Lm2940
StarMan
Posts: 306
I have a simple question.
What would be the harm if I powered a board that is regulated with a LM2940 with a 28 volt supply?
The spec sheet gives an input voltage range of 6 to 26 volts.
Thanks,
Chris Isaacson
What would be the harm if I powered a board that is regulated with a LM2940 with a 28 volt supply?
The spec sheet gives an input voltage range of 6 to 26 volts.
Thanks,
Chris Isaacson
Comments
·· Because that is outside the recommended input voltage, the answer would be yes.· That would be a bad idea.· Not to mention, the more voltage you put into the input, the more heat the VR will have to dissapate, and the more heatsink the VR will require.· That's bed enough when you're within the normal range of input voltages.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
·· Well, in reality, yes, over the input voltage or not, when you start getting that high, you start to experience thermal issues, which ultimately will limit the current you can draw from the regulator, because it will already be dissapating large amount of heat.
·· Even on an LM7805, which has a max input of 30V, at 24 volts input, they tend to be running quite toasty, even with a heatsink.· By the time you try drawing your 1 AMP from it, you're already experiencing thermal shutdown.· At this point you will most likely need an active cooling system (Read: Fan or Peltier).· And that's 6 volts below the max input voltage.
·· Just trying to save you from the possibility of thermal runaway, which can, and may render protection circuitry inside the regulator useless.· When that happens, you could possibly let the magic blue smoke out of everything connected to that regulator.·
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Chris Savage
Parallax Tech Support
csavage@parallax.com
Bean.
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"A problem well defined, is a problem·half solved."
·
·· Even doing that the regulator will be running hot, which will somewhat impair it's ability to handle full current draw of the device.· Will make a nice heater though!·
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Chris Savage
Parallax Tech Support
csavage@parallax.com
I hate the way they spec regulators (6 to 28 volts, 1 amp)... yeah I'd like to see that regulator disapate 23watts !!!
Bean.
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"SX-Video Display Module" Available Now.
www.sxvm.com
"A problem well defined, is a problem·half solved."
·
·· I guess I just, as a rule of thumb, try to keep my input voltages pretty close to the regulated voltage.· usually no more than 5 volts higher if I can help it.· I haven't actually had a regulator go into thermal runaway myself.· I guess I have been pretty careful.· However, one of the other Tech Support guys has described this in great detail, and I understand it's quite interesting.· I understand what's happening when it does occur, and as I said, I guess I have evaded that scenario.· Nonetheless it's important to be aware of it.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
That's 28 volts DC.· It's (to my knowledge) typical for aircraft which is where this would be going.· Although right now I can't say what this is for, it's not a critical application - meaning the plane won't go down if there is a failure.· However, I am guessing that introducing an appreciable·heat source on an aircraft would be more difficult to get approved by the FAA.
My device(s) will be placed above the ceiling panels so there is not much room for transformers, etc.· There will be a number of these distrubuted throughout certain sections of the aircraft - approximately 1 for every 10 square feet of ceiling panel.· Each draws about 40 milliamps at the regulated 5 volts.
Right now this is just in the proposal stage.
·· Okay, this is a job for a DC/DC converter.· These employ switching supplies (Typically) which can handle higher input voltages and current/volts.· Of course, these are a bit more expnsive.·
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Chris Savage
Parallax Tech Support
csavage@parallax.com
Have you built the circuit, and if so what is the minimum current that the entire circuit would use, and what is the maximum current the circuit would use?
A very BAD practice would be to put a resistor in series on the input to the voltage regulator,·however, ·you MIGHT be able to use the series resistor. It all depends on the min and max current consumption of the circuit.
The difference between the min and max current will dictate if the series resistor can be used.
Get back to us on the min and max current used, and from that information I/we can easily tell you if the series resistor will work.
A similar scheme can be used with a zener diode to reduce the 28 volts....but again we MUST know the min and max current that the circuit would use.
Ken
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Sid Weaver
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Chris Savage
Parallax Tech Support
csavage@parallax.com
Sid, I like the PT5101.· If I don't use it on this project, I see a perfect fit for a future project on the drawing board.
Chris, do you have a specific part number in mind?
Ken, my digital meter measures a steady .039 amps from a 6 volt wall supply.· I am sure there is some fluctuation but I can't measure it.· The circuit has been built and I produce small quantities now.
BTW, my 6 volt wall supply measures 9.0 volts when powering this board so a revised power consumption would be .351 watts.
Chris Isaacson
Assuming the 28 volt supply will not change......a 470 ohm resistor at 39 mA will drop just over 18 volts, leaving the input to your 5 volt regulator about 9.5 volts.
The potential problem occurs if the total current used changes (goes up 5 mA), so too will the voltage across the 470 ohm resistor..for example, if the current consumed increases by more than 5 mA, then the voltage to your five volt regulator would then be getting close to 'drop out'....
28volts - (470 * 0.044) = 7.32 volts into your regulator
OR if the circuit current drops by 5 mA...
28volts - (470 * 0.035) = 11.5 volts into your regulator
So as you can see, as the current in your circuit changes, so too does the input voltage to your regulator, and that is why an in-line resistor is usually a bad practice.
But again, if the total current in your circuit does not change very much, you can get away with the in-line resistor. If you went with a 470 ohm resistor, a 5 watt size would do. Even though the resistor would be just under 1 watt, 1 watt is too small, it would be quite toasty (have you ever run a 1/4 watt resistor at 1/4 watt and touched it???? TOASTY)
·· No I don't have a particular part in mind.· I try not to be too specific in applications like this.· Just try to help with extra useful information.· You seem to have gotten some good replies, which all should work.· Good luck!
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Chris Savage
Parallax Tech Support
csavage@parallax.com
1. Use a single 28V to 12V switching regulator (DC/DC converter) and feed its output to all the modules. That way you only have to buy one for the plane. The 28 volt supply is probably as variable as an automobile's 12V supply. It will mean you need to have a pretty good regulator.
2. Since the current draw is only 40ma, the power drop from 28 volts to 5 is less than one watt. That can be dissipated easily by an LM2940 in free air. If you are in a tight place, you need to do the thermals yourself to make sure it won't overheat. As far as dropping the input voltage, you could use a 1 Watt 10V zener diode in series to the input caps of the LM2940. That would drop 10 volts and dissipate 0.4W. Again you need to make sure there is adequate cooling for the diode.
3. Go on a website like National Semiconductor's (www.national.com) and use it to design separate small switching regulators for each module.
By the way, I was a Field Application Engineer for Fairchild and then National Semiconductor for many years and
I have not heard of an LM2940 going into "Thermal Runaway". You make them too hot, they shut down, they cool off, they turn back on. No problem - except your device was shut off for a while. Thermal runaway is usually associated with a destructive meltdown.
For more information on using voltage regulators, you can go to my website www.bluebelldesign.com and click on the Tech Center tab.
Harry