A light load LED driver

[Space Varmint]

I thought I would share this. I wanted a signal strength indicator to run from the output of an AGC loop but I did not want to change the biasing level of the AGC. So what I did was make a buffer amp configuration and coupled it through some series resistance. It did not load the circuit enough to change it all, and to my surprise, it worked!

Here it is:

Resistor R75 was carefully worked out to keep the LED barely ON during normal noise (no signal) condition so that it would immediately be noticeable when a station was being received as I tuned across the band.

 

[ericgibbs]

I thought I would share this. I wanted a signal strength indicator to run from the output of an AGC loop but I did not want to change the biasing level of the AGC. So what I did was make a buffer amp configuration and coupled it through some series resistance. It did not load the circuit enough to change it all, and to my surprise, it worked!

Here it is:

Resistor R75 was carefully worked out to keep the LED barely ON during normal noise (no signal) condition so that it would immediately be noticeable when a station was being received as I tuned across the band.

Very Nice. [ Like the pun in your thread title]

What colour is the LED.?
Reason for asking is that the human eye can differentiate more shades of Green than any other colour.

BTW: thats also why night vision goggles are green.

Look at these 'oldies'
Caution: Nigel dont look at the first one.



http://www.decadecounter.com/vta/tubepage.php?item=25&user=0

 

[Space Varmint]

Very Nice. [ Like the pun in your thread title]

What colour is the LED.?
Reason for asking is that the human eye can differentiate more shades of Green than any other colour.

BTW: thats also why night vision goggles are green.

Look at these 'oldies'
Caution: Nigel dont look at the first one.



http://www.decadecounter.com/vta/tubepage.php?item=25&user=0

Hey thanks ericgibbs. It's a water clear type "blue". Maybe I should change it.

 

[Space Varmint]

On the second one, we had an RCA magic brain that used that tube.

I like the Russian bar graph. That's good thinking

 

[audioguru]

The LED in your schematic is backwards. If its polarity is corrected then it would probably smoke and burn because it has no current-limiting.

 

[Space Varmint]

It is backwards...lol Thanks

 

[Space Varmint]

The LED in your schematic is backwards. If its polarity is corrected then it would probably smoke and burn because it has no current-limiting.

OK, fixed. That comes from being tired I think.

But you don't need a current limiting resistor. That's the beauty of it. It is configured as a buffer. A common collector amplifier. Again, it produces very little load on the driving circuit. I did not notice any changes to the AGC voltage swing at all.

 

[Nigel Goodwin]

Very Nice. [ Like the pun in your thread title]

What colour is the LED.?
Reason for asking is that the human eye can differentiate more shades of Green than any other colour.

BTW: thats also why night vision goggles are green.

I thought it was because green is by far the most efficient phosphur colour?, which is why most scopes have green displays, with blue been second most efficient (which is why some scopes are blue).

Look at these 'oldies'
Caution: Nigel dont look at the first one.

Nothing wrong with those, I've repaired many units over the years with all kinds of valve indicator displays.

 

[audioguru]

Your LED was drawn upside down so maybe the transistor has its emitter and collector connected backwards which results in a very low current gain and keeps the LED from blowing up without a current-limiting resistor.

Maybe the supply voltage is low enough (you don't show the supply voltage, the signal voltage and the LED voltage) so that the current in the LED is not too high.

 

[Hero999]

The current through the LED will depend on the current going into the base of the transistor. If the base current is very low, the LED will not be damaged.

Eric is right, the eye can differentiate between more shades of green than any other colour because there are more green cones on the retina than red and blue cones; I would assume that they chose green for night vision goggles because of this.

 

[Space Varmint]

Your LED was drawn upside down so maybe the transistor has its emitter and collector connected backwards which results in a very low current gain and keeps the LED from blowing up without a current-limiting resistor.

Maybe the supply voltage is low enough (you don't show the supply voltage, the signal voltage and the LED voltage) so that the current in the LED is not too high.

Look to the right. The VCC is coming from a switch. So as Hero999 said. IE is determined by Ib. In emitter follower arrangement it will be Ib times beta = Ie.

So the LED drive is more dependent on the power source then the input. I have never seen that configuration used before and I did not want to load the AGC, so I thought I would try running it as a buffer arrangement for maximum isolation. A light load. The load is just over 4K ohms. It is also very linear in it's response. It is not a hard saturation switch in which you need a current limiting resistor.

I was surprised it worked. In fact, it worked so well, that for now on, I'm sold. I'm doing all my LED drivers like that.

 

[ericgibbs]

I thought it was because green is by far the most efficient phosphur colour?, which is why most scopes have green displays, with blue been second most efficient (which is why some scopes are blue).
Nothing wrong with those, I've repaired many units over the years with all kinds of valve indicator displays.

hi,
The information about the shades of green came from a documentary from a night vision manufacturer also a military application.

"Nigel": I've repaired many units over the years with all kinds of valve indicator displays

I just didn't want you to go all nostlagic and start feeling antiquated.

