 5V @ 100mA, a 50 Ohm polyfuse in series will supply only 2.5V @ 50mA into your circuit!
The "hold" current is that which is guaranteed not to trip the polyfuse.
The "trip" current is that which is guaranteed to trip the polyfuse - but not quickly.
The "maximum" current is that which will trip the polyfuse within 150 ms or so and which you should not exceed in use. Since the polyfuse does have a certain ("cold") resistance, this corresponds to a maximum permissible voltage that the circuit should apply to the polyfuse under duress.
Refer to my previous explanation for an example of these figures with the model you cited.
A poly fuse should be considered only as a last resort protection. It should not be expected to be used. While it will recover it is never the same again and will need replacing after a few trips.
On the other hand, if used in a circuit that limits the current and voltage to well within Imax and Vmax, they seem to trip away indefinitely. I've used an RXE-100 to charge a 1F 20V automotive cap in a circuit operating for 5 years. At 7 trips per charge, 1 charge per day, 5 years = 12,775 trips (and counting).