Great 12V regulator chip for car...I think

[Steves908]

I assembled a 9 amp, remote turn-on power supply board for a particular car audio unit I'm building. Just wanted to share with you this regulator chip that I thought would be perfect to use. http://www.fairchildsemi.com/ds/KA/KA378R12CTU.pdf. I was looking for something that would regulate power when the car was running, but would have minimum effect on the voltage when the car was not running, when power is already clean. This chip has a dropout voltage of only 0.5V, and can handle up to 3A.

PS board image.jpg

When I first tested the board with a 12V input, the regulators got uncomfortably hot. The board wasn't powering anything, so there was no current draw. I could only keep a finger on one for a second. Oddly, when the input was changed at 13V (car alternators run around 14V) the things cooled down to room temperature. I looked at the data sheet again, and found the issue. Looking at Figure 2, at 11 or 12V, it has an 80+mA quiescent current, but as soon as it hits 12.5V it's greatly reduced.

So my question is, do you think this will be a problem? I don't think so, but each regulator will normally be supplying 2 amps on top of this idle temperature. I am soon planning on attaching a heat sink to the regulators. Maybe that'll make it ok, and they'll just always be hot when the alternator/car is not running.

[DJNUBZ]

I'm not a PS expert but I am a car audio expert and this might be a problem. If you ever listen to you car stereo while the car isn't running you could run into an issue. I think that your heat sink plan is the way to go, just make sure it is large enough.

Could you tell me a little bit more about your amp project. One of my long term goals is to build a F5 amp for my car to push my compression horns.

[infinia]

Why don't you disable it when the car isn't running? drawing current (getting hot) when the car is idle does not bode well for battery life. You need some sort of smart switch {bypass and disable the chip when the Vin reaches < 13 V}

[Steves908]

Hi DJNUBZ. Yeah, I'll have to look for a good heat sink. My project is not quite an amp, but a preamp of sorts. It's a miniDSP (used as an active crossover) followed by two of these Aikido LV as line stages for each output. Normally I wouldn't bother with tubes in a car, BUT these LV Aikido boards run on 24V! I got one and thought "this has to go in a car", so I bought another kit along and a 3A 12-24V step up converter. I'm also going to make a temperature switch that turns off the tube heaters if the ambient temp is too cold or hot, and uses opamps when the tubes are off. It's going to take to time to get all this together, but I thought I'd start with step 1: the power supply

I looked up an F5 amp, looks awesome and simple! Have you successfully made a car audio power amp before? Because I would love to but am worried it wouldn't sound as good as just buying one.

[kevinkr]

I think Infinia's suggestion to disable that regulator when the engine isn't running is a good one.

Also not sure why you would need to disable the aikido LV for any temperature extreme, should work fine at any temperature you do.. I don't think thermal shock is an issue even under very cold conditions unless you live north of the arctic circle or Alaska? (which you clearly don't)

[Steves908]

Originally Posted by kevinkr

I think Infinia's suggestion to disable that regulator when the engine isn't running is a good one.

Also not sure why you would need to disable the aikido LV for any temperature extreme, should work fine at any temperature you do.. I don't think thermal shock is an issue even under very cold conditions unless you live north of the arctic circle or Alaska? (which you clearly don't)

The disable option sounds like a good one, being that I don't even need the regulators with just the battery. As for a "smart switch" that will ground pin 4 when Vin < 11V, does anyone have any suggestions? A comparator possibly? Also, I don't have experience with this, but I heard that it's damaging to a tube to turn it on while it's cold, or below freezing I guess. I would think this is true with the risk of a rapid temp change breaking them, but I don't know. I don't want to risk breaking any of my eight 6GM8's because they're getting so rare and expensive these days!

[Steves908]

Originally Posted by Steves908

The disable option sounds like a good one, being that I don't even need the regulators with just the battery. As for a "smart switch" that will ground pin 4 when Vin < 11V, does anyone have any suggestions? A comparator possibly? Also, I don't have experience with this, but I heard that it's damaging to a tube to turn it on while it's cold, or below freezing I guess. I would think this is true with the risk of a rapid temp change breaking them, but I don't know. I don't want to risk breaking any of my eight 6GM8's because they're getting so rare and expensive these days!

I meant Vin < 13V.

[kevinkr]

Originally Posted by Steves908

<snip> Also, I don't have experience with this, but I heard that it's damaging to a tube to turn it on while it's cold, or below freezing I guess. I would think this is true with the risk of a rapid temp change breaking them, but I don't know. I don't want to risk breaking any of my eight 6GM8's because they're getting so rare and expensive these days!

Given the relatively low heater power I doubt the thermal stress will be significant given your locale, and prior to the common use of transistors in car radios tubes were used, and I have not heard of any significant issues relating to thermal stress..

A much bigger problem IMHO is going to be shock and vibration- that may be much more of a concern than heat.. Shock mounting those boards would be a good idea..

[Steves908]

Thanks for the suggestions kevinkr! Considering these tubes were made decades ago for car radios, they will probably be fine. Even though I'm usually in Florida as a student at the University of Miami, my hometown is in NW Ohio. So, I know what it feels like to crawl into a 5°F car some days . Maybe I'll have a manual switch that turns off the heaters and switches to opamps for those extreme days in case I get find a job up north when I graduate. The shock mounting sounds like a great idea.

Have you ever used pcb shockmounts? Is there a good place to get them from?