Test fixture has protection diodes for each internal wire why?

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ParkerMike

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I was looking at schematics of test fixtures and i have notices there is a diode placed in series for each internal wire. These series diode for each wire is it protect the wiring from getting to hot when there is a short or over voltage.
The test fixtures have a patch bay jacks or break out box connections. The series diode protects the wiring and components inside the test fixture in care of patching the wrong jacks together in different configurations. Also the diode protects the test fixtures wiring when there is a short circuit or shorted component on the circuit board under test

How do these series diodes work? when there is a short or when an older voltage is applied to them?

The wiring must be DC voltage if you have a Diode placed in series, but when you put a higher or lower DC voltage on both sides of a Diode in series it will open or short?

When connecting the patch bay jacks in wrong configurations the diode has DC voltage on both sides of it
or a shorting the DC voltage depending of the wiring of the patch bay jacks you patched in a wrong configuration

What are these types of diodes called for these types of protection?
 

Please learn how a diode works.
It conducts (it could pass a very high current) when it is forward biased and it DOES NOT conduct any current when it is reverse biased.
Since it is in series then it cannot limit current and it cannot prevent over voltage but it can prevent backwards polarity.
It is frequently called an idiot diode because it prevents damage when an idiot connects the power polarity backwards to a circuit.
 

Since it is in series then it cannot limit current and it cannot prevent over voltage but it can prevent backwards polarity.
Click to expand...

How can it prevent backwards polarity?

If the Anode of the diode has +12 volts and An idiot connects the cathode to +15 or -12 volts , what will happen?

Can a diode in parallel limit current and prevent over voltage? or only zener diodes?
 

Only special diodes called 'constant current diodes' can limit current, normal diodes can't.

Zener diodes in parallel may limit the voltage by drawing current but their safety relies on the current source being small enough that they ar not damaged.

You are missing the point about series protection diodes, they are not set to a voltage at one end, in a test fixture the thing being tested goes to one end and the measurment system goes at the other. Given the correct polarity, the measurment system does the test, if the polarity is reversed, it is isolated and the lack of measurement should indicate a fail.

Brian.
 

ParkerMike said:
How can it prevent backwards polarity?

If the Anode of the diode has +12 volts and An idiot connects the cathode to +15 or -12 volts , what will happen?
Click to expand...
When the anode has +12V and the cathode has +15V then is the diode forward biased so it conducts current or is the diode reverse biased so it conducts nothing??

When the anode of the diode is +12V and the cathode is -12V then is the diode forward biased so it shorts +12V to -12V or is the diode reverse biased so it conducts nothing??

Have you ever seen an electronic circuit connected to a power supply with backwards polarity? It burns or smokes. Electrolytic capacitors blow up like bombs.
When the diode is reverse biased so it conducts nothing then it is protecting the circuit from backwards polarity.
 

 


 

If "every" wire has a series diode then this would be one of
the most useless test fixtures I've ever heard of. I'm thinking
maybe these are some sort of "long wire" marker like -->>--
or something, like mechanical drafters use to make long boring
features fit on the page.

Maybe you could post a pic so we know what you're talking
about.
 

On rare occasions I've seen diodes wire-OR'ed to test points when only the presence or absence of voltage is being checked. In that case the diodes are to allow conduction to the measurement point but block connection from one test point to another.

Your schematic would clarify matters.

Brian.
 

On rare occasions I've seen diodes wire-OR'ed to test points when only the presence or absence of voltage is being checked. In that case the diodes are to allow conduction to the measurement point but block connection from one test point to another.
Click to expand...

Yes I have seen this too

In some manuals i read they are called blocking diodes , i guess this is the right term from it

Yes each jack on the test fixtures patch bays has a series diode or block diode

I still don't understand how it blocks the reverse polarity.

Because some of the patch bays jacks are outputs and some are inputs. The outputs have DC voltage or AC voltages. If you mixed two outputs it will blow the series blocking diodes. If you reverse the polarity how so? If you wrongly connect an output to an input on the patch bay it will blow the series blocking diode.

