Power supply for automotive application - Page 1
Forum section: electronics / beginners

author: etnel
author_rank: Contributor
timestamp: August 12, 2021, 01:57:24 pm
content: I am designing a small project for automotive use. I want the input to be protected from reverse polarity, over-voltage, and voltage spikes. My PCB space is limited and I want the circuit to be fairly small and simple. I plan to use the LM7805 because I have a lot of them in hand. I know that the LM7805 is not the most compliant for automotive use. What improvements and changes do you suggest?

author: wraper
author_rank: Unknown
timestamp: August 12, 2021, 02:38:49 pm
content: Placing TVS on the other side of the diode so it does not act on reverse polarity at all. You can use unidirectional TVS in that case too. Doing this without a fuse may cause some serious magic smoke. C1 is too small, and there is not input filter. Don't assume that input voltage will be stable.

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2021, 04:14:55 pm
content: Forget the TVS because shunt protection without series protection will just move failure somewhere else, which is still failure.If you insist on using a 7805, then add a high voltage transistor series element before it to shield it from excessive input voltage. A depletion mode power MOSFET is a simplest way and may be used as a current limiter in combination with a TVS at the 7805 input to limit the voltage, or the transistor can be used alone by biasing it from the 7805 output so that it limits the maximum voltage at the input of the 7805. Or a normal power bipolar or MOSFET can be used with a resistor and zener diode.If a sustained overload is possible, then an over-voltage shutdown which disconnects the 7805 input will prevent overheating.

author: John B
author_rank: Frequent Contributor
timestamp: August 12, 2021, 10:30:51 pm
content: Use a series resistance connected to your 12V source as well. This will provide some additional protection for your clamping components by reducing potential peak currents, reduce the voltage drop across your voltage regulator, and provide a little RC filtering at the input of the regulator.

author: CaptDon
author_rank: Super Contributor
timestamp: August 13, 2021, 01:33:19 am
content: You will be wasting 1.4X as much power as heat in the 7805 as what your intended5 volt powered circuit will be dissipating. Perhaps a simple SMPS is a better answer?

author: bdunham7
author_rank: Super Contributor
timestamp: August 13, 2021, 04:08:28 am
content: How much current does your circuit need?You might consider using a PTC 'polyfuse', along with moving the TVS to the other side of the diode as suggested. This will allow it to withstand very high voltages and if you set it up properly, perhaps ride them through without interrruption.

author: etnel
author_rank: Contributor
timestamp: August 13, 2021, 12:26:10 pm
content: New schematic. My current order is Diode>Fuse>Resistor>TVS. Which drawbacks come with this ordering? I'm not sure which fuse to pick, the LM7805 is rated for 100mA. But the circuit won't see over a 50mA current draw.Quote from: CaptDon on August 13, 2021, 01:33:19 amSMPS is not an option in this case.Quote from: wraper on August 12, 2021, 02:38:49 pmHow would I calculate the value of the cap? And what purpose is served with the electrolytic cap? I can't find anything on the LM7805's datasheet, rather than the typical application with 330nF at the input and 100nF at the output.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: August 13, 2021, 10:53:46 pm
content: Here's an App Note from TIhttps://www.ti.com/lit/an/snva681a/snva681a.pdf