Robust power supply design

Most regulator lines now have "HV" variants designed largely for automotive applications. The 7805 expires at 35 or 40V in. The HV variants are usually rated at more like 60V.

All that said, the 7805 itself is a dime. Maybe you want to worry about protecting the load, rather than protecting the regulator?

Start with a 1.5 amp fuse in series with a reverse prevention 3 amp diode. If low voltage drop put is not a worry, add a 2.2 to 4.7 ohm 5 watt resistor in series to take some of the heat load off the regulator. Replacing the resistor with a 1 amp capable series inductor like 2124, 1000 uH, .4 ohms, down to 559 uH at 1.3 A:

or for the deluxe model

5724, 4000 uH, 1.16 ohms, down to 2100 uH at 1.75 A: Will make the regulator much more immune to spikes and noise that might leak through the regulator.

Then tie to ground with a 1 watt 18 or 22 volt zener in parallel with the input cap (25 volt 1000 uF electrolytic and 1 uf or so of film or ceramic in parallel). Most of the spikes are not low impedance and are very short duration.

This kind of regulator will survive almost anything but a direct lightning strike.

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John Popelish

Apart from using the HV variant of regulators, perhaps clamping Z diode with a series polyfuse (PTC) towards the input.

Rene

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Ing.Buero R.Tschaggelar - http://www.ibrtses.com
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How about something like this to protect the LM7805's input side?

Unitrode? ST MUR1660 5 A Fuse --->!-- ST901T Battery --O-O--------+ +-+------ ---+--- To LM7805 --->!-- ! \ /e ! ! ------ ! / ! --- \ \ --- 10U /1K / ! 50V CDE \100W \ GND ! / ! ! 100R +--------+ 50W ! ! --- /---/ ---50V ^ 5KP20A ! 100u ! Rectron ! CDE GND GND

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kensmith@rahul.net   forging knowledge

I interpret that to mean the diode is shunted in reverse.

If the input is a few volts higher than the regulator's minimum input, then why not put that diode in series and eliminate the fuse, so you get the reverse protection without having to replace fuses?

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No. I meant that the fuse and diode were to be in series between battery positive and regulator input. For a 5 volt regulator running on 13.8 volts nominal, the volt lost across the diode just reduces the regulator heat and the fuse is there to keep the wiring from burning (or some component that might get red hot and start a fire) if the zener shorts. There may be something like a 20 amp fuse feeding the battery positive connection inside the automotive harness.

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John Popelish

Google for "Load Dump" and you will find Automotive waveforms. The really big stuff doesn't last long. Say 80V peak for 20 ms and 36V for DC is a good target.

The rule of protection is series - shunt. Use a Tranzorb or zener or MOV in shunt.

For good protection at currents under an amp, a couple of ohms power resistor in series with a 36V Tranzorb in shunt.

Here is one interesting setup which lets you get an amp from 12 V electrics but is rugged. Say a S20K17 MOV from EPCOS in shunt. This MOV will drag a hundred amps as it holds down the volts. Now you need some series impedance. A Polyswitch is one choice. If you glue the polyswitch onto the MOV it will open if the MOV gets hot for sustained voltage above 36 V or so. Use the HV version of your 7805 regulator. You may need some series resistance to control the current the Polyswitch has to open.

For reverse polarity you can do a diode in Series or you can do Shunt if you have a Polyswitch.

The EPCOS range of AUTO polyswitches can absorb a full load dump with no series impedance !

Roger