# newbie problems with current source

#### cgama

Joined Dec 23, 2011
19
Hi
For my project I have designed this simple current source:

So, I have my clocks going into the OpAmp to drive the LEDs.
Everything is ok, I get a nice clock signal going out of the transistor but when I actually connect the probe with the LED (connecting to VCC), to me (a unexperienced person) it looks like im getting a lot of influence back from the VCC to the transistor collector output.

in the oscilloscope, without the probe connected:

Rich (BB code):
3V     _     _     _     _
0    _/ \___/ \___/ \___/ \  (very rude graphics.....)

with probe:

12V ----------------------
'      '      '             (it gives tiny dashes at the intendend frequency but that's about it)
any suggestions?? thank you very much for your time!!

Last edited by a moderator:

#### cgama

Joined Dec 23, 2011
19
ohh the diagrams didnt come as intended.... sorry

the idea is a clock signal for the first one varying from 0 to about 3V, and, for the second one, a constant signal at 12V with tiny dashes at the clock frequency

thanks!!

Joined Dec 26, 2010
2,148
Your schematic shows two voltage buffers...no current sources that I can see...
The transistors NPN1 and NPN2 could function as current sources if loads (e.g.LEDs) were placed in their collectors.

In this sort of application, it is important to remember that the voltage used to set the emitter voltage will subtract from the voltage span available for the load. Normally the emitter resistor voltage would not be very large, perhaps less than a volt.

#### cgama

Joined Dec 23, 2011
19
ohh I see! so I get VCC minus the very low voltage from emitter, ence my 12V in the collector when I connect the load. right?
This was adapted from another diagram, used for the same purpose, and the other diagram is working fine... (the other had 3 LEDs, so 3 OpAmps, 3 transistors and 3 clocks). Any suggestions or ideas why this isn't going well?
thank you so much for your help

#### cgama

Joined Dec 23, 2011
19
oh and before, i meant "crude" not "rude".... sorry......

Joined Dec 26, 2010
2,148
ohh I see! so I get VCC minus the very low voltage from emitter, ence my 12V in the collector when I connect the load. right?
This was adapted from another diagram, used for the same purpose, and the other diagram is working fine... (the other had 3 LEDs, so 3 OpAmps, 3 transistors and 3 clocks). Any suggestions or ideas why this isn't going well?
thank you so much for your help
For current sources of this kind to be able to give a reasonably large load voltage, the voltage on the emitter resistor should not be too big. Typically, the controlling voltage fed to the amplifier would be attenuated to about half a volt by using a pair of resistors (a potential divider).

The emitter resistor is then chosen to give the desired current at this voltage. Note that the amplifier used must be of a type able to work with its inputs and output in this low range of voltage: this may be an issue if only a positive supply rail is used.

#### cgama

Joined Dec 23, 2011
19
thank you so much for all your help. I will try your suggestion

#### ErnieM

Joined Apr 24, 2011
8,006
Hey cgama.

2.7V / 562 ohms = .005mA
to
2.7V / 62 ohms = .045mA

It depends on what LED you are using if that is a good range.

Your text-oscilloscope is cool, but it would be more useful if it was more precise; as drawn the two frequencies don't match (at least in the courier new font) so I can't tell where the blip in the collector occurs. However, I would guess it blips are where the 3V rises, that may indicate the LED isn't connected correctly.

Depending on what is making these 3V pulses you may not need the op amps.

Joined Dec 26, 2010
2,148
It will be very much easier to judge what is going on if you could post a more complete diagram, showing where the "clock" signals come from, and exactly how the LEDs are wired.

Depending on how much voltage is dropped in the LEDs, the 2.7V or so at the emitters may or may not be excessive.

#### cgama

Joined Dec 23, 2011
19
thank you for all your help!!
after much hair pulling i found out the transistor was the other way around.... everything is fine now.
Once again, thank you very much for your help and time
tc