But how it connected to switching? in h-bridge i don't need the advantage of large amplify.Hi,
darlingtons have high current gain (emitter / base).
A darlingtom may have a gain of 1000, a usual bjt maybe has 25.
If you want to drive an 1A load, then:
--> with the darlington you need 1mA of base current
--> with the bjt you need 40mA of base current (wich the 754410 might no be able to drive)
(And for switching applications you have to "overdive" the base. Drive it with 3..10 x the calculated current)
Klaus
To reduce input current.My question is why there is a need in darlington transistors to create these h-bridges?
.....................................
if yes why darlington allow to switch high currents?
It is odd.A darlingtom may have a gain of 1000, a usual bjt maybe has 25.
It is odd.
If beta of single transistor is 25, beta of darlington connection wil be less than 25**2+2*25=675 at most.
https://en.wikipedia.org/wiki/Darlington_transistor
Beta of conventional single transistor is 30~100 in IC.
So beta of darlington connection wil be less than 960~10200.
Again obviously odd.What is odd?
It is no more than a forced interpretation.There is a wide range.
You say 960..10200. so 1000 is within your given range.
Hi,
What is odd?
* the 25? I gave just an example. There is a wide range. The worst i have on stock is BUL1102. datasheets says 12..20 @ 25°C, 2A, 5V
* the 1000? I gave just an example. There is a wide range. You say 960..10200. so 1000 is within your given range.
And it depends a lot on V_CE. With switching applications you have low V_CE. Resulting in very low gain.
Klaus
In conclusion, darlington is better than regular transistor in switching high-current because of the small base current that needed to saturate the transistor?Hi,
ahh,
you are complaining that the gain of 25 does not meet the gain of 1000.
So mathematically you are correct with the 675.
But in reality they won´t connect two bjts with exactely the same gain of 25.
Usually the power bjt (with external emitter connection) is much bigger than the other bjt, because it has to drive a lot more current.
and even if they are fo same size they both have totally different operating points, because the inner bjt (with external base connection) drives less current.
Therefore i expext the power bjt maybe has a gain of 30, while the other has a gain of 100, giving a gain of about 3000. (just raw estimations!)
Audioguru is correct: To compare a simple bjt with a darlington we need to know the exact types and compare their datasheets.
Klaus
It's better, if:In conclusion, darlington is better than regular transistor in switching high-current because of the small base current that needed to saturate the transistor?
Darlington needs small base current because of the large beta of it?It's better, if:
- the alternative is only between regular BJT and darlington (true 30 years ago when sn754410 was designed, but not these days)
- you don't have a means to provide a high base current
- switching frequency is low
Right.Darlington needs small base current because of the large beta of it?
if i want the transistor to be in saturate i need:
beta*Ib>Ic
so if beta of the darlington is higher Ib is smaller, Am I right?
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