Revers Polarity Protection

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MrAl
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Post by MrAl » Wed Aug 22, 2007 2:55 pm

Hi again,

philba:
If his common is *already* connected to the power supply then
there is nothing that will help.
Since he said he has to *connect* the power supply and doesnt want
to get the polarity backwards, i think we can assume that what he
means is that the common is connected to the circuit ground already
and the power supply isnt connected to anything until it's time to run.
In this case, i think the bridge rectifier is the best idea if the voltage
drop (say around 1.5 volts total) will not hurt the circuit being powered.
LEDs vs Bulbs, LEDs are winning.

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Chris Smith
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Post by Chris Smith » Wed Aug 22, 2007 4:54 pm

Its ambiguous and a conflict to say in one PS, diodes are bad, but in the next they are the cure?

In both circumstances they may cure the problem, or perhaps not, but as a learner all options should be on the board, and tried for the learning experiences of building that project.

Yes and No leaves mixed message for the learner, but a simple try will teach the learner all sort of plus’s and minus’s on this subject, and for future projects.

In his particular situation, like my timing light, we had the power to run the device, [Car Battery] and we had zero chance of mixing up the leads.

And when you hook things up in the dark where the timing light works best, it’s a real plus.

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haklesup
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Post by haklesup » Wed Aug 22, 2007 5:20 pm

I don't see a conflict, the two examples are noticably different applications of the component.

In this case the diode was used to block DC current when the power is connected properly and short it when it was not. In this case it is used strictly as a rectifier, blocking current one way.

In the dual LDO example, you wanted to forward bias the diode all the time to achieve equal voltage drops at the output both regulators in order to achieve load balancing. While a diode would work here it has drawbacks such as a voltage drop more than one would want at a power supply output plus a difficulty guaranteeing that both would have the same drop at a particular bias (think about how you're not supposed to parallel a bunch of LED without a current limiting resistor or you may get current hogging and one will be brighter than the others). In this application, the component is used as a load which IMTO (IM technical O) is an inefficient use of this component.

For example it is just as inappropriate to use a 5V zener to turn a 12V supply into a 5V one by using it as a clamp on the output. It may work if the supply didn't have too much power but it would be insanely inefficient. They should be used in conjunction with a current limiting resistor to create a low current voltage reference to be applied to a high impedance next circuit stage (in the most typical applications of).

There's nothing wrong with experimenting to determine a solution. IN most cases I'm just trying to offer a technical critique of the application (pros and cons) suggested but you get so defensive. In the former, I wasn't the first to be puzzled by the use of a diode but I was the only one who offered a comprehensive explanation of what was happening at the component level.

Fine experimenting but I think that backing up those experiments with a sound theory of why that will work is better than just trying stuff untill the smoke stops flowing. If you offer an atypical solution, you should offer an explanation of why that should work or it appears it was just a guess. If one can contrast that against other solutions, all the better.

Designing through experimentation alone can (not always will) lead to designs that are inefficient or unreliable. If one understands why a diode is appropriate or not for a praticular application and what are the important parameters for selecting such a component isn't that better than just blerting out "use a diode" without even suggesting how to connect it or what it will do to the signal.

If you put out an incomplete or incorrect answer and I know the complete or correct answer, I am going to offer it. If I want to add information to the topic I will, and it may conflict with your answer from time to time and if you choose to be sensitive about those comments, then that's fine with me. I'm not just flaming for the sake of it.

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philba
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Post by philba » Wed Aug 22, 2007 5:52 pm

MrAl wrote:Hi again,

philba:
If his common is *already* connected to the power supply then
there is nothing that will help.
Since he said he has to *connect* the power supply and doesnt want
to get the polarity backwards, i think we can assume that what he
means is that the common is connected to the circuit ground already
and the power supply isnt connected to anything until it's time to run.
In this case, i think the bridge rectifier is the best idea if the voltage
drop (say around 1.5 volts total) will not hurt the circuit being powered.
a lot of assumptions there al. and in his picture, the other equipment doesn't show a connection to power at all - just a serial connection. heck, we don't even know the environment. But hey, it's not my gear...

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Chris Smith
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Post by Chris Smith » Wed Aug 22, 2007 6:20 pm

In the hundreds of examples of using the diodes to block any cross fire back to a parallel circuit in a VR, I never had any problems. [heat yes]

It wasn’t like you ramp it up, and one VR conducts, the other is delayed, it was more like on and off, while both were protected from interference and cross fire talk.

I have used dropping resistors as well and often they work fine, but when two parallel pieces of equipment start talking when they shouldn’t, and screw up the whole enchilada, diodes are the choice.

Just like the digital probe, often a circuit gives you unwanted frequency or feed back that stops the other circuit dead in it tracks.

One digital I have hates running transistor checks, the other doesn’t care?

Often a piece of tape solves that.

I have even had frequency run up the digital VOM leads, and like lightning, make toast of the VOM even when it should have been completely protected and isolated. [20kv]

That’s were the diode can help,.... or not.

I have lost three VOMs this way in the last twenty years.
Flukes etc.
Unwanted, unknown harmonics out of the blue.

After one meter fried, I opened it up and repeated the test.

The entire PC board was a blue hue from some form of HV and voltage feed back despite the fact the over all voltage under test was low.

My guess was there was more HF more than just HV, and it walked like a skin effect past it all.

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