7135 drivers with higher voltages revisited

I really like your ideas tho I dont quite understand half of it.. but if you use 4 cells and 4 leds, why not just go direct drive or..

If it really is possible to use the AMC chips in series like shown in the picture above, why still use a driver board rather than just wiring up the AMCs without processor and stuff?

If you are referring to Downloads diagram(by the way, thank you ChicagoX for posting that image) what it shows is the need for reducing Vbatt(the voltage from the battery, to the processor not the 7135 chips. The diagram in the method used by Oldlumens/Techjunkie shows that the processor can be powered separately from the LEDs using a low current through the processor on a master board to control the led current on the slave boards.
In a nut shell, modes and runtime. I don’t need high output grinding slowly up hill but coming down I need to see every root, rock, and rut, and I need to have hours of run time without extra batteries. DD would work but have no modes and lousy run time. Actually, while I was searching info on these chips I ran across some Soeul stars with two 7135 chips and a diode. The first piece of info in the data sheet for the Amc 7135 reads “no external components needed”. Another tidbit was that the T0-252 package for this chip is rated at 1000 mW which would take care of the higher voltage drop I need without using a 1.1 ohm resistor or extra switch.

Actually, 700mW is the max without additional heatsinking. So with a good enough path to ambient(I’m talking ground tab soldered directly to solid copper, not the measly foil on these board, and definitely separate from the led heat sink, even the smaller sot-89 might handle the load. Before I do that I want to verify the input resistor value for the processor. Anyone ever heard of a 10k-15k ohm linear pot? If I start at 15k and reduce the resistance until the processor goes poof I’ll know the minimum value needed.

Actually, a reverse audio taper would give me more accuracy at the high resistance end of travel. I checked and these are not cheap. Better would be a 10k-12k ohm resistor in series with a 5k or 3k linear pot.

Indeed, a resistor is needed if using Download idea to drive 3-4 XMLs in series,
otherwise, too much heat may be shed onto the 8x7135 driver board & may burn it out.
I had a build a while back, tried out different resistors. Different battery & XML vfs may need fine tone the resistors

Example here:

I advise against using a resistor for the logic supply voltage (MCU and AMC7135s). The current of these can vary quite a lot (e.g. with PWM), which would result in a wildly hopping supply voltage.

Alternatives: Voltage regulator like 7805, available in various form factors, including TO-92 and SMD. Or resistor and zener diode.

The data sheet for 7805 indicates other components unnecessary but schematic shows capacitors on input and output. Are they needed with a battery power supply?

Very much agreed. When the logic is idle, it could be drawing half or less of the rated current - this would cause it to see a much higher voltage. Resistor and zener is the way to go. Choose a 5V zener and a, oh, I dunno, 1k ish resistor… That will allow the logic to pull a couple of milliamps when/if it needs to…

PPtk

Ok, it makes sense that the input voltage would fluctuate to the attiny pwm chip if I just use a resistor. What makes the resistor + Zener preferable to the “black box” approach of the 7805? Is it more efficient, cleaner, or what? Thanks for this input. I enjoy going outside the box but I fully realize this makes me vulnerable to mistakes. I’ll throw my pride under the bus for knowledge anytime.

So I found a half dozen 78L05 in a SOT-89 package(same as 7135). Operating voltage 6.75V-26V. Output current is 100mA. Atmel chip draws .24 mA and four 7135’s draw .8mA so there should be plenty. I thought I would desolder the 7135 closest to the Atmel chip,“stack” in on on of the others, and put the 78L05 in its place. It would not be soldered to the pin pads other than ground, I would have to jumper across/in place of, the reverse polarity diode from the 78L05 in/out pins. Again, the data sheet says no additional components needed, but could signal bounce from the pwm chip cause problems in the 78L05? Should there be a .1 microfarad cap from the output of the 7805 to ground?

The 78L05 is a good choice. The resistor and zener isn’t better - its just cheaper :slight_smile:

I wouldn’t worry too much about cross-talk and radiation from the PWM into the regulator. Yes, you should try to cobble a .1uF cap on the output though. If there’s room, a .1uF and a .01uF, in fact.

I only paid $.50 each for 6. Where would the .01 cap go? Also across the output? Or one on the input, one on the output? Saw a diagram with .33 microf on the input and .1 on the output. How critical are the values(other than voltage capacity)?

Values aren’t super critical for an application like this. You don’t really need cap on the input, but the .1 and .01 on the output (in parallel) is a good idea. If you don’t have any .01s, just the .1 will probably be fine.

