I'm going to be posting some information over the next couple of weeks with real world measurements of the PTC components in the VEX products.
First a refresher, PTC is an acronym for Positive Temperature Coefficient, and is a passive component used to protect electronic circuits from over current conditions. This device is actually a type of thermistor , current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device. (thanks wikipedia).
The PTC in the VEX power expander is the HR16-400, this part is also (as far as I can tell) used in the cortex and perhaps the PIC micro controllers. The VEX 2-wire motors also use a PTC but with a lower current rating, perhaps I will evaluate these in the future but for this post I'm going to concentrate on the HR16-400. As with many passive components it's not always possible to find the exact part used in a design, for the following tests I purchased a number of equivalent parts from digikey.
The key parameters for this part are as follows.
Hold current, maximum current at which the device will not trip at 25 deg C in still air. This is 4 Amps for the HR16-400
Trip current, minimum current at which the device will always trip at 25 deg C in still air. This is 6.8 Amps for the HR16-400.
I wanted to verify these numbers and also see how the device behave after repeated tripping. There is a theory that after repeatedly being tripped theses devices become "weak" and perhaps wear out, the plan is to try and prove this one way or the other.
To test the PTC devices I'm using a bench power supply that can operate in a constant current mode, the particular unit used is an Agilent E3633A . The power supply is set to the desired output current and connected to the PTC, an oscilloscope is used to measure the voltage accross the PTC as a means of easily determining the time from starting a test to the PTC tripping. A typical test will create a waveform on the scope as follows.
Although this trace shows voltage against time, it is a convenient way to measure when the PTC trips.
Test 1 - Can the PTC sustain a 4 Amp current.
The first tests were done with a new device, a 4A current was set on the power supply and the time to trip the PTC was recorded. Power was then removed from the PTC which was then given time to recover. The test was repeated for various recovery times.
The results are as follows.
New, never been tripped, 4A current, 59 seconds to trip
1 minute recovery time, 4A current, 29 seconds to trip
5 minutes rest time, 4A current, 40 seconds to trip
30 minutes rest time, 4A current, time to trip 42 seconds
So it's interesting to note that with this PTC the 4A current could not be sustained for more than 1 minute when the PTC was completely cold. This is different than the datasheet would have us believe.
Test 2 - Can the PTC sustain an 8 Amp current.
Similar to the first test but with 8 Amps of current, a different new PTC was used for this test.
New, never been tripped, 8A current, 6.5 seconds to trip
1 minute rest time, 8A current, 5 seconds to trip
As expected the time to trip is much shorter, however, even with a short recovery time it does not change much.
Test 3 - Can the PTC withstand a 3A current for an extended length of time.
The same PTC as used in test 2 was setup with a 3 Amp constant current, the device was still working after 10 minutes so the test was aborted with the conclusion that 3A is not going to trip the device.
Test 4 - Are all PTC devices made equally.
For this test three different devices were timed with a 3.5A current.
PTC-1, 3.5A current, 123 seconds to trip
PTC-2, 3.5A current, 57 seconds to trip
PTC-3, 3.5A current, 48 seconds to trip
So it appears that some devices will withstand a given current near to the rated minimum hold for longer than others.
Test 5 - As test 4 but run at 5 Amps.
PTC-1, 5A current, 15 seconds to trip
PTC-2, 5A current, 13 seconds to trip
PTC-3, 5A current, 13 seconds to trip
This time they are closer in performance but PTC-1 still lasts longer.
So what are the conclusions from this first series of tests.
1. Contrary to the datasheet, none of the devices tested could withstand the minimum hold current for more than 60 seconds.
2. Not all devices perform the same when near the minimum hold current.
3. After the device has been tripped, it takes a significant amount of time before the original performance is regained.
4. These devices seem to be better suited to a constant 3A current than the specified 4A.
In a few days time I will post results after these devices have been tripped at least 100 times over a period of several days.
I know it's not nearly as much fun as murdomeeks reveal, but hopefully is informative :)