Re: Stability and Lifetime Question concerning PHR
AoiShikaku said:
The Setup:
I ran direct power from my outlet to my driver and ran it for as long as I could.
[highlight]I kept the same settings 7.2v and 180ma.[/highlight]
Attached a Thermal Take water cooling system that I used for my PC a while back.
AoiShikaku:
What does 7.2V mean? Is that the voltage you are feeding the driver? If so, i'd assume it's a Rkcstr driver?
What confuses me is why you are mentioning the voltage at all. If you give the driver the voltage it needs, the voltage doesn't matter. The diode NEVER gets to see the voltage you are putting into the driver. What the diode gets from the driver is the exact voltage it needs for the current to flow. That is, IF the driver is regulating of course..
That's why when we talk about diodes, we only talk about the current. The voltage will be exactly what the driver decides it has to be. Only the current is important. (in simplified terms).
The voltage into the driver is either enough, or it isn't. If it is, it doesn't matter. If it isn't, then the driver isn't working right.
And if this is a Rkcstr driver, and you're giving it 7.2V, then it can't really regulate the current through the diode.
That would also explain why your power is climbing with heat.
The Rkcstr driver needs 2.25V (minimum) MORE than the diode. A PHR at 200mA will need somewhere around 6V. So the MINIMUM voltage that should be going into the driver is 8.25V. 9V, just to make sure it's enough, since the Vf's vary from diode to diode and with temperature and current.
If you are giving the driver too little voltage, then this is what happens:
1. The driver tries to give the diode the voltage it needs for the set current to flow, but it can't. It gives it the max it can give it (completelly unknown current)
2. The diode starts working at the unknown current, and starts warming up
3. As it warms up, it's Vf drops
4. As the Vf drops, the diode suddenly needs less voltage for the same current.
5. Since the driver is still giving it the max it can under the imperfect conditions (input voltage minus 2.25V), more current will now flow through the diode
6. The diode heats up more from more current, the Vf drops further, the current increeases, etc, etc.
7. Theoretically, the diode can heat up enough, for it's Vf to drop so low, that the driver can suddenly regulate the current (but only barelly).
8. Now the diode is actually running at the current you thought you set it to.
So if these are the conditions of the test, then that's why your power starts low and then climbs.
Unless i am completelly mistaken, and the 7.2V meant something else, or you are using a different driver.
In that case, another explanation for the power climb could be this:
1. The diode is set to a current where it has a kink
2. The diode warms up, and the kink moves up
3. If the kink moves up far enough, the diode is now at a power it would have, if there was no kink at that spot (if the graph was a straight line).
Again, this looks like a gradual power climb.
Otherwise, in your tests, how often is the diode turned on and off?
When i did my first tests, i left the diodes on continuously, and got too high numbers. That's why i use a cycler circuit now, which turns it on and off for a pre-set time, to simulate normal use. After that, they started dying much more rapidly, especially since they have time to cool off in the meanwhile. A hot diode can survive higher currents.
Also, did you by any chance plot these diodes? I would love to see what they look like, to try to find a similar one among mine, and replicate your testing.