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Automotive Generator Universal Digital Voltage Regulator

rjenkinsgb

Well-Known Member
Most Helpful Member
Did I go overboard with protection diodes and filter caps?
No, you can always leave out anything you later decide is redundant; it's a lot easier than adding something in later on.

I'd have stayed with the single transistor cut-out; having two transistors is only needed for bidirectional control and you never want any current from battery to D+. Two transistors will double the power loss and heat to dissipate.

The mechanical regs do not have any other way of cutting the connection than by reverse current, it's a weakness not a requirement.
The cutout worked as a "latching relay", turning on when D+ reached a certain voltage and connecting it to the battery. Based on voltage alone, that would then have stayed connected until the battery went flat, so the cut-out part is a second current-operated coil around the cut-in relay that counters the field from the voltage coil when current flows from battery to D+, allowing the relay armature to release.

You could also keep the B+ sense circuit at the battery side of the power switch; as you have the 0.1uF cap on the divider, the resistors could be eg. ten times larger and still get a good reading, avoiding any voltage loss in the power switch.
 

CK3

Member
No, you can always leave out anything you later decide is redundant; it's a lot easier than adding something in later on.

I'd have stayed with the single transistor cut-out; having two transistors is only needed for bidirectional control and you never want any current from battery to D+. Two transistors will double the power loss and heat to dissipate.
Thanks. OK, I went back to one transistor. I also made some other simplifications. herbie1200 suggested that I don't really need the whole ignition sense and idiot light control thing. The original warning light will still work the same as it always did if left connected to D+ as it is in the old regulator. And, the voltage regulator can go to sleep when D+ goes to 0 (for some time) and wake up on rising edge. (I just went to the garage and checked and D+ is at about 1v at low idle, so it's not 0 when the engine is running.) I can always add bells and whistles later.

I have just started playing around with PCB layout. The original enclosure is about 60 x 50 x 40 mm. Getting all this on a single 60 x 50 board looks tough! So, now I am suddenly enthusiastic about simplification. I think the prototype with through hole parts might have to go on a double-sized board. Later, I can switch as much as possible to surface mount, and maybe populate both sides? Or use long headers to stack a second board (like feathers and hats do)? Maybe a second board that is a metal plate that I can use for heat sinking the big components? Also, the MCU evaluation board takes up most of the space, although I guess all I really need is two 30-pin headers and I could place (short) things between those. But, just placing the components I need from that directly on my board should take up a lot less room. The MCU itself is tiny.
The mechanical regs do not have any other way of cutting the connection than by reverse current, it's a weakness not a requirement.
The cutout worked as a "latching relay", turning on when D+ reached a certain voltage and connecting it to the battery. Based on voltage alone, that would then have stayed connected until the battery went flat, so the cut-out part is a second current-operated coil around the cut-in relay that counters the field from the voltage coil when current flows from battery to D+, allowing the relay armature to release.
That's one area where I'm hoping to gain some efficiency. (Another big one being the regulator curve.)
You could also keep the B+ sense circuit at the battery side of the power switch; as you have the 0.1uF cap on the divider, the resistors could be eg. ten times larger and still get a good reading, avoiding any voltage loss in the power switch.
Sounds good. The 0.1uF value was kinda picked out of the air (and the resistor values certainly were). At one point I was considering using another analog input to measure that voltage loss, but then I decided to instead add a header pin for an external input that I can use for calibration. (E.g., voltage at the battery terminals?) I don't know how stable this will be with temperature. The MCU has an internal temperature sensor, which might be useful.

Anyway, here's the latest: https://oshwlab.com/carlk3/volt-reg-1-1_copy
Schematic_Volt Reg 1.2_2021-10-08.png
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I'd never consider anything less than double sided PTH boards, especially as they cost peanuts from such as JLCPCB now. You get much better joints and higher reliability, as well as being easier to lay out and having the possibility of a ground plane.

You are using easyeda for the design so that can create the correct board files.
 

CK3

Member
What am I going to need to heat sink?

Q3 (cut out) looks like one. From the AUIRF4905 data sheet, RDS(on) Static Drain-to-Source On-Resistance Max is 0.02 Ω at VGS = -10V, ID = -38A. So, if the generator is rated for 45A, I guess I'm looking at 40ish watts (worst case). What kind of heat sink do I need for that? Is it going to have to go outside the enclosure? The voltage regulator is mounted on the generator, which is mounted on the crankcase with a solid stand, so maybe my best bet is to try to thermally couple the TO-220 tab to the regulator mounting frame? The oil temp, which I guess could be a rough proxy for the crankcase temp, is normally below 125°C. If that won't work, there is very good air movement in the engine compartment (it's an air cooled engine and the cooling fan intake is at the end of the generator shaft).

