streeter@theory.lcs.mit.edu (Kenneth B. Streeter) (07/12/90)
Some of you may recall a previous query concerning building an electronic odometer for my car. (The application is TSD rallying.) Well, I've pursued a number of ideas, and still haven't really reached a decision. However, I am having power supply difficulties with one of my test circuits. In order to supply the 5V needed to drive the circuits, I am using a pair of 7805 regulators, using the car's 12V power as a supply. I have each regulator driving about half a dozen CMOS chips (uv erasable PALs, mainly) and five or six hexadecimal LEDS. (The regulators aren't in parallel, they're each driving separate sets of chips.) However, I seem to be having power supply difficulties. The regulators can just barely handle the current requirements, it seems. They get REAL hot (even with heat sinks) and can barely muster 4 volts. This makes the circuit VERY flakey. Driving the entire circuit from an external 5-volt supply (bypassing the regulators) indicates 1.26 amps of current being drawn. Basically, I'd like a trouble-free, reliable 12V to 5V converter that can push maybe 3 amps of current, and handle the switching transients in the circuit. Any suggestions? -- Kenneth B. Streeter | ARPA: streeter@im.lcs.mit.edu MIT LCS, Room NE43-350 | UUCP: ...!uunet!im.lcs.mit.edu!streeter 545 Technology Square | (617) 253-2614 (work) Cambridge, MA 02139 | (617) 225-2249 (home)
collie@ubvmsd.cc.buffalo.edu (collie) (07/12/90)
In article <1990Jul12.024412.122@mintaka.lcs.mit.edu>, streeter@theory.lcs.mit.edu (Kenneth B. Streeter) writes... >Some of you may recall a previous query concerning building an >electronic odometer for my car. (The application is TSD rallying.) >Well, I've pursued a number of ideas, and still haven't really reached >a decision. However, I am having power supply difficulties with one >of my test circuits. >Driving the entire circuit from an external 5-volt supply (bypassing >the regulators) indicates 1.26 amps of current being drawn. >Basically, I'd like a trouble-free, reliable 12V to 5V converter that >can push maybe 3 amps of current, and handle the switching transients >in the circuit. Any suggestions? ********************** Hi,Kenneth in your case the circuit is sagging the power supply.I would use a"To3"package 5 volts regulator instead of the 7805.I think with the correct regulator providing the voltage and current you might only need one To3 package for the job.The To3 package should be able to handle current close to what you want.However,I'm not sure about current output of the beast,as I don't have my stuff with me,and also make sure too filter the input line to the regulator from those nasty spikes.I used one to power about eight TTL(not CMOS mind you),and ten LED,and a display no problem. 'ps,you might need a heat sink for that beast !!' *********************** ****************************************************************************** collie. ****************************************************************************** x
grege@gold.GVG.TEK.COM (Greg Ebert) (07/12/90)
In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes: > > >Driving the entire circuit from an external 5-volt supply (bypassing >the regulators) indicates 1.26 amps of current being drawn. >Basically, I'd like a trouble-free, reliable 12V to 5V converter that >can push maybe 3 amps of current, and handle the switching transients >in the circuit. Any suggestions? > Try an LM78H05, that sucker will crank-out 5 amps. It appears that the regulator you are using is 'folding-back', ie lowering it's output voltage to keep the internal heat dissipation lower [ these guys are almost blowout proof !!]. You could also try a larger heat sink, or try dissipating the heat elsewhere. For example, you can hook a resistor in parallel with the regulator. You choose the resistor based upon the MINIMUM load cuurent and the MAXIMUM supply voltage. Ooops! This is in a car - very nasty electrical environment. You can also add some power rectifiers in series with the supply lead - they will gobble-up about 0.8 volts apiece. Four of them will get you down to 8-9 volts into the regulator, which will reduce the heat dissipated by the regulators. Be advised, though, that the diodes will be dissipating heat. You could build a DC-DC converter, but I honestly don't think it's worth the effort because you really don't have a tight power budget. The band-aid fixes I propose will work, waste a few watts, but are cheap and easy. By the way, I've hooked-up regulators in parallel (up to 4), without any trouble. My floppy drives are alive and well after 10 years of (ab)use.
brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer) (07/13/90)
In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes: > > ... [abridged] > >In order to supply the 5V needed to drive the circuits, I am using a >pair of 7805 regulators, using the car's 12V power as a supply. I >have each regulator driving about half a dozen CMOS chips (uv erasable >PALs, mainly) and five or six hexadecimal LEDS. (The regulators >aren't in parallel, they're each driving separate sets of chips.) >However, I seem to be having power supply difficulties. The >regulators can just barely handle the current requirements, it seems. >They get REAL hot (even with heat sinks) and can barely muster 4 >volts. This makes the circuit VERY flakey. I've seen several replies to Ken's question that missed the obvious solution, so I suppose I can toss my .02$ in: In the stated application, the logic ICs are the only components which require a well-regulated power supply, and, if they are indeed all CMOS, they could probably be supplied by a single TO092-packaged 78L05, with NO heatsink. The only significant consumer of current is the bank of "5 or 6 hexadecimal LEDs". Now, why insist on feeding the LEDS on regulated 5V power? I presume they're being driven by open-collector devices, so why not run them directly from the +12VDC supply? The same amount of power is wasted, only now it's ALL burned in the series dropping resistors, which, of course, must be re-sized for the 12V operation. Brian Rhodefer
jgd@rsiatl.UUCP (John G. DeArmond) (07/13/90)
streeter@theory.lcs.mit.edu (Kenneth B. Streeter) writes: >Some of you may recall a previous query concerning building an >electronic odometer for my car. (The application is TSD rallying.) >Well, I've pursued a number of ideas, and still haven't really reached >a decision. However, I am having power supply difficulties with one >of my test circuits. >In order to supply the 5V needed to drive the circuits, I am using a >pair of 7805 regulators, using the car's 12V power as a supply. I >have each regulator driving about half a dozen CMOS chips (uv erasable >PALs, mainly) and five or six hexadecimal LEDS. (The regulators >aren't in parallel, they're each driving separate sets of chips.) >However, I seem to be having power supply difficulties. The >regulators can just barely handle the current requirements, it seems. >They get REAL hot (even with heat sinks) and can barely muster 4 >volts. This makes the circuit VERY flakey. >Driving the entire circuit from an external 5-volt supply (bypassing >the regulators) indicates 1.26 amps of current being drawn. Your symptoms sure sound a lot like one or more regulators are oscillating. The pair should be able to handle 1.26 amps with room to spare if the load is partitioned well and the heat sinks are adequate. Have you hung a wideband scope on the output and looked for oscillation? I've seen these babies oscillate well up into VHF. You MUST have low ESR bypassing as near to the pins as possible. If there is no oscillation, then I agree with another poster that the problem is likely thermal foldback. You need more heatsink and/or to distribute the heat load. One trick I use is to hook several power diodes up in series with the raw power input to the regulator. Each diode will drop between 0.7 and 0.8 volts which at your current draw is about a watt. Move those watts off the regulator and onto the diodes. One thing you have to be careful with using this technique is not to drop your input margin too much or the voltage will sag during low input voltage conditions such as when you are cranking the car. I'd keep at least 7 volts on the input pin of the regulators under all conditions. John -- John De Armond, WD4OQC | We can no more blame our loss of freedom on congress Radiation Systems, Inc. | than we can prostitution on pimps. Both simply Atlanta, Ga | provide broker services for their customers. {emory,uunet}!rsiatl!jgd| - Dr. W Williams | **I am the NRA**
depolo@eniac.seas.upenn.edu (Jeff DePolo) (07/13/90)
In article <1163@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Greg Ebert) writes: >Try an LM78H05, that sucker will crank-out 5 amps. It appears that the >regulator you are using is 'folding-back', ie lowering it's output voltage >to keep the internal heat dissipation lower [ these guys are almost blowout >proof !!]. You could also try a larger heat sink, or try dissipating the >heat elsewhere. >You can also add some power rectifiers in series with the >supply lead - they will gobble-up about 0.8 volts apiece. Four of them will >get you down to 8-9 volts into the regulator, which will reduce the heat >dissipated by the regulators. Be advised, though, that the diodes will be >dissipating heat. Both good ideas for quick and dirty fixes. I'll throw in another one. Buy both a 78H09 and a 78H05 and run them serially. It's easier to mount and heatsink two regulators than worry about dissipation from the power diodes. Cascading the '09 and the '05 mounted with decent heat sinks should be able to provide 3 amps without a problem. I have a cheap-and-dirty 13.8 to 5 converter in the shack for powering small digitial circuits just made up with a 78H05 mounted right to the back of a small (2"h x 3"w x 2"d) aluminum box with just a heat sink pad (whatever those things are made of) with no heat sink and I can run it at 2 amps continuous (1 hour+) and it gets slightly warm (probably around 125 deg. F), which isn't all that hot for a regulator. It's been in service for a couple of years and I haven't baked it yet. The cost of a 78H05 is about $2.50, so what the heck. --- Jeff +-----------------------+-----------------------------------------------------+ | Jeff DePolo N3HBZ/AA | Internet: depolo@eniac.seas.upenn.edu | | Univ. of Pennsylvania | RF: 146.685- 224.40- 442.70+ 144.455s (Philadelpia) | | Computer Science Eng. | Twisted pair: (215) 386-7199 home | | Class of 1991 | Carrier pigeon: 420 South 42nd St. Philly, PA 19104 | +-----------------------+-----------------------------------------------------+
strong@tc.fluke.COM (Norm Strong) (07/14/90)
