Anyone feel like explaining how a 7135 chip works?

[psychbeat]

Im starting to kind of get how a linear regulator works (sounds pretty inefficient conceptually to me)

Im mainly wondering whats inside a 7135 and does it use a negative feedback loop or something or just
have a fixed output voltage? Im assuming this is where emitter VF comes into play?
I know the output current is fixed.

Most of my lights use these chips so I figure I should know whats going on in there

Also- does anyone use switching regulators for single cell lights or modules?

I think Im on the right track with understanding this stuff..

thanks in advance for any helpful insights!

 

[moderator007]

Have you looked at the datasheet. They act like a variable resistor trying to keep the current at 350ma constant. As the voltage gets above the led vf any of the extra voltage gets used up as heat in the 7135. So the more voltage above the load voltage will give you more heat and less efficiency. The closer you keep the supply voltage to the load voltage the more efficient.

 

[tylernt]

Im mainly wondering whats inside a 7135 and does it use a negative feedback loop

Most linear regulators use a current sensing resistor (may be internal to the chip, or a separate component). They don't sense Vf directly but they sense the effects of a changing Vf (Vf changes as the LED heats up).

Also- does anyone use switching regulators for single cell lights or modules?

For single-cell NiMH/alkaline you pretty much have too, to boost the low ~1.5V to the ~3.6V that the LED requires. Incidentally, you can also use a switching regulator instead of a linear regulator to "buck" a higher voltage down to a lower voltage. Bucking with a switchmode is generally more efficient than a linear regulator, which is why linear regulators aren't often used in battery-powered lights except at low current levels and when the input voltage is very close to the output voltage.

 

[psychbeat]

Oops yeah I meant single lion

I have a friend who works at SLAC and has a gift for anecdotal explanation of complex physics stuff.
Alas he's not very knowledgable about electronics or into lights

I looked at the data sheet but it's pretty dry.

All of my main lights use 7135 based drivers so I thought I should understand fundamentally how one works and why they're better than a switching type - but the single lion is so close to the emitter VF it makes sense I guess.
Some of these drivers are pretty high amperage-wise tho!

Thanks for the help guys I think I understand more clearly what's going on in there and will keep studying.

 

[BoarHunter]

Oops yeah I meant single lion

I have a friend who works at SLAC and has a gift for anecdotal explanation of complex physics stuff.
Alas he's not very knowledgable about electronics or into lights

I looked at the data sheet but it's pretty dry.

All of my main lights use 7135 based drivers so I thought I should understand fundamentally how one works and why they're better than a switching type - but the single lion is so close to the emitter VF it makes sense I guess.
Some of these drivers are pretty high amperage-wise tho!

Thanks for the help guys I think I understand more clearly what's going on in there and will keep studying.

The sheet is fine. What you need to learn is basic electricity and electronic circuits like voltage and current source/regulators. Plenty of learning material available on the net.

 

[tylernt]

I have a friend who works at SLAC and has a gift for anecdotal explanation of complex physics stuff.

Ok, here goes: An LED is like a car with the gas pedal stuck all the way down. In order to keep the engine from overspeeding and blowing itself up, you have to drive it with the brakes on. A linear regulator is the brake -- it keeps the car slow enough that the engine doesn't tear itself apart.

If this sounds wasteful, it is. Hence the popularity of switchmode regulators, which work like turning the ignition on and off repeatedly, to maintain a safe speed.

 

[Th232]

Very good analogy Tyler. To add to it, if you, say, want the car to go at 60 km/hr and the engine will only ever max out at 63 km/hr, you don't waste much. If however the engine can get the car up to 120 km/hr without any issues,

For the more technical side of things, a lot of linear regulators use a transistor, mosfet or similar. In the case of an N-type mosfet, there are three legs. Current goes in through "drain", out through "source", and whether it flows or not is controlled by the voltage across the "gate" and "source" legs, which alters the resistance between drain and source.

