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Potential divider leaves me with no current!

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chris414

New Member
Hey there. Okay so im new to electronics and am having a problem... I need to turn on a transistor when a certain variable resistor is at 50k ohms. By my understanding this would require me to make a potential divider with R1 = 300k ohms and R2 = variable resistor (50k) since I am running the circuit from a 5V power supply and my transistor's threshold is at 0.7 ohms. The only reason I can think of for why it's not working is that my 300k resistance means there's not enough current left to turn on the transistor... Is there another way to do this? Or should i just look for a transistor that takes a super low current?
 

dknguyen

Well-Known Member
Most Helpful Member
Is the transistor a BJT or FET? And are you trying to just use it as a switch (full on/off) or are you using it for some analog purpose?

Assuming you are trying to make it full on/off:

For a BJT, this would be your problem (since it is acting like a current amplifier). Yes, that's why low power (ie. high resistance) dividers cannot be used to power things or provide any real current. If you lower the resistance so you can power things you suddenly have a lot of power loss from current flowing through the resistive divider doing nothing. THe way to use a low power resistive divider as a reference to provide current at that voltage, is to buffer it with an op-amp.

However, if this is a FET it will turn on, just very slowly since current is very low (the gate capacitor needs to charges up to voltage). BUT it will not turn on because the "voltage threshold" of a transistor is where it BARELY starts to turn on. You want the transistor to saturate. Which means a higher voltage (10V for most FETs, 3.3V or 5V for more logic-level FETs). THe info should be in the datasheet and is normally the gate voltage they give that has a on-resistance (RdsOn). Anything between this voltage and the maximum gate voltage should work.
 
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dknguyen

Well-Known Member
Most Helpful Member
NPN and PNP are two complimentary types of BJTs. NMOS and PMOS are the two complimentary types of MOSFETs. THat is a BJT.

So, yes, current is your problem. For it to be used as a switch, you must be above the base threshold voltage (0.7V in this case), and be able to supply enough current so that the BJT can do it's job as a switch- amplifying the base current across the collect-emitter until it can do no more, at which point the current levels off at a maximum.

You don't need a resistive divider though. Think of the base-emitter pins as a diode connecting them. What happens when you just connect a voltage straight across a diode? Short-circuit (in reality, the resistance in the divider will limit it, but still not needed). Instead, just take a voltage higher than the BJT's saturation threshold voltage and connect it to the base through a resistor Choose this resistor so that a current large so that

(worst case current gain)*(base current) > (maximum collector-emitter current).

so the BJT can act as much like a switch as possible.

The current across this base resistor can be found with V=IR:
Ibase = (VDC - Vb_sat)/Rbase

This assumes emitter and VDC have the same reference (ground) or else you must take that difference into account as well. In this case, Vb_sat is 1.2V (base emitter voltage required for saturation). The 0.7V is for the collector-emitter voltage when the BJT is fully on (saturated).

Do you see how this works? It's exactly the same as a current limiting resistor used to drive an LED.
 
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chris414

New Member
I'm not sure exactly what you're suggesting? I'm using the potential divider because i need to trigger the transistor when the variable resistor (the x-axis in a joystick) goes above 50k...
 

dknguyen

Well-Known Member
Most Helpful Member
Then you should be using a comparator driving the base through a resistor. And the potentiometer is connected to one input of the comparator and the other comparator is your trigger voltage. The BJT is not going to suddenly turn on- it's going to gradually turn on with what you have in mind.
 
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Hero999

Banned
What load are you driving with the transistor?

If needs to be small >33k.
 
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