For Q1 (regulator switch), from the IRL40B209 data sheet, RDS(on) Static Drain-to-Source On-Resistance Max is 0.00125 Ω at VGS = 4.5V, ID = 50A, and I think Danwvw estimates the field current at less than 10A, so, 12ish watts?

This is the enclosure I'm talking about:
1633797374567.png


I could copy Dan:
1633798199047.png

1633798297304.png
 
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rjenkinsgb

Well-Known Member
Most Helpful Member
You need a better cutout FET, I think.. At 0.02 Ohms, 45A = 0.9V; a good high current schottky diode may have a lower voltage drop & power dissipation than that.
(Or a diode in parallel to handle high current peaks?)
 

CK3

Member
I'd never consider anything less than double sided PTH boards, especially as they cost peanuts from such as JLCPCB now. You get much better joints and higher reliability, as well as being easier to lay out and having the possibility of a ground plane.
Can one do a sort of hybrid? If I get to the point of mounting the MCU directly on my board, it would be nice to have JCLJCB Assembly mount at least the STM32L412KBU for me. I don't think you can get that in a through hole package (except the evaluation board).

Also, seems to me like those little SMT resistors and capacitors wouldn't shake off too easily. Maybe at some point I should consider potting or conformal coating.
 

CK3

Member
... a good high current schottky diode may have a lower voltage drop & power dissipation than that...
Well, that would be another nice simplification. [This is starting to look more like something I can actually build!]

Danwvw seems to have settled on STPS80115, but I can't find that anywhere. There is STPS61H100C which has a forward voltage drop of 0.63 at 30A. So, at 45A (probably never happen with my generator) that's roughly 30 watts.

Or, maybe a Kyocera KCQ60A03L at 0.50v for less than 25w.

EDIT: And now, I have seen that Dan moved on to the 82CNQ030 which has some impressive specs.: Maximum Average Rectified Forward Current: 80A, Forward Voltage Drop @ 80A, Pulse, TJ = 25 C (appears to be worst case): 0.55V.
 
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CK3

Member
I took apart an old regulator today:
1634012201699.png

1634012253232.png

1634012283939.png

1634012341869.png

The "floor" of the inside of the regulator seems to be a 1mm think sheet of something like phenolic board. I think it is about the size and shape that my board will need to be. I don't need all of those cutouts and holes, but I probably still want four mounting holes and the hole in the middle is handy for attaching the top cover.

I stuck it on my scanner:
1634013022318.png

(That's a mm ruler at the top.) Anyone got any good tricks for turning that into a Board Outline in EasyEDA? I think I will need to get all the space I can; there's not a lot of area to work with:
1634013791049.png
 

Buk

Active Member
Unpack the attached zip, there's a .dxf inside which ought to be close and should load into EasyEDA.

I've left all the holes in; presumably you can throw away the once you don't want once its loaded.
 

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CK3

Member
Unpack the attached zip, there's a .dxf inside which ought to be close and should load into EasyEDA.

I've left all the holes in; presumably you can throw away the once you don't want once its loaded.
Beautiful! Thanks! How'd you do that?
 

Buk

Active Member
How'd you do that?
Pasted your image from above into my CAD program; scaled it carefully until the 3-11 cm marks on a ruler matched a 80mm line draw in the cad; then drew around it.
export as dxf. Its a process I do all the time, so it only took a couple of minutes.

It actually took far longer to find a way to post/attach it here. .dxf isn't accepted as a picture nor can it be attached directly.
 

CK3

Member
Thinking about the power side of things some more...

The original regulator relied on those phenolic (?) sheets for insulation. Even where different circuits passed through the frame they just relied on getting the conducting rivet really centered. Given this:
1634144790067.png

1634148053485.png

could I replace that 1mm phenolic sheet with a PCB? Have a bottom layer (or two) with nothing on it as insulation? Put some copper pours under the bolt-on terminals and rely on those for thermal conduction away from the big, hot components (surface?) mounted on the PCB? How much thermal conduction would go through the PCB itself? What if I put some paste under it?

EDIT: Or, how about a copper pour on the bottom (in this view; top when mounted) of the PCB, some thermal vias, and a thermal pad between the board and the frame? The User Contributed Component 10MMPAD illustrates what I'm talking about.

[I made a bit of a mess taking apart the old regulator. I'll do better next time. But that thing was built to never, ever come apart!]
 
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