5v fixed Auto grate Regulator

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
i know automotive related stuff is very shunned here but i feel this falls in the "legal" part.Anyone have any suggestions as to a 3 amp positive, fixed or adj, Linear regulator that is automotive rated? i found the LD1085xx but it appears digikey and mouser do not carry them in the TO-220FP package, which is the only pkg designated for automotive use.Anyone know of any alternatives?
 
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ifixit

Joined Nov 20, 2008
650
Hi,

Here is what I did when interfacing to my Kubota tractor 12V power system. All the components used need to have an operating temperature range of at least -40C to +85C. Select fuse F1 to be 5 amps. D1 should be 5 amp also. The transorb you already have.

Good Luck,
Ifixit
 

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SgtWookie

Joined Jul 17, 2007
22,210
Looks like you're planning on getting 5v @ 3A out of an LM340.

I'm wondering how you're planning on getting rid of the (14v-5v)*3A = 27 Watts of power that the regulator will dissipate?
 

CraigHB

Joined Aug 12, 2011
127
Here's what I used to make a couple USB cig lighter adapters. They work great, can actually output 2A no problem even though 1.5A is specified.

I added a 24V TVS diode behind a Schottky diode and 220uF tank capacitor to provide protection for the electronics I power with it. Most cig lighter adapters have none and automotive electrical systems are hostile.


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Thread Starter

Geoffr67

Joined Dec 5, 2011
69
i was thinking possibly a simple switcher like the LM2576 adjustable. having Vout set to around 7 volts, Then feeding this into a linear regulator such as the LM2940 to provide extra smoothing and more stable operation. what do you guys feel about doing it this way?
 

SgtWookie

Joined Jul 17, 2007
22,210
I'd just get a couple of the regulators CraigHB pointed out, slap caps on the front and back ends, along with the TVS diode to guard against the HV transients, and let it go at that.

No point in going to a linear regulator; all you'll do is complicate the matter, increase power dissipation, and wind up with a larger, less reliable project. These regulators are switching at such a high frequency that you would never be able to hear any artifacts.
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
Craig, when you run it at 2 amps does it heat up allot? I dont think ill be exceeding 1.5 amps with it what are your recommendations as far as a heatsink?
 

CraigHB

Joined Aug 12, 2011
127
The OKI part runs plenty cool enough. My max loading on it is 1A so I have not run it for extended periods at 2A. When testing the completed adapter, I did some load testing on it and found no issue when checking output with a 2A load. The best thing about the part is the price. Most of the switching modules are more expensive. So, if you wanted to run two in parallel, cost probably would not be an issue.
 

Thread Starter

Geoffr67

Joined Dec 5, 2011
69
The OKI part runs plenty cool enough. My max loading on it is 1A so I have not run it for extended periods at 2A. When testing the completed adapter, I did some load testing on it and found no issue when checking output with a 2A load. The best thing about the part is the price. Most of the switching modules are more expensive. So, if you wanted to run two in parallel, cost probably would not be an issue.

Thanks for the info guys im going to order up 10 of these bad boys today, just waiting to hear some advice on my new question about a 20mhz crystal for a pic16F88 then will place the order from mouser! you guys have been a ton of help and i really appreciate it.
 

thatoneguy

Joined Feb 19, 2009
6,359
Most cig lighter adapters have none and automotive electrical systems are hostile.
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That is a pretty huge understatement.

An automotive circuit is exposed to about the worse case of all components it is made from. sub-freezing to +150 degrees Farenheit, vibration, condensation or spills directly onto the device, and then you have the electrical problems, sometimes way over voltage, sometimes way under, sometimes both in the same few millisecond span.

In a lighter jack based circuit, intermittent power is common when the jack chosen doesn't make good contact, or the vehicle's contact is corroded. Don't forget the spikes from spark plug EMI in older systems, or alternator regulator failure in newer systems. It's amazing more of the cheapo adapters don't simply start on fire more often.

The list above is just one of many reasons automotive circuits aren't allowed for discussion here. The other reasons are safety and the extremely varied regulations required across the globe.

--ETA: Correction: Motorcycles and boats are in the absolute worse environment. Cars/tractors/combines/etc are just about the "worse place imaginable" to be installed if components had feelings.
 

CraigHB

Joined Aug 12, 2011
127
Definitely on boats. Lots of electrical horror stories to tell there. I use to own a 32 ft. deep water boat with a couple partners when I lived on the coast. We each had our maintenance responsibilites. I took care of the electrical system and it kept me busy. Seawater corrodes stuff faster than you can imagine. Then there's all the vibration from a couple high torque engines running at heavy load for long periods.

The main problem with alternator powered DC systems is the response time on the alternator/regulator. It's pretty slow with all those high currents and all that inductance. You get big spikes in output from load transitions. The most common source is load dumping when shutting off accessories. When you have an AC inverter on a power boat running all kinds of creature comforts like microwave ovens and refrigerators, you constantly get huge load dumps that make big spikes. For a car, jump starting another car with a discharged battery is the worst, huge spikes when you disconnect the cables. Always jump another car without your engine running and the ignition off.
 
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Thread Starter

Geoffr67

Joined Dec 5, 2011
69
Definitely on boats. Lots of electrical horror stories to tell there. I use to own a 32 ft. deep water boat with a couple partners when I lived on the coast. We each had our maintenance responsibilites. I took care of the electrical system and it kept me busy. Seawater corrodes stuff faster than you can imagine. Then there's all the vibration from a couple high torque engines running at heavy load for long periods.

The main problem with alternator powered DC systems is the response time on the alternator/regulator. It's pretty slow with all those high currents and all that inductance. You get big spikes in output from load transitions. The most common source is load dumping when shutting off accessories. When you have an AC inverter on a power boat running all kinds of creature comforts like microwave ovens and refrigerators, you constantly get huge load dumps that make big spikes. For a car, jump starting another car with a discharged battery is the worst, huge spikes when you disconnect the cables. Always jump another car without your engine running and the ignition off.
did you use filter caps on the input and output of this regulator Craig? If so what values did you end up with? I was going to take this and test with a 10uf on the input and a 2.2 ceramic SMD on the output and go from there, but id like to hear what you ended up with and go from there.
 

CraigHB

Joined Aug 12, 2011
127
Are you asking about the OKI-78SR regulator I mentioned earlier? I used a 50V 220uF electrolytic capacitor on the input only to absorb voltage spikes. A 100V 3A Schottky diode behind the capacitor blocks negative spikes and a dual sided 600W 24V TVS diode behind that absorbs anything over 30V, negative or positive. That was the main reason I built my own USB charger adapters, for reliable surge suppression and high efficiency. Most adapters have neither.

No input or output filter capacitors are required for the OKI-78SR. The switching module already has them on-board. They are optional for improved performance which is discussed on page 10 of the data sheet. The performance is good enough for my purposes without them, just for charging stuff. If you need better dynamic response or less ripple, then follow the recommendations in the data sheet.
 
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