protection circuit is diodes the best way?

[porthill]

All,
I am getting back into electronic design after being out of if for a while. I have designed a voltage regulator circuit and it works well as designed, and is a power supply. about 24VDC and could pull as much as 5A. I need to be about to protect it from someone connecting the terminals in backwards. I have an LED that will come up when the system is working properly. Are diodes the best way?

 

[KMoffett]

Please post a schematic.

Ken

 

[porthill]

KMoffett,
here is the schematic that is connected to the bus. There is some other circuits down stream from this on but I figured if I could protect this one then the rest will also be protected.
Thanks for the help...
View attachment 63854

 

[KMoffett]

A single diode in series with the (+) input would prevent damage due to reversal of input polarity. A low Vf Schottky diode would be the most efficient. There are active polarity-reversing protection circuits with almost no voltage drop, but for your boost circuit it would be a waste.

Ken

 

[MrAl]

KMoffett,
here is the schematic that is connected to the bus. There is some other circuits down stream from this on but I figured if I could protect this one then the rest will also be protected.
Thanks for the help...
View attachment 63854

Hi,

You say you want to protect it, but you dont say whether or not you want to protect the input, output, or both. You show a diode on the output so i am guessing the output but you should specify anyway.

 

[porthill]

My goal is to protect the circuit from a user error. The most likely one will be connecting the positive terminal of the battery to gnd of my circuit and the gnd terminal of the battery to the vin of my circuit. I am engineering controls into the installation instructions but they are not always followed as they should be and don't want to be replacing the board all the time.
Thanks!

 

[porthill]

I tested a shottky diode in this configuration and it did not protect my circuit when I reverced my input terminals.
View attachment 63857

 

[RCinFLA]

Just about all IC's will take a negative supply of 0.7 vdc. You can use a large amperage rated diode but you need an input fuse that will blow when the diode conducts due to reverse polarity. The diode amperage rating needs to be higher then the fuse amperage.

Most DC-AC inverters use this approach to protect against reverse polarity connection.

 

[KMoffett]

I tested a shottky diode in this configuration and it did not protect my circuit when I reverced my input terminals.
View attachment 63857

So what happened when it did "not protect my circuit "?

I had suggested that you place the diode in series with the (+) input, not in series with the ground. With the diode in the ground input, the output ground will be ~0.3V more positive than the input ground. This will be a problem if any of the load circuits needs to have a common ground with the 24V input.

Ken

 

[porthill]

Ken,
what happened was that it fried the the vreg chip.
I know that you said to put it series with the + input. I have not had a chance to test that. I am wondering how that will work to protect it? Do you think it needs a fuse? If so what would be the configuration. I really appreciate this I am trying to learn it has been almost 10 years sence I have done circuit design.
Thanks!

 

[Lund_Pa]

This works good and I have used this approch a few times. Place Diode across input see attached if connected backwords F1 blows,

 

[porthill]

is this is a shottky diode is that okay? or what diode would be the best?

 

[unclejed613]

what was the reverse breakdown voltage of the schottky diode? mbrd835l reverse voltage is 25. try something with a higher reverse voltage, and put it on the positive lead.

 

[jimy245]

which kind of protection you need,the schematic you provided is about voltage regulator,either you want over voltage protection or diode polarity ?

 

[KMoffett]

I can't see how your regulator could have been smoked...if you connected the actual circuit as you indicated in your schematic.

Ken

 

[MrAl]

Hi,


There is a chance that leakage current from the diode caused a problem, but this usually only happens when the circuit to be protected has relatively high impedance for minus inputs. Just in case that happened, the solution is two diodes: one to protect against reverse polarity, and the other to protect against the first diode negative leakage current with reverse polarity.

   +  o-----D1*-----+------o to regulator + input
                    |
                    |
                    *
                    D2
                    |
                    |
   -  o-------------+------o to regulator - input 


    * Asterisks mark cathodes

The other type of solution, the diode across the input with fuse trick, has to be applied very carefully because the PC board traces leading to the diode can actually vaporize while the fuse is in the process of blowing open. I dont think that should be needed here anyway as that should only be necessary when there are two power sources that can connect to the circuit such as a regular input and a battery (as in a battery charger). The series diode solution doesnt work on the output of the battery charger, so a diode across the output has to be used, with the proper precautions of course.

Another cute trick that is used:
Provide the end user with a switch that connects the input power to the input of the regulator circuit. Before the input power is turned on, the switch is moved to the 'open' position. Also connect an LED with resistor across the input before the switch, preferably a big red LED, that only lights up when the polarity is reversed. Provide another green LED that only lights up when the polarity is correct. The user, seeing the green LED is lit, then turns the switch to the 'on' position. If the user sees the red LED lit, he reverses the input polarity and checks again for the green LED.
This solution is used when very high currents are involved so that no diodes with their wasteful voltage drop need to be used. Only the switch has to be rated for the whole current, and only then in the 'closed' position (it doesnt have to actually switch high currents, just be able to conduct them).
This solution can save lots of power and avoid having to use very high current diodes but it does rely on the correct operation by the end user. If this is a problem, a relay can be used instead with the coil powered from the input supply voltage and a diode to allow only one polarity to energize the relay coil.

 

[RCinFLA]

With up to 5 amps of current, putting a series diode would not be a good idea. Beside the extra voltage drop the diode will have 0.8v to 0.9 volts of drop resulting in about 4 watts of heat to dissipate with a sizable heat sink.

You won't gain much in voltage drop reduction at that current with a Schottky diode as the series resistance of this type of diode is higher. You are also getting near the breakdown voltage of a single diode layer. Higher voltage Schottky's are stacked diodes which defeats any voltage drop improvement right off. Primary advantage of Schottky is high speed reverse recovery which is not needed in this application.