why these resistor have been using????

[olemp]

hfe uln2803

hi! in this picture circuits work also without RX1, RX2 and RX3...
but they have been using.....what is the reason???

 

[papyaki]

Hi,

These resistors add a threshold before transistor conduction.

In the case of RX1 ans RX2, the needed current to saturate the transistor is greater than the one needed without RX1 or RX2.

In the case of RX3, led ilumination must be greater than without RX3.

By using those resitors, you can avoid spurious activations of the transistor by increasing the needed current to saturate them.

 

[olemp]

thank you papyaki....
than if we use a transistor which has lower hfe we don't need to use these resistor...

for example:
bc337(45V 500ma)
hfe min 100
hfe max 600

and for mpsa42(300V 600ma) these values
25 and 40 respectively.........

so if i use mpsa42 i don't need theese resistor. because hfe very small.... am i right????

 

[papyaki]

Hi,

Those resistors are not required. They are mainly used as security to avoid spurious activation of the transistor.

Another point I have not detailed is the the difference between Hfe min and Hfe max of the transistors.

Whithout RX, the transitor is current activated (you inject ib). Whith Hfe min/Hfe max = 6 like with BC337, the voltage difference into R could vary from 1 to 6.

Whith the resistance RX, the transistor is voltage controlled. The current flowing trough R is, let say 10 to 20 times the ib current, so the voltage trough R is less ib dependant. You need only to have 0.7V between RX ends to activate the transistor.

In your case, you can try without RX as the delta between Hfe min and Hfe max is lower than for BC 337.

But if I would have to build the schematic, I would use RX and maybe use a darlington transistor to reduce ib and with this maner reduce the current flowing through R and RX (10 to 20 times ib).

In this case why not use an ULN2003 or ULN2803 which include several darlingtons with all the needed resistors.

 

[STOYANOV]

Hi,

In the first case , using a resistor between the base and ground is nessesary
if:
- the control signal does not go to ground ;
- if the signal circuit can go in hi-impedance state,
for example, if it is delivered from a uP during reset.
The second case is different, RX resistor may help if control circuit can not give voltage, high enough to keep the pnp transistor off (driving the circuit from a ttl for example and 5V supply, for example)
In the third case RX is a current limiting resistor.

BR,
Stoyanov

 

[FANT]

The purpose is to kill the ICB0 of the transistor; if the base is left unconnected, the ICB0 will be amplified by the HFe of the transistor and the collector will not be open but will pass a small amount of current.
Depending on the application that "small" amount of curren can be enough to switch from 1 to 0 or so.
Also a second purpose of the base resistor is not to leave the base terminal floating and so sensible to static disharges, parasitic currents, electric fields, RF fields and so on.
Also often is added a capacitor ( some hundred of pF ) form base to emitter to short the RF field ( of course depends on applications ).

Mandi

 

[metal]

Hello

In the first case, the two resistors form a voltage divider, by which the voltage at which the transistor turns on is controlled more specifically, and accurately.

The same thing applies for the second case.

Regards

 

[SolarTorch]

RX1 and RX2 is required if the control signal has Z or undefine state. They make the transistor completely turn off. Also they make the off time faster. For RX3, since the LED is a current device, if no current basically no light to trigger the transitor. RX3 can be eliminated. For noisy envoriment, add RX3 should be better.

 

[nand_gates]

So many answers posted here!!! Which one is correct???
I think FANT's explanation is the correct one!

 

[arcaphill]

For current limitation