Running a microcontroller in car - power supply aspects

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Updated the power input part of my schematic. I'm not sure about that inductor+diode jobbie I did there and whether I should leave it in place or drop it entirely. I placed the TVS after it, which I'm not sure is a good idea, since L1 would take the full surge current if the TVS D10 breaks down and conducts.

[sam_sam_sam]

Re: Running a microcontroller in car - power supply aspects

i dount you had any spikes, firstly the battery doubles as a huge cap,
second, the alternator wouldnt be turning enough to generate anything.

Well that is mostly true but you can have voltage spikes so you do need have something to absorb the voltage spikes and smooth them out over time

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Thanks. This is great stuff to know.

But it's only good to 50mA before the IC heats up. It would be enough for your Arduino Pro-Mini.

I was really excited to hear this at first, but then realized it may not be enough: I'm also using some ACS712 current sensors to work out the current draw of the windows and know when to stop them when they reach the end of travel up/down. I had a look at the datasheet just now and these have a disappointingly high current draw of 10mA EACH ! This immediately puts an end to my "low-power" requirements. I'll have to power up the sensors only when needed and put them to "sleep" when idling. This shouldn't be too hard, since I can hopefully power them off one of the pins on the arduino directly, take a reading, stop the window if necessary, then cut the power again.

D1 blocks -ve spikes which can be -150V in a 12V car, -450V to -600V in a 24V truck.

Can this be a 1N4007 ? Mostly out of laziness, since I have plenty on hand already and don't need to source 5s

TVS1 20V clamps +ve spikes to at most 29.1V@21A (Vreg 30V max rating) and does not leak current in cold weather. A bigger 5,000W TVS is 1.5KE20 a little different specs but tougher 27.7V@54.9A.

Got a bit confused here What am I looking for in a TVS ? The Breakdown figure or the Maximum Clamping figure ? The schematic shows a P6KE TVS, which if I understand correctly has a power rating of 600w and would have to be a P6KE20A variety with a maximum clamping voltage of 27.7v, despite the Br voltage being 19-21v. The math also confirms that 27.7v*22A maximum current = ~600w. The 1.5KE20A is the same thing, but can take more current to go up to 1500w if the formula stands - I don't think it's 5000w like you said, unless I misunderstood.

Car ECU/anti-lock brake controller etc. uses special 6,600W SM8A27 TVS - which can take 700A spikes, load dump and all that. Seriously tough part.
BUT polyfuse F1 has some DC resistance that is useful to limit current through the TVS if a huge spike happens. Without some resistance there, the TVS tries to protect the entire car, which is not a good idea due to the possible energy being more than a small diode can take. So the resistance helps a lot.[/QUOTE]
That diode sounds good, but I'm not sure I can source it. Considering my current "design" employs a glass fuse, which offers no resistance, I could put a small resistor in series with the circuit to account for it. This also answers a question I had in the back of my head this whole time, since this circuit is running straight off the battery and was worried what might happen when the car is jump-started (possibly with reverse polarity, to make it a worst-case scenario) or when it's cranked (in the winter). It's a 1.6L petrol engine, not a diesel, but still takes some current I imagine

[redwire]

Re: Running a microcontroller in car - power supply aspects

A TVS diode operates basically as a zener diode, but designed for high energy pulses so they have loose ratings +/-10% and are not precision parts.
Most transients in a car are from inductive loads- ignition system, blower fan, solenoids, fuel pump, ABS solenoids etc.

This LDO circuit is proven for powering a small MCU in a vehicle, I've used it on cars and trucks for many years, with zero problems. But it's only good to 50mA before the IC heats up. It would be enough for your Arduino Pro-Mini.

D1 blocks -ve spikes which can be -150V in a 12V car, -450V to -600V in a 24V truck.
TVS1 20V clamps +ve spikes to at most 29.1V@21A (Vreg 30V max rating) and does not leak current in cold weather. A bigger 5,000W TVS is 1.5KE20 a little different specs but tougher 27.7V@54.9A.
Car ECU/anti-lock brake controller etc. uses special 6,600W SM8A27 TVS - which can take 700A spikes, load dump and all that. Seriously tough part.
BUT polyfuse F1 has some DC resistance that is useful to limit current through the TVS if a huge spike happens. Without some resistance there, the TVS tries to protect the entire car, which is not a good idea due to the possible energy being more than a small diode can take. So the resistance helps a lot.
Quiescent current of LP2950 is 1mA with 30mA load, down to 0.1mA with lighter loads.
If you used a LM7805, it's max. input is 25V (use lower 18V TVS but not good in winter) and quiescent current 5-8mA. Some brands need a minimum load of 5mA.

In the lab, when you test a TVS for car transients, say you hit it with +100VDC impulse most TVS will instantly short. If you have a couple ohms resistance (line wiring+fuse/polyfuse) in series, the TVS diode does not clamp so hard and does not fail.

[stj]

Re: Running a microcontroller in car - power supply aspects

i dount you had any spikes, firstly the battery doubles as a huge cap,
second, the alternator wouldnt be turning enough to generate anything.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

My car battery just died after 6 years of service and I had to push the bastard to get it started yesterday This got me thinking about my project here and how bad the power output must've been when the car was being pushed and I popped the clutch, causing the car to judder really hard a couple of times before it finally turned over - spikes up the a$$...

