# Constant current LED drivers

Discussion in 'General Electronics Chat' started by Macnerd, Apr 27, 2015.

1. ### Macnerd Thread Starter Member

May 22, 2014
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This is a new post that I copied from another post of mine. The moderator suggested that I start a new thread. My comments will be in bold & DL324’s comments will be in italics. I apologize.

Macnerd said:

OK, I wish to make a slight detour & ask questions on a different but related subject.

You should probably start a new thread for your detour. That way others will know if this thread interests them.

I've googled constant current LED drivers & I've seen 2 different kinds - single transistor & double transistor.

There are many types of constant current drivers. Post a schematic of any you have questions about.

I've read on the 'net that transistors are temperature sensitive so that a single transistor would not make very good constant current drivers.

I can't think of any electronic devices that aren't temperature sensitive. If you really need a circuit that is less sensitive to temperature, there are ways to minimize sensitivity.

Based on the description of how the circuit works, the driver is basically a feedback loop.

Some have no feedback.

So, the driver isn't literally a "constant" current driver. The current goes up & down based on the voltage.

If there's enough voltage, the current source/sink will provide the designed current; assuming there's a path for it to flow. In a current source, current is constant but voltage may vary.

I also read that there's a lot of voltage lost on the FET whatever that means.

That will be some power dissipation in the FET, but it doesn't have to be a lot. The FET has some "ON" resistance whenever it's operating.

Based on my very limited knowledge of how a FET works, it gradually turns on & gradually turns off.

FETs can also be used as switches.

A BJT, on the other hand, is either on or off.

BJTs have other modes of operation; they were invented as amplifiers to replace vacuum tubes...

So, of the two 2-transistor drivers is one better than the other & why?

All circuits have compromises. Post schematics.

2. ### crutschow Expert

Mar 14, 2008
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So do you have any further questions?

3. ### AnalogKid Distinguished Member

Aug 1, 2013
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Actually, BJT's were invented as switches to replace relays. The team had been working on developing a FET, but couldn't get it to work and wanted to get something done before the Christmas holidays.

Many of the bolded statements above are incorrect. No matter what transistor type is used, a well-designed constant-current circuit provides a *very* constant current. Holding a set output value to within 1% over a 10,000:1 change in load impedance is only medium-hard. If you have specific questions about a circuit, post a schematic.

ak

4. ### dl324 Distinguished Member

Mar 30, 2015
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I thought they were invented as a triode replacement.

5. ### AnalogKid Distinguished Member

Aug 1, 2013
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The team was supposed to be working on Shockley's pet project, a FET to replace a triode. Couldn't get it working, blew him off, went after something that could pass DC with a little audio riding on it. Shockley was *not* happy, forced his name onto the design, the patent, and the Nobel Prize. Later, he did get the FET working, only to have the patent application denied because it already was under patent - from 1929 (!!!).

ak

6. ### Macnerd Thread Starter Member

May 22, 2014
37
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What about using digital buffers instead of transistors to drive LEDs?
The following is a quote from Wikipedia: A digital buffer is a special electronic circuit that is used to isolate the input from the output, preventing the impedance of one circuit to alter the impedance of another. The output drive capability is generally much higher than their input signal requirements and is used to drive high-current loads. So, buffers provide power amplification to the digital signal, as they have a high fan-out capability.
It would be a whole lot simpler than using transistors & resistors & such.

7. ### dl324 Distinguished Member

Mar 30, 2015
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I selected my first college based on the fact that Walter Brattain was a member of the faculty. I never saw him once...

I had the pleasure of meeting and working with a couple of Shockley's "traitors".

Oct 27, 2012
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9. ### dl324 Distinguished Member

Mar 30, 2015
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Depends on what you want. Buffers aren't current sources. They also sell quad/hex LED drivers that will sink/source 200mA which should be sufficient for most applications. The only rub is that when you're running multiple LEDs in series and want to match brightness with a single LED, you need to use a current source; in addition to "matched" LEDs if you want to be a purist...

10. ### ian field Distinguished Member

Oct 27, 2012
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CMOS buffers are almost current limiting - sometimes the sink current just happens to be convenient for LEDs you want to drive.