Those 'cats eye' indicators worked very well for tuning indicators.[dip meters]

EDIT; reference link added.
https://www.hownightvisionworks.com/

 

[audioguru]

Is the current gain of the transistor 175 (2N4401)? Then for 40mA emitter current to blow up the LED, the input to the 10k input resistor is only 0.23mA and its voltage is only 2.3V higher than the base voltage of the transistor. If the input voltage is higher then the current in the LED is higher. If the current gain is higher then the LED will burn out sooner.

If the input resistor is 20k or more then the current gain of the transistor would limit the LED current. Then the current in the LED would depend on the wide range of current gain of the transistor.

 

[Nigel Goodwin]

Is the current gain of the transistor 175 (2N4401)? Then for 40mA emitter current to blow up the LED, the input to the 10k input resistor is only 0.23mA and its voltage is only 2.3V higher than the base voltage of the transistor. If the input voltage is higher then the current in the LED is higher. If the current gain is higher then the LED will burn out sooner.

If the input resistor is 20k or more then the current gain of the transistor would limit the LED current. Then the current in the LED would depend on the wide range of current gain of the transistor.

Any way you look at it, leaving out the LED's series resistor is a silly idea, and a stupid design choice.

 

[Space Varmint]

Any way you look at it, leaving out the LED's series resistor is a silly idea, and a stupid design choice.

Why would you say that Nigel? The current is limited by the bias. As long as the bias is not exceded the LED has a long way to go from being blown open. In this particular case the AGC is what sets the bias and the biasing resistors set the limits of the bias. The design is a perfectly safe one with one huge major advantage. It produces and extremely light load on the driving circuit.

As guru was talking about. That's if you drive the transistor into hard saturation as is often done in a common LED driver using a CE amplifier configuration. This is not a CE amp, it is a CC amp. Big difference. No current limiting resistor is needed.

 

[Nigel Goodwin]

Why would you say that Nigel? The current is limited by the bias. As long as the bias is not exceded the LED has a long way to go from being blown open. In this particular case the AGC is what sets the bias and the biasing resistors set the limits of the bias. The design is a perfectly safe one with one huge major advantage. It produces and extremely light load on the driving circuit.

As guru was talking about. That's if you drive the transistor into hard saturation as is often done in a common LED driver using a CE amplifier configuration. This is not a CE amp, it is a CC amp. Big difference. No current limiting resistor is needed.

As AG also said, LED current is completely dependent on the gain of the transistor - which means you will have to individually test and calibrate for every single transistor you might use in the circuit. This is an extremely poor design choice - when a single current limiting resistor, as used by any sensible design, will not only make the LED perfectly safe, no matter what the transistor, but also even out the gain of the transistors used.

It will also give an even lighter load on the AGC line - your 'design' saves ONE RESISTOR, gives no advantages apart from that, but gives many disadvantages and poorer performance.

 

[Hero999]

I agree with Nigle, resistors are cheap and it doesn't have to be a good resistor - cheap and nasty 0.25W 5% carbon film would do.

 

[audioguru]

As guru was talking about. That's if you drive the transistor into hard saturation as is often done in a common LED driver using a CE amplifier configuration. This is not a CE amp, it is a CC amp. Big difference. No current limiting resistor is needed.

You don't know the actual current gain of the transistor. A few have a gain as high as 900.
You don't know the actual forward voltage of the LED. 1.8V? 3.5V?
You don't know the max voltage from the AGC circuit. 5.0V? 9.0V?
Maybe you know what is the supply voltage but it is a secret to us.
When you put them all together then the max current in the LED might be much too high.

Your emitter-follower has a variable voltage output but an unlimited amount of current.
The LED needs a limited current.

 

[Space Varmint]

I agree with Nigle, resistors are cheap and it doesn't have to be a good resistor - cheap and nasty 0.25W 5% carbon film would do.

OK, both you all. All I did was build an emitter follower amplifier and replace RE with an LED. That simple. If I drive the base with the supply, the base bias will do the current limiting.

The purpose of my post was to show that (A) I personally have not seen this done before and so I wanted to demonstrate that the common transistor switch configuration (driving a Xsistor into hard saturation) is not the only method to drive an LED. And (B) it puts a light load on the driving circuits. I have had need of the latter in many situations such as tapping of the AC house current, power line and making a small rectifier circuit (no Xformer) to get enough DC to run some logic and ultimately driving a fairly hefty relay which could shut the AC house current off and on. In the voltage converter section I was unable to use anything but an opto-isolator which doesn't have allot of drive. So it would have been nice had I used a buffer circuit to turn on the relay Pre-driver Xsistor.

 

[Ubergeek63]

put the LED on the high side and a resistor in the emitter to gnd line.

you get a current source dependent on signal level that is independent, of the most part, of transistor beta for any signal that generates more than 0.6V on the signal strength line instead of having to exceed 2V or more and then most likely pop the LED.