Redwire suggested a TVS diode on the input of my regulator from the battery (somewhere in the red circle, in place of ZD5, after the fuse). I had a look at the datasheet myself to better understand what that's for and what it'd do. If I understand correctly, it conducts very little current when the "reverse standoff voltage" across it is less than 15.3v and "breaks down" and conducts when it goes above 17.1v-18v. Is a TVS diode in this configuration used to protect against spikes all the time ("clamping" ?), or is it a single-use device intended to blow the fuse if the input exceeds a "safe" level ? Would it be a good idea to go relatively high with a 1.5KE27A, assuming we take the worst-case breakdown at 28.4v ? It would essentially go right across the battery, so it might not be enough....

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

download "open camera" for it
you can get it from f-droid store

Already have it. I got it for when I need the torch to be on at the same time as the camera - the stock camera app doesn't let you do that. The first shot was taken with O/C. The later shot is the stock camera app in "pro" mode so I could mess around with the aperture...

[stj]

Re: Running a microcontroller in car - power supply aspects

download "open camera" for it
you can get it from f-droid store

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

clean the flux off, it's hard to tell if i'm looking at reflections or solder bridges!

Probably went a bit overboard with the lighting, but I can assure you there are no bridges Here's a hopefully better shot. This Galaxy S10+ that I'm using seems to have a lot of potential, but I can't seem to get stuff just right. Seems it's either too dark or too bright and not in focus....

[stj]

Re: Running a microcontroller in car - power supply aspects

clean the flux off, it's hard to tell if i'm looking at reflections or solder bridges!

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Here's what the solder side looks like. The perfboard is janky, I know, it was a leftover piece that I just had to use to get rid of The leads to the left are the 2k resistors in series with the anode of the LED in the optocouplers and essentially serve as our "inputs" - when pulled to 12v, the LED turns on and the pin on the arduino is pulled low via the C-E path in the optocoupler.

The leads to the right are the 10k pull-ups which hold the pins high to 5v again - they'll all go to a "bus" rail when I get to that part.

The yellow wire is a GND wire which will be soldered once I sort out the layout of the power section....not sure whether I should go with the switching regulator or a simple 7805 as suggested....

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

I'll take a shot of the back as well. The way I do it is leave as much of the component's lead as possible and bend in the direction I want to form a track and then solder over it, which ensures the leg of the component IS the track It's safer but can get ugly when you need to replace something....VERY ugly

I WAS thinking of a 7805 when I first started, but thought a switching regulator would be more efficient in the long run. Noise on the 7805 would still exist though, as I learned from a video of Dave. It doesn't quite help filter out the noise AFAIK, though probably less than the switching one.

[redwire]

Re: Running a microcontroller in car - power supply aspects

It looks fine to me, I have put far uglier builds in cars. They worked great until I hit a pothole lol. Just need a way to solder wires to the protoboard that don't flex and break.

I'm surprised we haven't beat the LM7805 yet for powering the board.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Just for those who are curious, here's what the project currently looks like on its perfboard The "power" section will go where that empty space is, opposite the optocouplers and the unused headers will be trimmed off, of course. 7 inputs, so 7 optocouplers: UP, DOWN, UP, DOWN (left+right), IGN. LOCK, UNLOCK.

[stj]

Re: Running a microcontroller in car - power supply aspects

spikes are spikes - very short.
just clamp them with a big mov

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

I think the lowest TVS in a car would be 18V such as 1.5KE18A. They start to conduct 15.3V and at -40C they drop and start to leak current at 14.3V which is OK. They clamp to 25V with a big transient 59A which you could lower with a fuse having high resistance (an ohm or two) in series, which limits TVS current and it would clamp to less than 25V.

This is good info. A TVS would go across the input to the regulator ? I think I'll stick with the larger LM regulators in the end, which can supposedly go up to 30v, but clamping to 25v would suffice I think...

[redwire]

Re: Running a microcontroller in car - power supply aspects

MP chinese datasheets are misleading, they give quiescent current drain when the IC is off lol "VFB=0.9V" "Static Current 0.85mA" "97% efficient" trickery. Or the IC's are fake. All of these modules have too small in/out capacitors so they can go unstable. What a hassle.
The AAM Power Save resistor on pin 1 sets when it does pulse-skipping. It seems to be a voltage threshold.

Other modules MINI360 uses MP2307 340kHz, sync rectifier but this guy measured 41mA drain with 12V in and no load
Other modules use MP1584EN 1MHz, Schottky rectifier, max. 28Vin.

None of them are that great or beat an LDO, although some people got 3.3mA from your MP2315 modules, so it might have another problem?

I think the lowest TVS in a car would be 18V such as 1.5KE18A. They start to conduct 15.3V and at -40C they drop and start to leak current at 14.3V which is OK. They clamp to 25V with a big transient 59A which you could lower with a fuse having high resistance (an ohm or two) in series, which limits TVS current and it would clamp to less than 25V.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Ok, I cut the track and indeed it works correctly now: bridging the 5v pads, actually gives me 5.1v, since the pot is out of the loop now. Still, the power consumption is 11mA...maybe it's related to that low power state on pin 1. I'm not sure these modules are suitable though because of the 24v limit of the MP2315 IC.

[stj]

Re: Running a microcontroller in car - power supply aspects

exactly, it's in series so you can fine-tune the resistance but only by 5k

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

the mod i do on some is to replace the pot with 5k (original is something like 200k)
and cut that track - then solder a resistor across it i salvage from the top for 5v.

this gives me something like 5v-5.5v ajustment range!

Where does the resistor go exactly ? Across the track you just cut, so it's in series with the pot now ?