7135 drivers with higher voltages revisited

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!

I had wondered about that(not really. I’m obviously not an engineer) but postulated, perhaps incorrectly, that the same situation occurs in Downloads method without negative effects and that maybe the location of the LEDs in the circuit was relavent only to the input voltage of the Attiny chip and the Vdd pin on the 7135’s. I’m just using a different method to lower ther processor input voltage than him.

While we are waiting for DX,… could you post a pic of the sink with AMC’s?

Did you reflow as you would reflow a led on the star, or did you first reflow the sink with tin and then soldered the AMC’s to it?

There is a picture of the sink in post #23 before I cleaned it up with one of the amc’s set in place. I pretinned the chips and the sink, cleaned up the sink a bit more, and then set the sink with chips on another layer of pretinned cu and reflowed the whole thing with a small butane pencil torch. When I saw the chips settle I removed the heat. The sot-89 plastic is undamaged so there’s hope yet for the ic’s Not at home for a few days but will check output when I am. Here are 2 pics with the chips wired. I used a dremel to grind off the ground pin in the middle for more room. To get a nice circle I pretinned the wire and then wrapped it around a screwdriver.

It’s a bit ugly and I’ll likely redo the outer Vdd wire but this should be fine for testing. Still needs a ground wire to the sink itself.

Oh my… no guts no glory !

Seems adequate for only 4 of them… hope they’ve no hard feelings from heating up. Thx for the update.

I wonder if what goes on in the Download version(extra LEDs in series with the driver to lower battery voltage) is that as the current goes to zero in the pwm cycle and the drop across the chip rises, since the current is going to zero, the power loss(and heat gain) is also going to zero. I don’t know, but his method DOES work so it must not be too much for the chips to handle. I expect someone with an o-scope could spot any voltage spike and recommend a possible solution if necessary. I don’t have one so I’ll be using the “if it ain’t broke, don’t fix it” solution.