I’ll be waiting awhile for the 78L05 chips to arrive but in the meantime I’m working on a drawing that shows how this works. I’ve studied close ups of a few different boards and I like what I see. In each one that I examined, it appears that the 7135 chip closest to the processor and led+ pad could be replaced with a 7805 SOT-89 voltage regulator. I would need to cut the traces leading to the other 7135 chips and jumper the led+ pad to the input pin on the 7805 and the output pin to other side of the reverse polarity diode. An smd cap across the output pin to the ground pin and done I’ll try an make the next post with fewer words and more pictures. Check back in a week or two. This could be a way to use these boards with more options on battery voltage.

Still waiting on the 5V regulator chips but I’ve made up a dedicated board for the 4 7135 chips which is just a copper sink with a crenelated cylinder for the chips. I want to see how much excess voltage they can handle when they are mounted this way. They are rated to 700mW per chip in free air but the specs don’t give an absolute limit on wattage, just temp at 150C. As the chips are in parallel, each has to drop all of the excess voltage. One of the graphs in the spec sheets plots dropout voltage(voltage burned by the chip) vs output current and tops out at ~4V! At 350mA that’s 1.4 watts per chip folks! The heat sink capacity reccommended for them is WAY more than can be supplied by the boards we typically use and I suspect that even when stuck to a large heat sink the traces constitute a bottleneck in the flow of heat. Stay tuned.
Here are some pics:
The first is a virginal Dx 7612 1A 16-mode board

The second is another 7612 with the input diode and the three 7135 chips removed and the traces cut to the Vdd and output pins of the Q3 pads

And the third shows the heat sink with one of the chips placed

I have several battery voltages that I can test and I want to find out how well this heat sink works. The sink will get some clean up work and pretinning done to it before assembly.

Running the LEDs in series is no more or less efficient than in parallel. Power loss = voltage drop across the 7135 chips x the current through each of them.

4 xpg in parallel from 4 Li-ion and 4 boards(1master, 3slaves) at 1.4-1.5A =
(4.2Vb - 3.3Vf) x 16(350mA) = 5.4W

4 xpg in series from 4 Li-ion and 1 board =
(4 x 4.2Vb) - (4 x 3.3Vf) x 4(350mA) = 5.4W

The difference between the two is the battery current (6A vs 1.5A) and the heat being dissipated across 16 chips vs 4. The input voltage to either the Attiny 13A(multi mode) or 7135 Vdd pin(single mode) is a separate issue with a separate solution. I believe it is the boards as currently used that limit the range of use more than the chips. While I don’t expect to be able drop almost 4V into each chip, I suspect we can do much better than 1V. If not, I’ll be sure to post the sauce I use on my crow dinner.

The voltage regulator chips should be in tomorrow. I reflowed the 7135’s to the sink last night so I might have some test results by the weekend.

Sorry, I have not read the whole thread carefully but the way I understand this, in a PWM situation the voltage across the 7135’s can switch between 0V and 16.8V.
Have you considered that the 7135 are only specified to 7V max ?

Well, this could easily be remedied by a zeener diode across the 7135’s (z6.3V or like) to limit the voltage so I think that your circuit is sound enough.

Cpf Download shows in the thread kindly referenced by ChicagoX above how LEDs can be used to lower the voltage drop across the 7135 chips. The dropout voltage of the chips is the limiting factor here, not the total voltage of the system. I first read that thread before I even joined BLF and have reread it several times since then. Also, OldLumens thread, “the much sought after master/slave mod”, in which he collaborated with Cpf Techjunkie, has been a great resource. The ground work for this idea was laid in those two threads. The voltage drop in a system is equal to the sum of all of the voltage drops in that system. 7135 chips were designed as led drivers. It is assumed that at least some of the 6V max will be dropped across the “assumed” led. From the spec sheets, the actual max Vdrop is 4V and that only with very good heat sinking. As in Downloads version, I am using a series of LEDs to provide some of the voltage drop in the system. I am also using the Techjunkie method of splitting the Led circuit and the driver circuit and using a link wire to control the 7135’s as slaves. What I am adding is a separate method of lowering the voltage in the driver circuit.

When PWM switches the current off, the voltage drop across the LED’s will go towards zero at a speed determined by the capacitances in the system. Hence the voltage across the 7135 will rise towards the battery voltage.
But good luck with your project!