The Series blocking diodes protect the wires inside the test fixture from getting hot and starting a fire when mixing different voltages hooking the patch bay up wrong?

If there is a short circuit on the board under test the series blocking diodes will blow open?

Each Plug on the test fixtures has multi pins 30 pins per cable. So the patch bays has 30 jacks for each plug that goes to the board under test.

If there is a short circuit on any of those pins , a series diode will blow open. How does this series diode work when there is a short on it?
 

You are talking about a "test fixture" with a patch bay and jacks, it is highly unlikely the voltages or currents will be anywhere near high enough to cause damage. I have designed and built may test fixtures, some with as many as 3,000 test points and design philosophy dictates you never do anything that can overstress the device under test or the test equipment itself.

If you need a component that would 'blow open' to protect something, you use a fuse not a diode!

Brian.
 

Brian here is the pictures of the diodes that directly to each jack on the patch bay of the test fixture. The diodes have 3 different configurations

I'm not sure what these configurations are and why they used them this way, but maybe you can tell me



 

Still difficult to be sure without more complete schematics but I would guess they are being used as steering diodes. That is nothing whatsoever to do with protection!

I think what you are seeing is a wiring checker, it feeds voltages in through the diodes and looks to see where the voltage reappear at the output. When done in the right combinations it can be used to test that none of the wires are shorted, open or connected to the wrong jacks. The DC supplies are routed by the diodes so each input feeds voltage to one or more jack circuits. The other diodes route the voltages from the jacks to the measurment device.

For example, and that's all this is, in your case the routing will be different:

Feeding voltage in at point A should produce voltage out at points B and C.
Feeding voltage in at point B should produce voltage out at D, E and F
Feeding voltage in at E should produce voltage out at A, C and G.

Any missing voltages or ones on other outputs would indicate a wiring error.

I could be wrong but I'm sure they are nothing to do with any protection system. Using diodes that way can dramatically reduce the number of connections needed to do the tests. In the worst case scenario, without diodes you would need one DC supply for each input and one measurement system for each output.

Brian.
 

'Steering' isn't a type of diode, it's a description of how an ordinary diode is used. The idea is to use them to create "one way" routes to conduct the higher voltage from the supply to a lower voltage measurement point. They allow the current (and hence voltage at their output side) to follow a determined path but prevent it flowing backwards, away from the path, down another route.

Try picturing it this way: if you wanted to test the wiring in a system you could simply join all the inputs together, provide a voltage at them and check the voltage is conducted to all the outputs. That would confirm that there were no breaks in the wiring but would not be able to tell you if any of the wires were shorted or connected to the wrong output. By using diodes to selectively route the voltage it becomes possible to isolate individual paths through the wiring and hence improve testability.

Brian.
 

The idea is to use them to create "one way" routes to conduct the higher voltage from the supply to a lower voltage measurement point.
Click to expand...

How does a diode in series convert a higher voltage to a lower voltage measurement point? i don't get it

They allow the current (and hence voltage at their output side) to follow a determined path but prevent it flowing backwards, away from the path, down another route.
Click to expand...

Well the cathode side of the diode goes direct to the output jack on the patch bay

If there is a Negative voltage that gets injected into the output jack from crossing wiring or incorrectly wired the patch bay wrong , this will inject a negative voltage into the output jacks on the patch bays and the series diode will conduct?

The Anode side of the diode has a Positive Voltage supply

If the Anode side of the diode has No voltage supply, but a Negative voltage got injected into the output jack of the patchbay it will still allow the negative voltage to pass through the series diode

This is what happens when crossing wires externally on the patch bay which causes the series diodes to open and blow right?
 

You are not noting what I said:
How does a diode in series convert a higher voltage to a lower voltage measurement point? i don't get it
Click to expand...
I never said it converted anything.

Is the purpose of your test fixture to test something or is it to destroy diodes?

Please stop thinking of diodes as being some kind of sacrificial part in case the test fixture should produce completely inappropriate voltages. A test fixture is supposed to test something and as such would only inject voltages and/or currents necessary to establish functionality of the device under test. YOU should be in control of the testing and ensure only suitable test conditions are established.

Brian.
 

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