# Has anyone used a capacitor to provide the Vbe for an NPN BJT.

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
Hi

Has anyone used the capacitor n an RC circuit to provide the source for the Vbe of an NPN BJT?

Any problems or quirks? Anything I need to know about?

I'm having a few.

The cap voltage only gets to 640 mV which is just short of the 700 mV that is required to close the C to E connection .

Just to be clear, I do understand why the cap voltage is limited to the voltage of whatever it is that is tapping it. An LED for example.

That's not the question. My question is why does the cap not reach the 700 mV rather than the 640 mV that I am observing.

I have used 6 new 2N2222 NPN transistors with a variety of RC combos and they all produce the same result.

Can anyone shed some light on this situation?

Thanks

#### dl324

Joined Mar 30, 2015
11,904
My question is why does the cap not reach the 700 mV rather than the 640 mV that I am observing.
Without seeing a schematic, we can only guess. A transistor doesn't have an abrupt turn on voltage. The junction behaves like a diode and will start conducting before the junction voltage gets to 0.7V; which is what I was taught to approximate the "fully on" junction voltage.
1. What is the capacitor value?
2. What voltage was it charged to?
3. Do you have a resistor to limit current from the capacitor?

#### Alec_t

Joined Sep 17, 2013
11,749
In your circuit (so far as I understand it) the capacitor charge current via the resistor matches its discharge current via the base junction, once the base votage reaches 640mV.

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
Without seeing a schematic, we can only guess. A transistor doesn't have an abrupt turn on voltage. The junction behaves like a diode and will start conducting before the junction voltage gets to 0.7V; which is what I was taught to approximate the "fully on" junction voltage.
1. What is the capacitor value?
2. What voltage was it charged to?
3. Do you have a resistor to limit current from the capacitor?
Quick sketch attached. Three components. It's an RC circuit. An electrolytic capacitor so it's polarised. With a transistor attached to the +ve pin of the capacitor via the base and the emitter to negative of the power supply. You wil notice that the collector is also attached to the +ve pin of the capacitor. The cap and resistor sizes are given but they don't matter. I have tried various combos on a just in case basis. But I wouldn't expect it too. All that affects is the time constant of the RC circuit ie: the length of time it takes to get to 640 mV.

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Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
In your circuit (so far as I understand it) the capacitor charge current via the resistor matches its discharge current via the base junction, once the base votage reaches 640mV.
That's my understanding too. But that's not my issue. I'll give you a different example. If, instead of the transistor, I put an LED in the circuit that starts to emit light at, say, 2.8V to 3.8V the light will come on. When I measure the voltage across the capacitor it never exceeds the minimum 2.8V requirements of the LED for current to flow no matter what the power supply t the RC circuit is. That makes sense. It's the same if I put the coil of a relay in the circuit. If it takes 4.5V to energize the coil that's the maximum voltage that the capacitor stores. But when I put this transistor in the same circuit I get the 640 mV accross the capacitor even though, when I test this transistor on its own in 10 mV increments it takes 700 mV before a current will pass from C to E.

#### dl324

Joined Mar 30, 2015
11,904
Quick sketch attached.
You're operating the transistor as a diode. The forward voltage will depend on the current through the resistor.

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
By the way. The reason for the RC in this circuit is to enable me to configure the cap/resistor combo to give me a particular time constant to delay switching on the CE. In case anyone was wondering.

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
You're operating the transistor as a diode. The forward voltage will depend on the current through the resistor.
Yes. See my previous post.

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
You're operating the transistor as a diode. The forward voltage will depend on the current through the resistor.
Actually, now that I think about it, why not just use a diode? That just came to me while I was making a brew. Thanks for that. I'll give it a go tomorrow.

#### crutschow

Joined Mar 14, 2008
25,675
Below is the LTspice simulation of your circuit (without the capacitor since it has no effect on the DC voltages).
The transistor voltage, V(out), is 0.65V which is close (within about 11mV) to what you observed.

Last edited:

#### BobTPH

Joined Jun 5, 2013
2,600
Change the resistor to 1K and see what voltage you get.

Bob

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
Change the resistor to 1K and see what voltage you get.

Bob
Tried it. All that does is alter the time constant of the RC combo. Vbe remains at 640 mV

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
Below is the LTspice simulation of your circuit (without the capacitor since it has no effect on the DC voltages).
The transistor voltage, V(out), is 0.65V which is close (within about 11mV) to what you observed.

View attachment 198626
Now that's interesting. I thought I had invented it. LOL!! It's actually 642 - 644 mV that I observe. The problem is it's not enabling CE which is inconsistent with that chart which shows Ic > 0. Not by much, mind you, but it's there. Having said that none of my equipment can measure an Ic < 1 mV so maybe it is being enabled but I just can't see it. Thanks for that diagram. It gives me something else to think about.

#### djsfantasi

Joined Apr 11, 2010
6,966
I know I’m missing something. What’s Vbe? Isn’t that the voltage across the Cap? It’s roughly 0.7V, very close to 640mV.

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
This is what I am working on. I have brought it this far using the same circuit but using a relay to do the switching. I want to replace the relay with a couple of transistors to make it silent. I'll then replace the Rs in the RCs with pots so I can adjust the output voltage and frequency. Getting there. Just need to figure this out.

http://www.smegateway.co.uk/SMEgateway/videos/flashingled.mp4

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Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
I know I’m missing something. What’s Vbe? Isn’t that the voltage across the Cap? It’s roughly 0.7V, very close to 640mV.
Its the voltage across the transistor base/emitter pins that enables current to pass between the collector pin and the emitter. Vbe usually needs to be a minimum of 0.7V. Same as some small diodes.

#### BobTPH

Joined Jun 5, 2013
2,600
Tried it. All that does is alter the time constant of the RC combo. Vbe remains at 640 mV
That's funny, in simulation, it goes from 650mV to 712mV when I change from 10K to 1K, which is what I would expect.

Bob

Thread Starter

#### RAMBO999

Joined Feb 26, 2018
247
That's funny, in simulation, it goes from 650mV to 712mV when I change from 10K to 1K, which is what I would expect.

Bob
Are you using a capacitor to supply the Vbe in your simulation?

#### ci139

Joined Jul 11, 2016
1,696
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it (the Vc) goes from 380mV up to near 1V . . . the apx. for Vb being 700mV has it's roots far in the history (when the computers looked like that)
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#### BobTPH

Joined Jun 5, 2013
2,600
Yes, I am giving you the steady state Vbe. The time it takes to get there varies from 0.28 s for 1K and 2.6 at 10K, which is again as I expect.

Interestingly the voltage rise is pretty close to linear, which makes sense because the exponential behavior of the diode and the logarithmic behavior of and RC counteract each other.

Bob

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