The simple circuit that Nelson Pass published in his 1993 article "Matching Devices", even though it was pretty thorough, left me with some questions.
I wondered how he'd arrived at the 15V input value for his tests, and the 2.2k resistor value. The math for choosing the resistor value was in the article, but it mentioned that the input devices should "share the current to within 2mA", and this was based on an equivalent source resistance of 15ohm. No indication of how he got that value, so I looked further back in his publications. I found that the whole thing was a reprint of a portion of his A75 amplifier two part article from the previous year. In that article, Nelson showed his calculations were based on testing the IRFD110.
So now my first question is, how were the choice of input voltage (resistor value plus V at the supply) arrived at for matching these input pairs?
I've looked at how people are matching parts for Nelson's modern designs. The Aleph J uses the IRFP240, and folks are still using 15V with a 2.2k resistor. Just glancing at the data sheet I see they have identical Vgs. So is that the main factor for picking the test voltage and resistor? If so, how would I arrange the test for different Vgs? If it was 2.0 or 3.0 or 5.0, what resistor would I place in there?
Or, since these are for input pairs, should the value stay at 2.2k with a 15 volt input no matter what the Vgs is?
I wondered how he'd arrived at the 15V input value for his tests, and the 2.2k resistor value. The math for choosing the resistor value was in the article, but it mentioned that the input devices should "share the current to within 2mA", and this was based on an equivalent source resistance of 15ohm. No indication of how he got that value, so I looked further back in his publications. I found that the whole thing was a reprint of a portion of his A75 amplifier two part article from the previous year. In that article, Nelson showed his calculations were based on testing the IRFD110.
So now my first question is, how were the choice of input voltage (resistor value plus V at the supply) arrived at for matching these input pairs?
I've looked at how people are matching parts for Nelson's modern designs. The Aleph J uses the IRFP240, and folks are still using 15V with a 2.2k resistor. Just glancing at the data sheet I see they have identical Vgs. So is that the main factor for picking the test voltage and resistor? If so, how would I arrange the test for different Vgs? If it was 2.0 or 3.0 or 5.0, what resistor would I place in there?
Or, since these are for input pairs, should the value stay at 2.2k with a 15 volt input no matter what the Vgs is?
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Thanks for that. If I decide to build the "Test Everything" jig that article is going to be a great help. However, the section relevant to my question doesn't have an answer as to how the input voltage and resistor value were arrived at.
He speaks of the resistor value in generic terms P1, etc, and never a concrete value, only "set P1 full clockwise". There's no mention of what the "given current" needs to be. I understand all of this is project-dependent, but going back to the more generic and simpler Vgs-ony jig mentioned in my first post, Nelson said "You might consider running lots of tests on these transistors, but only one is essential: measuring gate-source voltage versus current". He used 15V and a 2.2k resistor for matching Vgs, with no real explanation as to how those two values were arrived at. I should be able to take any Mosfet useful as an input device and match it. I just need to know how to arrive at the proper testing values.
He speaks of the resistor value in generic terms P1, etc, and never a concrete value, only "set P1 full clockwise". There's no mention of what the "given current" needs to be. I understand all of this is project-dependent, but going back to the more generic and simpler Vgs-ony jig mentioned in my first post, Nelson said "You might consider running lots of tests on these transistors, but only one is essential: measuring gate-source voltage versus current". He used 15V and a 2.2k resistor for matching Vgs, with no real explanation as to how those two values were arrived at. I should be able to take any Mosfet useful as an input device and match it. I just need to know how to arrive at the proper testing values.
general rule is - test devices at currents set in intended circuit
so, if mosfet is going to have Iq of 250mA, that's it
if it's going to have Iq of 1A5, that's it ........... even if those tested at smaller Iq will be close enough for most circuits where Iq is higher
when you know this, all you need to know is some of Ohm's or Kirchoff's Law
........ test, gate circuit - commanding voltage , if you're going that way, varying that - there is no current so that circuit/pot can have highish values; in short - 10K pot will cover most of your needs
so, if mosfet is going to have Iq of 250mA, that's it
if it's going to have Iq of 1A5, that's it ........... even if those tested at smaller Iq will be close enough for most circuits where Iq is higher
when you know this, all you need to know is some of Ohm's or Kirchoff's Law
........ test, gate circuit - commanding voltage , if you're going that way, varying that - there is no current so that circuit/pot can have highish values; in short - 10K pot will cover most of your needs
15V is the power supply he used. You could use a higher voltage if you do not have 15V.
