If you would read A75 article carefully you would find the answer. It all depends on the current you want to run through tested devices.
You find the appropriate resistance from equasion:
R=U/J
Where U is power supply voltage less 4V VGS (in your case 10 - 4 = 6V), the current depends on the type of amp you are building. If you want to run 170mA the resistor's value would be 35 ohm and 2W power.
Don't use heat sinks and measure mosfets for 3-5 seconds maximum. Trust me, that's the easiest way to do it.
You find the appropriate resistance from equasion:
R=U/J
Where U is power supply voltage less 4V VGS (in your case 10 - 4 = 6V), the current depends on the type of amp you are building. If you want to run 170mA the resistor's value would be 35 ohm and 2W power.
Don't use heat sinks and measure mosfets for 3-5 seconds maximum. Trust me, that's the easiest way to do it.
Peter,
if you look on the first page of this thread, you will note that I used the circuit and formula you just posted, unfortunately, my results are what they are. My 10V power supply yields me 6V and so with my 36ohm (measured) in series to the drain my current is in spec. My questions seem relevant under the circumstances and the A75 article doesn't answer them. Were you able to open the file I sent you?
if you look on the first page of this thread, you will note that I used the circuit and formula you just posted, unfortunately, my results are what they are. My 10V power supply yields me 6V and so with my 36ohm (measured) in series to the drain my current is in spec. My questions seem relevant under the circumstances and the A75 article doesn't answer them. Were you able to open the file I sent you?
yldouright said:
OK, I did not read carefully, your prcedure is OK, the problem is stated above.
Don't put the mosfets on a heat sink. Put them on a table to settle down and acquire uniform, stable temperature. Use a setup with a socket so you connect and measure your mosfets fast. After placing mosfets in a socket, wait 3 seconds and take the measurement. You will see the numbers going up, but as long as the initial temp is the same for all mosfets and the time interval until you take the reading the same, you should have properly matched devices. I've read about different approaches to the task, with some people placing devices on heat sinks and waiting 30 min. before taking reading, but what I'm suggesting here is much simpler method, which worked for me and should work for you as well.
Peter Daniel
I don't know if that is such a good idea. I think what we want is a set that will "settle" at the same vgs since that will be scenario when the amp is fully warm. I have noted that some mosfets wake up and drop their Vgs quickly to let's say 4.6V and others don't even wake up for a full 10 minutes and then drop to 3.8V. With the method described, it's very possible to have one mosfet of the pair drawing 90% of the current when the amp is warm. Your method sounds convenient but it doesn't sound like the right way to me.
yldouright said:
This is what happens when you use substitutes. I always use International Rectifier parts and never observed what you describe. Pass Labs is using similar technique to measure devices. One other member compared both methods (few seconds and longer time to measure) and said that the results were very similar indeed.
It is repeatable and should be written with a marker on a case (in case you have to replace the device). After I measure 20 and 50 devices I go back and measure the first one to confirm that I'm doing everything right.
If you really want to be really accurate, measure the ambient temperature in case you have to do more measurements in a future to replace devices.
After I put devices on a table, I wait at least an hour to let them settle down.
If you really want to be really accurate, measure the ambient temperature in case you have to do more measurements in a future to replace devices.
After I put devices on a table, I wait at least an hour to let them settle down.
Hi Peter.
1. There is another one, Do you power on setup-device first and then plug mosfet in to connector, or do you first plug the mosfet to connector and then power setup-device on. ?
It may sound stupid
sorry but trying to match IRFP244 for my Aleph 4, gets me confused as well. I have never done matching before.
As well if I use a different resistor it will only affect the readings on the instrument and as well temperature??? or am I wrong.??
Thanks.
Trigon.
1. There is another one, Do you power on setup-device first and then plug mosfet in to connector, or do you first plug the mosfet to connector and then power setup-device on. ?
It may sound stupid
sorry but trying to match IRFP244 for my Aleph 4, gets me confused as well. I have never done matching before. As well if I use a different resistor it will only affect the readings on the instrument and as well temperature??? or am I wrong.??
Thanks.
Trigon.
Hi Trigon,
I have just sent you an e-mail to check this thread since I know you are maching devices.
The PS is turn on all the time. If you are using 15V supply in your case 22ohm resistor shoul be used.
Different resistor shouldn't affect VGS that much, but you will have different current running through the mosfet. The higher the current the faster it heats up and the reading changes faster. The key is to use the same timing with ea. device.
I have just sent you an e-mail to check this thread since I know you are maching devices.
The PS is turn on all the time. If you are using 15V supply in your case 22ohm resistor shoul be used.
Different resistor shouldn't affect VGS that much, but you will have different current running through the mosfet. The higher the current the faster it heats up and the reading changes faster. The key is to use the same timing with ea. device.
you have already tested your mosfets and I trust you and will do the same.
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