Electronics > Beginners
Drive LED from mcp23008. BJT vs Mosfet
aheid:
--- Quote from: David Hess on March 28, 2019, 06:57:55 pm ---Bipolar transistor collector-emitter saturation voltage is much lower than 0.6 volts.
--- End quote ---
I should have checked more thoroughly, for some reason the 0.6V figure had gotten stuck and the first datasheet I pulled up (BC337) had 0.6V as well. My bad |O
But AFAIK higher current capability is associated with higher saturation voltage, and given the LED was 12V I assumed it was a fairly high current thing.
Audioguru:
Yes, the collector-emitter saturation voltage of a bC337 transistor is 0.7V max, but only if it is almost overloaded with a collector current of 500mA.
David Hess:
The BC337 is designed specifically for audio and not saturated switching unlike the similar 2N4401 which uses a different process. On Semiconductor shows a 50 millivolt Vce(sat) at a forced beta of 10 up to 100 milliamps. Even at a forced beta of 100, it is still below 200 millivolts.
A modern saturated switching transistor under the same conditions would be more like 15 millivolts maximum or 0.15 ohms. The best similar MOSFET I could find is 0.3 ohms and costs more than twice as much.
aheid:
--- Quote from: David Hess on March 30, 2019, 01:13:19 am ---The BC337 is designed specifically for audio and not saturated switching unlike the similar 2N4401 which uses a different process.
--- End quote ---
On Semi says linear and switching for the BC337, copy/pasta error? https://www.onsemi.com/PowerSolutions/product.do?id=BC337
Checking further I see other manufacturers specifies it as an audio output driver.
--- Quote from: David Hess on March 30, 2019, 01:13:19 am ---On Semiconductor shows a 50 millivolt Vce(sat) at a forced beta of 10 up to 100 milliamps. Even at a forced beta of 100, it is still below 200 millivolts.
A modern saturated switching transistor under the same conditions would be more like 15 millivolts maximum or 0.15 ohms. The best similar MOSFET I could find is 0.3 ohms and costs more than twice as much.
--- End quote ---
I stand corrected. I looked at the max Vce(sat), I see the curves show much lower typical values.
Though, as I said, given that the LED was 12V I assumed much more current than 100mA (more like a 10W LED).
David Hess:
The comparison is difficult to make outside of specific applications. I used TO-92 packages but checking SOT-23 devices now, MOSFETs have a general advantage in saturation voltage and price which I suspect has more to do with the economics of production than technology. At 10 watts and 12 volts, the MOSFET has a clear advantage though because the base current of a bipolar transistor becomes excessive for a logic output.
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