As the gate-source voltage increases you start off with an extremely high resistance. Then at some stage, called the linear stage, drain-source resistance will decrease about linearly as the gate-source voltage increases. After that the resistance is pretty much minimal.

Normally this linear stage should be kept within as tight a voltage range as possible, less time spent in there means less power is lost during switching, and the closer we can reliably get to that boundary, maybe leading to faster switching times (all things being equal it's usually quicker to change from, say, 3.3 to 3.4 volts than 3.0 to 3.7 volts).

In a linear regulator however, we want to operate in that linear region (hence the name, surprise surprise!). A simple feedback loop is used so that if the voltage (or current in the case of a 7135) increases, the gate voltage is decreased to increase the drain-source resistance and vice versa. This will then reduce the voltage/current flowing through the device and keeps the desired output steady.

 

[psychbeat]

Awesome guys!
I feel like this is stuff most of us flashaholics should learn.

I thought there was probably a neg feedback loop...

And the microcontroller either turns the whole thing on and off super fast (PWM) or somehow increases the resistance in the 7135 for modes?

 

[Mr Happy]

Ok, here goes: An LED is like a car with the gas pedal stuck all the way down. In order to keep the engine from overspeeding and blowing itself up, you have to drive it with the brakes on. A linear regulator is the brake -- it keeps the car slow enough that the engine doesn't tear itself apart.

Actually, a car is an even better analogy than that. A car engine really does want to run as fast as possible and blow itself up. To stop that happening and to regulate the engine speed (LED brightness) you have to hold it back, which is what the throttle does. The throttle basically restricts the flow of air to the engine (flow of current to the LED) so that the engine can only run as fast as we want (LED run as brightly as we want).

So an LED driver is like the throttle in a car. The simplest possible regulator for an LED is a series resistor, which is like a regular throttle used with a carburetor. It just restricts the airflow and has no smarts. Modern cars have electronic engine management systems that can regulate the air and fuel flow in a much more calculated way, and these are more like LED drivers with electronics in them (like the 7135).

 

[PhotonWrangler]

Good analogies guys!

The way I see it is that an LED acts like a dead short (forward biased diode) and without any regulation, it would allow an unlimited flow of current and become a NED (noise emitting diode). The linear regulator with it's variable resistance decides how much of that energy to throw away as heat in order to prevent it from driving the diode into thermal runaway.

The thermal runaway phenomenon can happen really fast in a semiconductor junction. If too much current flows, the resistance to current flow goes down, allowing more current to flow, causing it to resist the flow even less, and the cycle keeps escalating until the junction (or the bond wire connection) simply melts. Or explodes.

Anyway, the upside of linear regulators is that they're small, cheap and simple. The downside is that they're inefficient and tend to run hot.

 

[moderator007]

The downside is that they're inefficient and tend to run hot.

They can be very efficent if the input voltage is close to the output voltage. Sort of like a direct drive setup but current regulated.

 

[PhotonWrangler]

They can be very efficent if the input voltage is close to the output voltage. Sort of like a direct drive setup but current regulated.

Good point. The efficiency depends on how much excess voltage it has to burn off.

 

[hank]

Does anyone have an X-ray of the innards of a 7135 chip?

I'm wondering how much of the black stuff I can file off the corners where someone squeezed one onto a driver.

I tried tightening retaining rings down on the top of the 7135s and when I dropped a flashlight, the impact broke a 7135.

https://www.flickr.com/photos/wossname/36845689954/

I've filed notches in retaining rings to clear the chips, but that gets tedious.

https://www.flickr.com/photos/wossname/22033940286/

 

[PhotonWrangler]

You could de-encapsulate a spare 7135 with nitric acid but that's rather dangerous. You could also find someone with a thermal camera that includes macro focus and get some images of one that's been in operation. You might be able to see the heat traces of the leads. Or you could file/sand down a spare one until you see metal or silicon to get a fix on the layout.