Supply 12V auto V to a 7805 regulator

[stuhagen]

I am installing a digital LED Volt Meter to monitor Auto 12v voltage and this unit requires 5V source. I was going to use a 7805 with simple 2 caps. But I hear that a 12V source (car V) is a bit too high of an offset for a 7805, that it should be a few volts from the output (5V) I dont think this Digital Voltage display is all that high in quality, and obviously doesnt draw that much current. Will a standard 2-cap 7805 work? or will I fry the 7805 because of the large V offset. I also read that a filter might work, but I am really thinking simple hear.

TIA

Stu

 

[dknguyen]

12V is the limit for most low-dropout linear regulators. BUt the 7805 is not a low-dropout device and requires an input about 2V higher than the output (varies with load current).

THe 7805 can actually handle up to ~37V input. HOwever, what limits it is usually heat. How much current do you need to supply? Because 12V-5V = 7V is being converted to heat for the regulator to do it's job. So if you draw 0.1mA, then 0.1A*7V = 0.7W is being dissipated by the device. At around 50C/W, this means the device would have a temperature rise of about 35C above ambient. At 0.2A, the temperature rise is twice that. So it adds up very quickly, especially with higher input voltages.

With a something like a 7V input, only 2V are being burned off as heat. So it would take 0.35A to get the same 35C increase.

 

[Speakerguy]

Yes, a standard 7805 will work.

It should be fine if it doesn't draw too much current. If it gets hot, add a cheap clip-on heat sink.

 

[stuhagen]

Yes, a standard 7805 will work.

It should be fine if it doesn't draw too much current. If it gets hot, add a cheap clip-on heat sink.

This is good to know. I cant imagine this thing drawing hardly anything. It is a 1-3/4"x1" x 1/2" thick 3-digit display LED/LCD digital readout. It can read 0-99v DC. It states that it requires 100% DC for it to operate. Whatever this means......I wonder how sensitive it is for the supply V. Hopefullly +/- 0.5v I would think.

 

[ericgibbs]

This is good to know. I cant imagine this thing drawing hardly anything. It is a 1-3/4"x1" x 1/2" thick 3-digit display LED/LCD digital readout. It can read 0-99v DC. It states that it requires 100% DC for it to operate. Whatever this means......I wonder how sensitive it is for the supply V. Hopefullly +/- 0.5v I would think.

hi stu,
Ideally the 7805 requires a 100nF [0.1uF] on the input and output pins.
Also recommended are electrolytics on the input and output, say about 100uF thru 470uF at 25V working.

It would be advisable to connect a 1N400x diode in series with the 7805 line in order to protect against reversed supply line.

 

[Ubergeek63]

you need an automotive rated part if you want good reliability:

**broken link removed**

Dan

 

[Noggin]

Use an LM2937. Its basically a 7805 made for automotive purposes.

 

[q5101997]

Bolt it to your chasy

As dkngyen said, the greater the voltage drop from Vin to Vout, the greater the power dissipation from the regulator (heat is P=IV). It is very similar to a resistor. Put some sort of heat sink on it, and you shouldn't have a drama. The metal tag is ground, so bolt it directly to your chasy (if it's grounded).

 

[chrisjpitt]

You should be able to use a 7805 to provide a reasonably clean 5V in an automotive application - as long as some sensible precautions are taken. Back in the good/bad old days, National Semiconductor produced a book called “The Voltage Regulator Handbook” – this was full of useful information regarding using linear regulators in all sorts of applications. I am not sure if they do a web-based version these days, but it’s worth a Google?

Some protection against reverse polarity spikes would certainly be good (due to starter motor spikes and alternator load dumps). This would also protect against a reverse connected battery - but killing your LED/LCD module would be the least of your worries if this happened! Not just reverse polarity voltages are a problem, but reverse current flow through a regulator can also kill it. This can happen if the input voltage is shorted (or falls rapidly) and there is a reasonable amount of capacitance on the output circuit. Protecting against this can easily be done, as mentioned, with a 1N400X diode in the input circuit to the regulator (before the input caps, not between them and the 7805).

Filter caps – as a general rule, 2200uF per amp drawn is a good guide for the electrolytic input capacitor, bypassed with a 0.1uF in ceramic (or mylar) close to the regulator pins. Regards voltage, 25V will probably be enough, but I’d go for 35V in a 12v volt automotive application, if physical size is not an issue. Some application notes mention using small value tantalums close to the pins, although they are high performance caps, I’d be wary of them in circuits that may have voltage surges or spikes - as they are prone to going short-circuit if over-voltaged. Regards the output cap, a similar 0.1uF close to the pins in required, but do not put a large electrolytic on the output circuit, as this can lead to the regulator becoming unstable. If the LED/LCD module requires a certain value cap, then put that on the module supply pins (I’d fit a 0.1uF regardless, nothing is advised).

