Which transistor to use?

[Nightstaber]

I am looking for a Darlington transistor with a hFe at around 10,000 which can deliver around 6-7 amp 12 v.

Do you have any ideas of which to use?

Would be lovely if it aint a very expensive one.

 

[Hero999]

What about using a power MOSFET like the IRL540?

 

[Roff]

I am looking for a Darlington transistor with a hFe at around 10,000 which can deliver around 6-7 amp 12 v.

Do you have any ideas of which to use?

Would be lovely if it aint a very expensive one.

As Hero suggested, there may be better or more available alternatives. Tell us what your application is. What is your load, and your supply voltage? Where/what does your drive signal come from? Is it an analog or switching application? If switching, what is the frequency and duty cycle?

 

[Space Varmint]

You could use a couple of 2n3055s or something rated about the same and make you own Darlington Amplifier like this....

Pay no attention to the transistor types in Diagram.

 

[Roff]

A beta of 10,000 at 6 amps is gonna require more than two transistors.

 

[Hero999]

Except you wouldn't normally draw it like that and you'd use one low power transistor like the BC548 and a high power transistor like the 2N3055 rather than two 2N3055s.

 

[Space Varmint]

A beta of 10,000 at 6 amps is gonna require more than two transistors.

It depends on the beta of the individual transistor. A '3904 which of course is a small signal device has a beta of about 300. So 300 X 300 is 90,000. I would say about anything you use would work as long as it's rated high enough in amps.

And to the other guy, that's a Darlington. You don't have to like it.

 

[Roff]

It depends on the beta of the individual transistor. A '3904 which of course is a small signal device has a beta of about 300. So 300 X 300 is 90,000. I would say about anything you use would work as long as it's rated high enough in amps.

And to the other guy, that's a Darlington. You don't have to like it.

But the problem is that the high-current (6 amps) BJTs have betas of around 10 - 20 at that current level. This leaves a base current of at least 300mA, and the other transistor will needs a beta of 500 at that current level, which you're probably not going to find.

 

[Nigel Goodwin]

And to the other guy, that's a Darlington. You don't have to like it.

True it is, but you wouldn't connect two transistors like that for this use, it's a VERY poor way of doing it - you would feed the first collector from a much higher voltage, with a current limiting resistor. The conventional darlington (as you drew) drops far too much voltage across it because the two collectors are joined together, it also seriously reduces the gain if you use it as a switch.

For this requirement (very high gain) I would probably use three transistors, with the middle one a PNP.

 

[Space Varmint]

True it is, but you wouldn't connect two transistors like that for this use, it's a VERY poor way of doing it - you would feed the first collector from a much higher voltage, with a current limiting resistor. The conventional darlington (as you drew) drops far too much voltage across it because the two collectors are joined together, it also seriously reduces the gain if you use it as a switch.

For this requirement (very high gain) I would probably use three transistors, with the middle one a PNP.

Rarely ever do you take a theoretical circuit and build it for your particular application and it works beautifully. I always find I have to do a little tweaking here and there using my volt meter & scope. Notice I didn't even put any values to the resistors? Imagine if I did. You guys would use me for target practice.

Basically what I was telling the guy is you don't have to buy a Darlington package like a TIP120. If he needs more beta he can use another emitter follower circuit. There are only too many different solutions. I was just getting him to think outside the box a little.

 

[Willbe]

I am looking for a Darlington transistor with a hFe at around 10,000 which can deliver around 6-7 amp 12 v.

Depending on your freq. response, could you use a three terminal voltage regulator with an extra pass transistor to get up to 7 amps?