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Old 2nd October 2011, 10:44 AM   #1021
AndrewT is offline AndrewT  Scotland
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Lazy & Fred.
The DUT is passing 100mA and has 4.9Vce when hFE=100.
This is a dissipation of 490mW. The junction will be changing temperature rather quickly.

Need to refine your "method" to help reduce the effect of different Tj on your results.
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Old 2nd October 2011, 10:47 AM   #1022
AndrewT is offline AndrewT  Scotland
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Esp,
I like the look and simplicity of your fault detection circuit.
Can you go back to that thread and tell us more on how well it copes with different types of faults, different lengths of transients and at different frequencies?
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Old 2nd October 2011, 12:17 PM   #1023
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Quote:
Originally Posted by AndrewT View Post
Lazy & Fred.
The DUT is passing 100mA and has 4.9Vce when hFE=100.
This is a dissipation of 490mW. The junction will be changing temperature rather quickly.

Need to refine your "method" to help reduce the effect of different Tj on your results.
Hi Andrew ... a lot of Andrews are lurking in this thread

Look I know about the power dissipation in this case but (there's always some but) for a real comparison of power transistors you need to put them near bias collector current (100mA) conditions and compare them in that circumstances. The measurement has to be short one, 2 seconds max, just to read out the voltage value, afterwards voltage on 1 ohm resistor will of course rise up. But the first two seconds are relevant to get realistic comparison. One could even change Vce voltage from 4,9V to lower level with use of a zener in series with 1 ohm resistor but than BJT's Vce/Ic parameters wouldn't be in linear area of the graph any more - out of working point. So proposed measurement is OK, only it has to be performed for a short time.

P.S. This measurement is not about absolute mA (hFE) values but relative ones between output power transistors to find as close matches as possible.

Last edited by Lazy Cat; 2nd October 2011 at 12:26 PM.
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Old 2nd October 2011, 02:10 PM   #1024
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Esp,
I like the look and simplicity of your fault detection circuit.
Can you go back to that thread and tell us more on how well it copes with different types of faults, different lengths of transients and at different frequencies?
Thank-you. I will try to answer your question on my thread, it looks like this does not interest L.C. at all.
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Old 2nd October 2011, 02:28 PM   #1025
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Hi fredlock

You can use multimeter with hFE function which calculates hFE=Ic/Ib at Ic=1mA.

If you don't have such meter than you can use attached schematic.

Thanks Lazy Cat. I've seen some hFE matching circuit in the internet and also in the forum but just curious what you were using. I was thinking of building this project but I think your simple schematic would be good enough.
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Old 2nd October 2011, 02:38 PM   #1026
AndrewT is offline AndrewT  Scotland
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Testing hFE at constant Ic is a little more consistent than using constant Ib.

I always use constant Ic now, for hFE measurement even though I don't know Tj.

Multimeters, with a give away for free hFE tester built in, generally use constant Ib.
They are no better than a batching tool. The problem is that one does not know what Ic is.
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Old 2nd October 2011, 02:55 PM   #1027
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Testing hFE at constant Ic is a little more consistent than using constant Ib.

I always use constant Ic now, for hFE measurement even though I don't know Tj.

Multimeters, with a give away for free hFE tester built in, generally use constant Ib.
They are no better than a batching tool. The problem is that one does not know what Ic is.
Could you share us you hFE matching circuit then? Which might benefit me and the others to match it perfectly.
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Old 2nd October 2011, 05:45 PM   #1028
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by Lazy Cat View Post
Hi Andrew ... a lot of Andrews are lurking in this thread

Look I know about the power dissipation in this case but (there's always some but) for a real comparison of power transistors you need to put them near bias collector current (100mA) conditions and compare them in that circumstances. The measurement has to be short one, 2 seconds max, just to read out the voltage value, afterwards voltage on 1 ohm resistor will of course rise up. But the first two seconds are relevant to get realistic comparison. One could even change Vce voltage from 4,9V to lower level with use of a zener in series with 1 ohm resistor but than BJT's Vce/Ic parameters wouldn't be in linear area of the graph any more - out of working point. So proposed measurement is OK, only it has to be performed for a short time.

P.S. This measurement is not about absolute mA (hFE) values but relative ones between output power transistors to find as close matches as possible.

Thanks for the explanation LC............




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Could you share us you hFE matching circuit then? Which might benefit me and the others to match it perfectly.
+10
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Old 2nd October 2011, 06:18 PM   #1029
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Thank-you. I will try to answer your question on my thread, it looks like this does not interest L.C. at all.
Hehe don't worry I looked at yours Crescendo modification to CFB and I must say I'm impressed. I saw you have very small Ic in input pair, is that OK, cause usually input transistors should work at 1-5mA collector current. That also means smaller influence of parasitic capacitances if the collector current is larger, which finally brings you wider bandwidth and higher slew-rate, all though I saw yours sim already shows 380V/us.

Please continue thread in solid state part of the forum I think it will bring more interest from enthusiastic amp builders.
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Old 2nd October 2011, 06:32 PM   #1030
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Thanks Lazy Cat. I've seen some hFE matching circuit in the internet and also in the forum but just curious what you were using. I was thinking of building this project but I think your simple schematic would be good enough.
Yes, this circuit test a transistor at constant collector current level you choose with SW combinations, but the same temperature dependency appears here, base current will go down with temperature rising, so matching is again time dependent. For simplicity and relative only comparisons sake I suggest you to built simpler one.

Last edited by Lazy Cat; 2nd October 2011 at 06:36 PM.
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