Author Topic: Double fused battery input, replace or bypass ground blown fuse?  (Read 602 times)

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Offline videobruce

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The equipment is a Tektronix RFM151 SLM (around 20 years old). Long story short, the battery input has two SM fuses (2.5a) one on each side. The ground side blew due to a wrong polarity of a battery connector. The meter works off a AC adapter fine.

I never saw a fuse used in both sides before and it's the ground side that blew. Being a very odd (to me) fuse and the fact it's on the ground side with the same fuse on the positive side, can just bypassing that blown fuse be ok? There doesn't appear to be any diode protection as one would think.

See attachment, it's the SM left side fuse (F1 now removed) in the upper left corner of the pic.
 

Online georges80

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #1 on: October 06, 2018, 04:05:56 pm »
Fusing + and - is common in some situations, such as for example HAM radios used in a vehicle where the +/- cables of the radio connect directly to the battery.

The reasoning is, that if the radio is mounted so its chassis is grounded say in the cab and then a main battery cable is disconnected, the radio ground wire now becomes the ground path for the vehicle between the body (via the radio chassis and radio's ground wire) and the battery and other loads (like the starter motor...) will push high current through that path.

With your tek, the dual fusing may be for a similar situation or combination of possible mis-wires (or reverse polarity connection of your battery...)

cheers,
george.
 
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Offline videobruce

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #2 on: October 06, 2018, 04:20:19 pm »
I wouldn't/couldn't think this was were intended to be used in a vehicle. Surely not as an entertainment device since it sold for between $2k & $2.5 k in the late nineties when it came out.  ;)

Ok, but this didn't affect the coaxial jack input from a AC supply, just the battery. If it did, that wouldn't still power up the meter. Correct?  ???
 

Online schmitt trigger

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #3 on: October 06, 2018, 04:43:49 pm »
What Georges80 said :-+

The Tektronix engineers must have applied a DFMEA analysis, during which they identified a particular failure mechanism which could trigger an excessive battery negative terminal current.
The simplest mitigation for that fault may have been a fuse.

Case in point: the fuse blew because an incorrect connection.
I would not bypass the fuse.
 

Online georges80

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #4 on: October 06, 2018, 04:44:38 pm »
Yeah, didn't imply it would be used in a vehicle :)

Just pointing out that fusing both +/- is important where you have cables that carry +/- and that there can be unintended currents that can flow through a -ve wire due to losing a ground connection somewhere else when there is a common chassis (in the tek situation the common chassis is the metal case). In that situation the fuse is to protect the wire from burning out in a high current situation versus protecting the actual electronics etc.

No idea in your Tek, you would need to see whether a scenario as above is possible. Maybe there were just being safe and following company policy when running such cabling...

cheers,
george.
 

Offline videobruce

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #5 on: October 06, 2018, 04:49:25 pm »
Could a diode be substituted for that fuse?
 

Online georges80

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #6 on: October 06, 2018, 05:11:35 pm »
You could just use a nail  :-DD

cheers,
george.
 

Offline videobruce

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #7 on: October 06, 2018, 07:31:46 pm »
I mean to protect against reverse polarity.  ::)
 

Online georges80

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #8 on: October 06, 2018, 09:53:24 pm »
Add the diode and you've added a voltage drop (say 0.5V), what will the charger think is going on with the battery?

Put a fuse back in there.

cheers,
george.
 

Online schmitt trigger

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #9 on: October 07, 2018, 12:22:58 am »
The battery is a rechargeable one, correct? If so....
How are you going to connect the diode?  anode or the cathode to the battery?

Think your answer for a second.
 

Offline videobruce

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Re: Double fused battery input, replace or bypass ground blown fuse?
« Reply #10 on: October 07, 2018, 05:36:47 am »
I did forget about the voltage drop.
I went ahead and wired a small glass fuse in place (same amperage) secured it to the ribbon cable header right next to those fuses. Trying to order those 'odd' fuses would of been a pain, then waiting a week wasn't worth it.
 


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