As for the values, that is explained in the article:
The test is simple and requires a power supply, a resistor, and a DC voltmeter. Figure 12 shows the test hookup for N- and P-channel types. The supply source resistance (R1) is nominal, and is found from I = (V - 4)/R1. Consistency is the most important thing here. The given voltage is 15 and, adjusting for about a 4V VGS, we will see about 11V across the resistor.
We are looking for as much matching of the input MOSFETs as possible at a current of 5mA.
The 4V is an approximate value of Vgs of a mosfet. That can be found in mosfet specification sheets. The calculation given provides the value of R1 for a given current and voltage, basically Ohm's Law.
Ben Mah said:
At first I thought the same thing, but his statement was made in the context of using the IRFP110 in the First Watt A70 amplifier. I needed to know if that context controlled the test parameters or not, or if it was even a slight consideration. Specifically, he mentioned "adjusting for about a 4V Vgs" as you mentioned, but that doesn't tell me how to get to I in that formula.
Living that aside for moment.. say I'm testing parts that have a 3.0 or 2.0 Vgs rating, for example, instead of the 4.0 Vgs Nelson used. Is that 4.0Vgs where he got the "4" he used for I=(V-4)/R1? Should I then use I=(V-2)/R1? Or is that 4 a constant?
Assuming its a constant, then using a 15v supply I already know this means that my resistor value is R1=11/I, so how do I figure out the proper current and then decide on a resistor? Is there just a median current I can allow for (mode, really) so I could put pairs "on the shelf" for when I need them, and test more specifically if I have a circuit to use?
you need to learn to read and understand datasheets
easy nowadays, tutorials are everywhere ..... "how mosfet works" etc, dozens on ootoobe
most of them good enough, while shorter than time needed to type one post on forum
https://www.youtube.com/results?search_query=how+mosfet+works+
easy nowadays, tutorials are everywhere ..... "how mosfet works" etc, dozens on ootoobe
most of them good enough, while shorter than time needed to type one post on forum
https://www.youtube.com/results?search_query=how+mosfet+works+
In his article Nelson suggested 5mA for small input mosfets, 20mA for mosfets in TO-220 packages, and 170mA for output mosfets as current to use in the test.
Alternatively, as Zen Mod wrote in post #4, test them at the current that the device will run at in its intended circuit.
Alternatively, as Zen Mod wrote in post #4, test them at the current that the device will run at in its intended circuit.
I'm sure I need to learn data sheets, and I'm glad you brought that up.
Nelson solved for R1 by inserting a current value of 5mA. Where in the datasheet for a given Mosfet would I find the basis for that value? IS that value just for the IRFP110, or is it just for the A70 amplifier? Or both? Or neither, and I can use that value to match any Mosfet pair?
Ben Mah said:
Again, Nelson made that statement in the context of the A70 project, specifically in sourcing and matching Mosfets for that project. My question was does that rule apply across all Mosfets for those three purposes, or is it specific to the A70. And again, is that 4,0Vgs number the source of the 4 in the I=9V-4)/R1 equation, or is it a constant?
appropriate current for size of transistor
meaning voltage/current/dissipation figures, and intended values of same for actual circuit they're tested for
meaning voltage/current/dissipation figures, and intended values of same for actual circuit they're tested for
4V is treshold Vgs for actual mosfet, he was using in A70
put your efforts in article I linked earlier, watch some vids
put your efforts in article I linked earlier, watch some vids
And yes, the 4 in the equation is the 4V.
Again, what Nelson wrote (bolding is mine):
The test is simple and requires a power supply, a resistor, and a DC voltmeter. Figure 12 shows the test hookup for N- and P-channel types. The supply source resistance (R1) is nominal, and is found from I = (V - 4)/R1. Consistency is the most important thing here. The given voltage is 15 and, adjusting for about a 4V VGS, we will see about 11V across the resistor.
