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Troubleshoot Buss Resistor packs

Danny Daviss

Mar 16, 2013
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When you have circuits that use those resistor packs that are bussed not isolated resistors

The Resistors inside are bussed, how does a tech troubleshoot them?

If one or two resistors inside the resistor pack are shorted or open , how does a tech know that since the resistors are bussed together? How can i use my DVM to tell which resistor in the resistor pack is shorted or open?

I have seen these resistors packs that are bussed in logic circuits, they have diodes in parallel with each resistor in the resistor pack, what are these diodes used for with a bussed resistor pack?

I have seen them used for logic gates that have multiple inputs
 

john monks

Mar 9, 2012
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Most of the time the circuitry around resistor packs are similar. So what I do is put one probe on the common and run the other leads along the rest of the leads. If one is open it usually shows up. Usually all the resistors are the same. Other than that your stuck, you just have to know the circuitry.
But one thing you seem to hit on is that the failure for resistor packs seems to be a bit high.
 
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john monks

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For an open resistor it measures either open or just higher than the rest. In what I work on the resistors are usually used as pull up or pull down resistors. So the direction I use the ohmmeter may make a difference. So I just pay attention to that.
For a shorted resistor you would get maybe 1 ohm. But I never had that happen. Usually they open rather than short.
 
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Danny Daviss

Mar 16, 2013
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Open would measure what? close to 1 meg?

A shorted resistor in the bussed resistor pack would measure? 0.2 ohms?
 

john monks

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That depends on what the rest of the circuit. I don't remember measuring close to 1 Meg. Sometimes the resistor only comes out maybe 30% higher than the rest of the resistors. So I usually am only looking for a resistance out of the ordinary because sometimes I don't know what the rest of the circuitry is like especially if the resistors are feeding a chip.
I have never seen a shorted resistor pack but I assume if you see 1 ohm or less it is shorted. I have seen lost of solder splashes between resistor pack leads but that's another issue.
 

Danny Daviss

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so each resistor should measure the same value when the are bussed? and if one is higher or lower value that means there is something wrong right?
 

Danny Daviss

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My Manager measure the resistor packs "incircuit" and some of the values are higher and lower and he says it changes the impedance of the circuit

But I have no idea which is normal and what is not with these resistor packs if they keep change the impedance of the circuit

do you know what he means?
 

john monks

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Usually that is the case but before replacing the resistor pack I would look a little closer at the rest of the circuit. Frequently I would take a regular resistor and place it in the location of the resistor I suspect is open. And if that fixes the problem I may just solder the resistor in place. Resistor packs are a pain to replace.
 

Danny Daviss

Mar 16, 2013
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you move the resistor back and forth until it breaks , the leads are left in the solder holes and I move the PCB vertically to do through holes
 

john monks

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Sorry, I don't know what your manager is talking about. I never heard of a resistor pack that is supposed to change in value.
The resistor packs I usually deal with are 330 ohm, 470 ohm, 1kohm, or 4.7kohm. And I know that placing meter leads on these are not going to change the impedance significantly. So I'm not sure what your manager is talking about.
To read a resistor is not to difficult. For example if it has a 472 in it's part number it is most likely 4.7kohm.
 
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Danny Daviss

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some of the resistors in the resistor pack, some are higher and lower in values , taking them

When i change a resistor pack, i just move it back and forth until it breaks, the leads are let in the soldered holes, and i suck the solder out of them
 

john monks

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You can do that but usually I just run solder across all the leads and pull the resistor pack out as a unit so I don't pull any barrels out of the pc board. But I guess it doesn't make much difference. But just placing a resistor across an open resistor is certainly an easy fix.
 

Danny Daviss

Mar 16, 2013
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If one of the resistor is open or short, doesn't it change the values for all of the other resistors since the are bussed?
in circuit testing or out of circuit testing?

It makes in circuit testing for me when one of the resistors is open, it changes all the values of the other resistors when they are in circuit
 

john monks

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It may very well change the rest of the readings but I am only looking at the one that is different. In my case I never noticed the rest of the readings way off because of one resistor. Usually the common is connected to a power supply and the rest of the leads are connected to some logic gate or gates.
 

Danny Daviss

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Usually the common is connected to a power supply and the rest of the leads are connected to some logic gate or gates.

Yes I see that a lot

The use resistor packs for Rin and Rf , one resistor pack for multiple op amps

Pull up or Pull down resistors or for the current limiting resistor for LED's
 

john monks

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If you are using resistor packs in op-amp circuits then you just have to know the circuit. This is sort of an unusual application for resistor packs.
 

Danny Daviss

Mar 16, 2013
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yes the company i work for uses them like that

They use SIL resistor packs with logic gates that have multiple inputs

example: a 8 input NOR gate

They will have an 8 SIL resistor pack with diodes in parallel one for each input of the NOR gate

So each input of the NOR gate has a Diode and Resistor but they are bussed

1.) First question is why they used a bussed SIL resistor pack and Not an Isolated ISL resistor pack

2.) What does the Diode do that is bussed just like the SIL resistor pack in parallel with it?

I don't get why they use Bussed SIL resistor packs VS isolated SIL resistor packs , isolated resistor packs are not bussed together

So have the resistors bussed together does what ? for logic gates, LED's, pull up and pull down?

Why not use isolated resistor packs for pull up and pull down , LED's current resistors, logic gates?
 

john monks

Mar 9, 2012
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Without seeing the circuit I can only guess.
A busses sil resistor pack has all of one end of the resistors connected together and connected to one pin unlike the resistor packs that have individual resistors. So there is some savings there. Ax for the diodes I haven't a clue. Does this connect to the outside world. Maybe they are for protecting the gates. I just don't know.
 

Danny Daviss

Mar 16, 2013
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The Resistors are pull down or pull up resistors using the SIL pack for pull up and down down resistors, but they place diodes in parallel for each pull up and down down resistor

What does the diode do in parallel with a pull down or pull up resistor? the diode is not zener its silicon switching diode
 
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