Does it need a heatsink? As the input voltage is more likely to be around 14V (rather than the nominal 12V of the battery), the regulator would be dropping around 9 volts. So probably not, if the current drawn by the module is under 100mA (although, at 100mA, it would run very hot with a Rthj-amb of 50-60C/W). Large pads and thick tracks around the regulator would help here in keeping the temperature down.

 

[creakndale]

You should calculate and provide adequate power dissipation based on voltage delta and maximum current draw (if something fails then hopefully a fuse would blow). A 3-digit "LED" display could require 20mA per LED segment. Being "3-digit", does the display have the potential to display "888"? Are there decimal points? Each "8" may be comprised of 7 individual segments each drawing 20mA when lit. So each digit could be 7 segments x 20mA = 140mA. Multiplied by 3-digits = 420mA. 14V - 5V = 9V x 0.42A = 3.8W.

This doesn't include any LED decimal points or operation of the ADC.

If the display is "LCD", then disregard the above.

creakndale

 

[geko]

If you use an LM2937 it needs a 10uF or greater capacitor on the output otherwise you may get high frequency oscillation on the output.

I speak from experience from using one as a substiture for a 7805 in an automotive application. 'Scope showed about 1 volt of noise on the 5 volt output until I put 22uF capacitor across it.

 

[stuhagen]

If I use my existing circuit where I am installing this V Meter, it already has a 47uf cap on the 12v input. So all I need is to determine the size of the output cap. Sounds like anywhere from a .1uf to a 22uf. I will research this LM2937 to see if this is a better choice over the 7805. I am sure this display is a LCD, and not individual LED's. Heck....it only cost like $15 of ebay~!

Thanks for all the inputs, it is always appreciated.

Stu

 

[eng1]

IfSo all I need is to determine the size of the output cap. Sounds like anywhere from a .1uf to a 22uf.

If you'll use a LM2937, a critical parameter of the output capacitor (>10 uF) is the ESR. Choose a capacitor with a known ESR (provided in its datasheet); the acceptable range of ESR is plotten in the datasheet of the LM2937.
Also consider that ESR strongly depends on temperature. Tantalum capacitors have a more stable ESR vs. temperature. I like the solution suggested in the datasheet:

Solid tantalum capacitors have a more stable ESR over temperature, but are more expensive than aluminum electrolytics. A cost-effective approach sometimes used is to parallel an aluminum electrolytic with a solid Tantalum, with the total capacitance split about 75/25% with the Aluminum being the larger value.

 

[audioguru]

First you talked about an LED display that uses a lot of current for its 27 LEDs.
Then you talked about a low current LCD that needs a high current backlight.

Decide on which one, look on its datasheet for its max current then calculate how hot will its voltage regulator get.

 

[stuhagen]

First you talked about an LED display that uses a lot of current for its 27 LEDs.
Then you talked about a low current LCD that needs a high current backlight.

Decide on which one, look on its datasheet for its max current then calculate how hot will its voltage regulator get.

ha, ya.....I am aware of this missing info. I have not received these as yet, and this info is unavailable on there website. As soon as I get them, I will post up on the design and current consumption.

Stu

 

[audioguru]

Don't buy garbage that doesn't have a datasheet.

 

[BeeBop]

Don't buy garbage that doesn't have a datasheet.

This is some of the very best advice! It should be in the header at the top of the page...

 

[davew]

12V is the limit for most low-dropout linear regulators. BUt the 7805 is not a low-dropout device and requires an input about 2V higher than the output (varies with load current).

THe 7805 can actually handle up to ~37V input. HOwever, what limits it is usually heat.

Yes, A couple of weeks ago I was playing with a 7805.
Using a DC bench supply I had 13.8V to a 7805 with a small clip-on heatsink (no other components) with a 10Ω load.
So I got an output of about 4.9V at 500ma, I measured the case temperature of the 7805 which was about 80C. I increased the input voltage up to 20V and the output voltage of the 7805 quickly went down to about 2.4V, as the 7805 case temperature went up to 123C !
Which is interesting as the maximium junction temperature of the 7805 is stated as 125C. when blowing on the 7805 to cool it down the output voltage went up.

 

[audioguru]

The 7805 is supposed to shutdown when it gets too hot. Maybe yours is bad or is a counterfeit one.

I think it was at the hairy edge of shutting down.

 

[shokjok]

Based upon what has been written, a run-of-the-mill 7805 should suffice for this application. If stuhagen is worried about current draw and/or overheating the 7805, a TIP41/ TIP42 bypass transistor with a 1.5-ohm/ 5W power resistor connected to the transistor base and voltage supply input will help ( piggy-backed to the 7805). The transistor will begin conducting when the circuit draws more than 500mA, and both the transistor and 7805 will get warm but not too hot for this circuit. Both will fit on the same heatsink.
I can't imagine a 3-digit voltmeter drawing more than 300mA when active, even if the main component is an ICL7107A/ 7106A, unless it has large displays.