Again, what Nelson wrote (bolding is mine):
The test is simple and requires a power supply, a resistor, and a DC voltmeter. Figure 12 shows the test hookup for N- and P-channel types. The supply source resistance (R1) is nominal, and is found from I = (V - 4)/R1. Consistency is the most important thing here. The given voltage is 15 and, adjusting for about a 4V VGS, we will see about 11V across the resistor.
For large mosfets like say a IRFP240 150 etc, you can set an amount of time that the mosfet is under load as the initial cold result will change from the result at say 50 degrees celcius. If you pull your result at 30 seconds (or what ever feels fairly warm to the touch) that should yield more consistant results.
With that being said, I tested some IRFP240's at a very low load (what you would test say a IRF610 with) and repeated the test at what I intended to use them with and the results were pretty similar. Out of 100, a couple may have switched from their original spot in the lineup to maybe one or two over (out of the 100 positions).
The original test was me just pulling results immediately. The 2nd high current test was me pulling results after 45 seconds (going off of memory)
With that being said, I tested some IRFP240's at a very low load (what you would test say a IRF610 with) and repeated the test at what I intended to use them with and the results were pretty similar. Out of 100, a couple may have switched from their original spot in the lineup to maybe one or two over (out of the 100 positions).
The original test was me just pulling results immediately. The 2nd high current test was me pulling results after 45 seconds (going off of memory)
The test is simple and requires a power supply, a resistor, and a DC voltmeter. Figure 12 shows the test hookup for N- and P-channel types. The supply source resistance (R1) is nominal, and is found from I = (V - 4)/R1. Consistency is the most important thing here. The given voltage is 15 and, adjusting for about a 4V VGS, we will see about 11V across the resistor.
My setup uses a 12V regulated supply.
I test at 170mA for 240's
R1 = (V - 4)/I
R1 = (12-4)/.170
R1 = 47R
I test 9610's at 16mA
R1 = (V - 4)/I
R1 = (12-4)/.016
R1 = 500R
I have a switch for the 47R and the 500R on the jig. It also has a timer relay board that fires up a "warming up" LED and a "Done" LED that I use to make consistent measurements.
My setup uses a 12V regulated supply.
I test at 170mA for 240's
R1 = (V - 4)/I
R1 = (12-4)/.170
R1 = 47R
I test 9610's at 16mA
R1 = (V - 4)/I
R1 = (12-4)/.016
R1 = 500R
I have a switch for the 47R and the 500R on the jig. It also has a timer relay board that fires up a "warming up" LED and a "Done" LED that I use to make consistent measurements.
This doesn't have a fancy timer relay etc but I started drawing this up a while ago. It is pretty simple.
-3w resistors with holes on the PCB below to keep them cool. People commonly have 3w resistors floating around You can use a variety of different values to get the current you want.
-On the TO-220 side, I put three resistor spots so you can add and remove resistors to get a specific value if needed.
-Three switches to swites to turn off power to the devices you are not testing so you don't put things in the wrong spot. The same switches that are in the ACA mini. LEDs to show you what is powered
-The DIP 6 pads are there because the spacing is appropriate for to-220 devices. So you can use a DIP 6 socket.
I haven't had an excuse to order PCBs for a while so haven't had them drawn up yet. I have the parts sitting in my drawer. I thought of adding a JFET testing section as well. The DIP 6 pads are there because the spacing is appropriate for to-220 devices.
I was thinking this could be screwed to a piece of ply or something with a small transformer.
-3w resistors with holes on the PCB below to keep them cool. People commonly have 3w resistors floating around You can use a variety of different values to get the current you want.
-On the TO-220 side, I put three resistor spots so you can add and remove resistors to get a specific value if needed.
-Three switches to swites to turn off power to the devices you are not testing so you don't put things in the wrong spot. The same switches that are in the ACA mini. LEDs to show you what is powered
-The DIP 6 pads are there because the spacing is appropriate for to-220 devices. So you can use a DIP 6 socket.
I haven't had an excuse to order PCBs for a while so haven't had them drawn up yet. I have the parts sitting in my drawer. I thought of adding a JFET testing section as well. The DIP 6 pads are there because the spacing is appropriate for to-220 devices.
I was thinking this could be screwed to a piece of ply or something with a small transformer.