cyamamot@girtab.usc.edu (Cliff Yamamoto) (11/28/89)
In article <1989Nov27.160950.13937@sj.ate.slb.com> poffen@molehill writes: >My friend bought a Mazda MX-6 last year. Earlier this year the regulator on >the alternater went berserk. As near as the dealer can figure, the voltage >out of the alternater probably went as high as 100V! Everything, and I mean >EVERYTHING electrical in the car blew out. Virtually every light bulb, all of >the computers (engine, transmission, turbo boost, etc) blew out. The stereo >smoked, the air conditioner fryed, the windshield wiper motor melted, the >battery exploded. In addition, several wiring harnesses burned up. > >Russ Poffenberger DOMAIN: poffen@sj.ate.slb.com This got me thinking about the audio/ham-radio gear I have installed in my car. Can anyone confirm if the following would work: (-) 12 Vdc (+) 30 Amp +--- battery ---o-fuse-o--+----> to your favorite & expensive equipment | | | - | ^ 30 Amp +15 Vdc Zener diode ( just match the | | diode rating with +-------------------------+----> car chassis the fuse rating ) Are there any better ideas out there? What can be done to clamp a large voltage surge or at least blow out the fuse when a surge occurs? Thanks, Cliff
forbes@aries.uiuc.edu (Jeff Forbes) (11/28/89)
How about MOV or ZNR transient/surge protectors?
wrf@mab.ecse.rpi.edu (Wm Randolph Franklin) (11/28/89)
In <6709@merlin.usc.edu> cyamamot@girtab.usc.edu (Cliff Yamamoto) writes: >This got me thinking about the audio/ham-radio gear I have installed in my >car. Can anyone confirm if the following would work: > > (-) 12 Vdc (+) 30 Amp > +--- battery ---o-fuse-o--+----> to your favorite & expensive equipment > | | > | - > | ^ 30 Amp +15 Vdc Zener diode ( just match the > | | diode rating with > +-------------------------+----> car chassis the fuse rating ) > That looks reasonable with a few caveats. The fuse should be smaller than the Zener, and be quick blow, since you don't want the Zener to blow first. If you want to be fancy, run the car's ignition off this circuit also, or have some other indicator, so you know when there's an overvoltage. A high tech version of a Zener diode is an LM385 (approx) voltage regulator. I used to use one to give 6V 2A out (from 12V in). It got a 2" long heat sink practically too hot to touch, but survived. -- Wm. Randolph Franklin Internet: wrf@ecse.rpi.edu (or @cs.rpi.edu) Bitnet: Wrfrankl@Rpitsmts Telephone: (518) 276-6077; Telex: 6716050 RPI TROU; Fax: (518) 276-6261 Paper: ECSE Dept., 6026 JEC, Rensselaer Polytechnic Inst, Troy NY, 12180
mberns@cygnet.UUCP (Mike Bernstein) (11/28/89)
Please excuse my lack of a diagram but if you want to buld a circuit to blow a high current fuse at a specific overvoltage it will work a lot better to use an SCR triggered by the zener. A very small SCR can blow a 30 amp fuse. You wil need a very big Zener, as overvoltage problems will tend to creep up on you and melt the zener. Also a 15V zener is too low. Keeping tolerances and temperature variations in mind a threshold of at least 17 volts is required. I can't draw the circuit but I will email a futurenet DASH4 file of it to anyone interested.
kristi@hpscdc.scd.hp.com (Kristi Bittner) (11/29/89)
I'd say an 18-20V trigger would be better - do you know how high the voltage goes when your running the alternator? Usually around 15V. Most ICs that I've seen for automotives (that want to hang right on the battery) are rated to around 28V. Kristi Bittner hplabs!hpscdd!kristi
djw@hpldsla.HP.COM (11/29/89)
>EVERYTHING electrical in the car blew out. Virtually every light bulb, all of >the computers (engine, transmission, turbo boost, etc) blew out. As a broader issue (probably better for something.misc), what did the car do without it's computer? Did it just die, was it safe? David Williams djw@hpldsla.hp.com, (djw)hpldsla/HP1900/00, (415) 857 6100
wrf@mab.ecse.rpi.edu (Wm Randolph Franklin) (11/30/89)
In <2571CD5D.238F@rpi.edu> wrf@mab.ecse.rpi.edu (Wm Randolph Franklin) writes: >In <6709@merlin.usc.edu> cyamamot@girtab.usc.edu (Cliff Yamamoto) writes: >>This got me thinking about the audio/ham-radio gear I have installed in my >>car. Can anyone confirm if the following would work: >> >> (-) 12 Vdc (+) 30 Amp >> +--- battery ---o-fuse-o--+----> to your favorite & expensive equipment >> | | >> | - >> | ^ 30 Amp +15 Vdc Zener diode ( just match the >> | | diode rating with >> +-------------------------+----> car chassis the fuse rating ) >> > >That looks reasonable with a few caveats. The fuse should be smaller >than the Zener, and be quick blow, since you don't want the Zener to >blow first. If you want to be fancy, run the car's ignition off this >circuit also, or have some other indicator, so you know when there's an >overvoltage. I just realized a serious omission in my answer - I assumed something that might not be obvious. What your proposed circuit does is this: Your favorite equipment draws an insignificant current compared to the battery's capability, so before the fuse blows you essentially have a Zener in series with the battery. Now if the voltage rises, that poor little Zener is trying all on its own to drag the battery's voltage down. Unless it's a 200A 2000W Zener (do they exist?) it'll blow pretty fast. Your circuit works, if it does, because the fuse will blow first. If the Zener blows first, you have no protection, and no indication that the protection failed. That's why I said to use a small, quick blow fuse. Standard voltage regulation circuits use a resistor in series with the Zener, and maybe use the output to control a power transistor instead of feeding straight to the load, if the load is significant. The problem with using a resistor is that all the load current goes through it so your equipment doesn't get the full voltage. It's probably better to use a regulator chip, like the LM385. Here's a crazy idea that should work, but which I haven't seen anywhere. If the voltage is high for awhile you might want to know about it even before the fuse would blow normally. If you use a big zener or regulator, and wrap them together, as the voltage rises, the regulator will get hotter and the fuse will tend to blow earlier. -- Wm. Randolph Franklin Internet: wrf@ecse.rpi.edu (or @cs.rpi.edu) Bitnet: Wrfrankl@Rpitsmts Telephone: (518) 276-6077; Telex: 6716050 RPI TROU; Fax: (518) 276-6261 Paper: ECSE Dept., 6026 JEC, Rensselaer Polytechnic Inst, Troy NY, 12180