Electronics > Beginners
24VAC to DC
T3sl4co1l:
LED in series -- not recommended, even a red LED is 1.5-2V drop. Not much left for the 0.7V Vbe out of a 3.3V supply.
China can't really screw up the voltage, but they might deliver parts that are fully shorted or open (a dark emitting diode).
hFE(sat) is typically chosen as 20, where'd 3 come from?
Or if you mean minimum required is 3 (but, based on what, I don't know?), then yes, and you can expect to find much better in real parts out there. :P So much better in fact that you can use a higher value for the resistor calculation.
--- Quote from: Zero999 on January 26, 2020, 08:15:30 pm ---there is no need for pull-down resistors, because the BJT will turn off, if there's no base current, so if the MCU's I/O ports are set to inputs, the BJTs will be off.
--- End quote ---
FYI, better to have them: faster turn-off, lower leakage, more noise immunity.
BJTs aren't controlled by current so much as voltage, or charge; it just so happens the base is...rather leaky, in a rather predictable way in fact, so we can drive it as a current amplifier in certain situations. This does happen to be one of those situations, but that's no excuse to fully ignore the transistor's nature. :)
Tim
joeyjoejoe:
MUN5230DW1T1G says the hFE min is 3.
https://www.onsemi.com/pub/Collateral/DTC113ED-D.PDF
T3sl4co1l:
Yes, measured hFE will be very low at low currents and linear operation. Note Vce = 10V, so Vin has been controlled (to about 1.5V as it happens) to set Ic as specified. This doesn't affect the hFE of the transistor itself.
Pretty sure I've seen prebiased transistors before, that are rated as the bare transistor (so, hFE ~ 200 at whatever currents). Which is actually more strange than this, as the resistors are monolithic (right on the transistor die itself), there's not really any way to measure that parameter apart from the resistors.
Tim
Zero999:
--- Quote from: T3sl4co1l on January 28, 2020, 12:50:42 am ---
--- Quote from: Zero999 on January 26, 2020, 08:15:30 pm ---there is no need for pull-down resistors, because the BJT will turn off, if there's no base current, so if the MCU's I/O ports are set to inputs, the BJTs will be off.
--- End quote ---
FYI, better to have them: faster turn-off, lower leakage, more noise immunity.
BJTs aren't controlled by current so much as voltage, or charge; it just so happens the base is...rather leaky, in a rather predictable way in fact, so we can drive it as a current amplifier in certain situations. This does happen to be one of those situations, but that's no excuse to fully ignore the transistor's nature. :)
--- End quote ---
Yes, base-emitter resistors are often a good idea, but I think they're superfluous in this application. The BJT is switches a relay and its base is driven from a CMOS output which is push-pull, so when it's low, the base will be pulled to the emitter anyway. The only time it could help is if the output goes into a high impedance state, but that's something outside of normal operation, it shouldn't last long and the leakage current will be tiny.
--- Quote from: joeyjoejoe on January 28, 2020, 12:55:18 am ---MUN5230DW1T1G says the hFE min is 3.
https://www.onsemi.com/pub/Collateral/DTC113ED-D.PDF
--- End quote ---
Yes, that does look low, but I think it will be fine at 50mA. Figure 6 shows IC = 50mA, when the input voltage is 3V and output voltage 0.2V.
You might be better with a 10k base-emitter resistor, which will rob less base current from the BJT.
https://assets.nexperia.com/documents/data-sheet/PIMC31.pdf
https://au.mouser.com/datasheet/2/348/dtc113zebtl-e-1664170.pdf
Navigation
[0] Message Index
[*] Previous page
Go to full version