In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes: } }Some of you may recall a previous query concerning building an }electronic odometer for my car. (The application is TSD rallying.) }Well, I've pursued a number of ideas, and still haven't really reached }a decision. However, I am having power supply difficulties with one }of my test circuits. } }In order to supply the 5V needed to drive the circuits, I am using a }pair of 7805 regulators, using the car's 12V power as a supply. I }have each regulator driving about half a dozen CMOS chips (uv erasable }PALs, mainly) and five or six hexadecimal LEDS. (The regulators }aren't in parallel, they're each driving separate sets of chips.) }However, I seem to be having power supply difficulties. The }regulators can just barely handle the current requirements, it seems. }They get REAL hot (even with heat sinks) and can barely muster 4 }volts. This makes the circuit VERY flakey. } }Driving the entire circuit from an external 5-volt supply (bypassing }the regulators) indicates 1.26 amps of current being drawn. }Basically, I'd like a trouble-free, reliable 12V to 5V converter that }can push maybe 3 amps of current, and handle the switching transients }in the circuit. Any suggestions? Boy, this is your lucky day! What you need is a National Semi LM2575-5 switching regulator. They're sampling them now; call your local NS rep. -- Norm Strong (strong@tc.fluke.com) 2528 31st S. Seattle WA 98144
phil@brahms.amd.com (Phil Ngai) (07/14/90)
In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes: |In order to supply the 5V needed to drive the circuits, I am using a |pair of 7805 regulators, using the car's 12V power as a supply. I |have each regulator driving about half a dozen CMOS chips (uv erasable |PALs, mainly) and five or six hexadecimal LEDS. (The regulators |... |Driving the entire circuit from an external 5-volt supply (bypassing |the regulators) indicates 1.26 amps of current being drawn. 3 amps times 7 volts drop = 21 watts. I don't think you can do that with the To-220 case the 7805 comes in, especially without a heatsink, which you didn't mention you were using. You'd have a better chance with a To-3 but you still need to put some effort into heatsinking. This might be overkill but way back in 1979 I built a DC-DC switching converter to go from 5 to 12 volts. It worked very well and would avoid this heat dissapation problem you have. Lower temperatures== better reliability. There's a lot of single chip solutions available now. I did wind my own toroidal transformer but that wasn't hard. -- -- Phil Ngai, phil@amd.com {uunet,decwrl,ucbvax}!amdcad!phil PALASM 90: it's not the same old PALASM any more!
logan@titan.tsd.arlut.utexas.edu (Cary Logan) (07/14/90)
Streeter, It is my impression that automotive voltage levels often exceed 12 volts. My guess is that it could reach 16 to 18 volts while the vehicle is running. This would mean that at 3 amps you need to disapate more that 30 watts in your power supply. I strongly suggest that you consider a dc-dc converter. Interpoint makes a wide variety of dc-dc converters, I'm sure they could sell you one to meet your needs. Their address is: Interpoint 10301 Willows Road Redmond, WA 98052 (800) 822-8782 Order and Applications Hotline I have no idea what their prices are like, but they will certainly be more expensive than linear regulators. An alternative would be to check Radio Shack. I belive they have voltage converter that would plug into the lighter socket and supply you with either 3, 5, 9, or 12 volts out. I don't recall what kind of power they they can deliver. I beleive that they cost on the order of $10.00. -- Cary Logan logan@titan.tsd.arlut.utexas.edu ARL:UT/TSD P.O. Box 8029 10000 Burnet Road Austin, TX 78713 (512) 835-3118
depolo@eniac.seas.upenn.edu (Jeff DePolo) (07/14/90)
In article <887@titan.tsd.arlut.utexas.edu> logan@titan.titan.tsd.arlut.utexas.edu (Cary Logan) writes: >It is my impression that automotive voltage levels often exceed 12 volts. My >guess is that it could reach 16 to 18 volts while the vehicle is running. The supposed standard is 12 VDC with no load on the battery - vehicle not running, 13.8 VDC with no load on the battery - vehicle running, both values +/- 15%. However, there is probably quite a few cars out there that fall outside of this spec range. Most car electronics manufacturers realize the problem and have some sort of voltage regulation built in, particularly car stereos, radar detectors, portable TV's, etc. I'd suggest checking your electrical system with a volt meter with the car running and not running, under heavy load (e.g. lights on, heater fan running, rear defroster on), under no load, etc. to make sure it falls within specs. If it's higher than normal, expect more heat dissipation from the regulators. --- Jeff +-----------------------+-----------------------------------------------------+ | Jeff DePolo N3HBZ/AA | Internet: depolo@eniac.seas.upenn.edu | | Univ. of Pennsylvania | RF: 146.685- 224.40- 442.70+ 144.455s (Philadelpia) | | Computer Science Eng. | Twisted pair: (215) 386-7199 home | | Class of 1991 | Carrier pigeon: 420 South 42nd St. Philly, PA 19104 | +-----------------------+-----------------------------------------------------+
macminn@powertool.crd.ge.com (Stephen R MacMinn) (07/16/90)
Please note that the 12V automotive bus has to be one of the WORST regulated voltage sources there is. If you're planning to interface your voltage regulator to it you'd better add some front end protection such as an MOV. Transients on this 12V bus come in all varieties, the worst ranging up to 200 Volts (with relatively low energy content). In fact there is a whole spectrum of defined transients, decreasing in energy as they increase in voltage. So unless you want to see some unexplained failures of your power supply, best to design some protection in from the start.
tomb@hplsla.HP.COM (Tom Bruhns) (07/17/90)
>Boy, this is your lucky day! What you need is a National Semi LM2575-5 >switching regulator. They're sampling them now; call your local NS rep. But rated at (alas) 1 amp...