Author Topic: Fluke 89IV components blown off pcb  (Read 765 times)

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Offline mattyelec

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Fluke 89IV components blown off pcb
« on: August 31, 2018, 10:50:13 pm »
Hi there,

Have just purchased a faulty 89IV, which actually operates normally on basic functions (resistance, voltage AC and voltage DC). After pulling apart, I've noticed a few components have been vaporised off the pcb. I believe these to be a couple of resistors, a bridge rectifier & a diode (compared with my meter and photos online), but am unsure on their specs/values. Can anyone help me out in that department?
Also, what is likely to have caused this?

Thanks in advance,
Matt
 

Offline 0xdeadbeef

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Re: Fluke 89IV components blown off pcb
« Reply #1 on: August 31, 2018, 11:53:09 pm »
The vaporized bridge rectifier should be a something like DF04S or DF06S and the diode should be something like an S1G rectifier.
Trying is the first step towards failure - Homer J. Simpson
 

Offline mattyelec

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Re: Fluke 89IV components blown off pcb
« Reply #2 on: September 01, 2018, 04:41:05 am »
Thanks for that! Two down!
Any idea on what the resistor values might be? I guess I'm hoping that someone's got either a 87IV or a 89IV that they've had open and could tell me...
 

Offline FRR

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Re: Fluke 89IV components blown off pcb
« Reply #3 on: September 01, 2018, 08:22:04 am »
I just so happen to have a unit on my desk I finished refurbishing. This should be everything you need.







 

Offline mattyelec

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Re: Fluke 89IV components blown off pcb
« Reply #4 on: September 01, 2018, 10:35:15 pm »
Thanks for that FFR!  Any idea on what would have caused this? Perhaps testing a valve with their high voltage?
 

Offline 0xdeadbeef

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Re: Fluke 89IV components blown off pcb
« Reply #5 on: September 02, 2018, 08:39:34 am »
The bridge rectifier is meant to protect the shunt (and ADC) before the HRC blows when measuring currents. Typically the bridge is connected between the AMP and Com inputs in parallel to the shunt(s) and either shortened (two diodes forward/backard) or additional diodes are used to increase the voltage drop. In case of a shortened bridge, there are two diodes in parallel to the shunt resistor, so the voltage drop over the shunt is limited to ~1.2V. Now the DF06S is designed to withstand peaks of 600V and 50A or so for a short time which means that a higher current and/or a higher voltage was applied to it before the HRC blew. Like I guess something like this is supposed to happen if you connect the Com/Amp inputs directly to a voltage supply that can deliver 100A or so.

[Edit]
When looking again at the picture above, you can see that the SG1 diode is used to shorten the bridge, i.e. it's connected between the "+" and  "-" terminals of the bridge.  So there are three diodes between Amp and Com and the total forward voltage is something like 3.3V. And the 10k resistor is obviously connected in parallel to the shunt (between the two "~" terminals of the bridge). Its location indicates that it's related to the protection circuit and its green color makes me think it has some specification beyond its resistance but honestly at this very moment, I'm a bit puzzled what it's meant for exactly.
« Last Edit: September 02, 2018, 09:06:29 am by 0xdeadbeef »
Trying is the first step towards failure - Homer J. Simpson
 


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