FlexRadio and fuse or fuses?

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Yesterday John K ask a question about using a high power supply with his flexRadio. The question got me thinking about how the 6400 and other flexRaios are designed. Without a schematic (which must be old school now) I don’t know if these radios have a common chasse ground or a floating system ground.

Which makes me wonder if both the negative and positive power leads should be fuse at 30 amps, and a non-fused earth ground to the radio chassis.

I am assuming that if both positive and negative power leads needed to be fused that flexRadio would have done so.  I do wonder because I know many cost saving deviations have been incorporated in the new 6400.  What would be best practice for our power leads and are fuse additions necessary?  

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Rick l

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Posted 2 years ago

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According to the manual the 6400 and 6600 have internal blade fuses, I think 30 amp.

I put a 25 amp inline fuse as manual says max 23 amps.

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Tim - W4TME, Customer Experience Manager

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The negative power lead is chassis grounded,

The radios have an internal fuse(s) depending on the model.  There is a main 30A automotive blade type (ATO) fuse on the positive lead of the 13.8VDC power input.  And there may be an additional lower amperage fuse (5A) in the board that provides the non-RF and low power RF functions of the radio.  When paired with the recommended power supply of 25-30A, this configuration is adequately fused.  When used with a higher amperage power supply, the p/s is usually fed into some sort of distribution block.  And many times that block has individual line outs fused so that you can properly size the fuse to the amperage requirement of the device being fed from it. A Rigrunner is one such example.

If you decide to provide in-line fusing, I recommend fusing both the (+) and (-) legs of the connection as an extra precaution.
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Mark Thomas

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I don't agree that fusing negative wires on ham shack equipment is generally a good idea.

In an automotive environment, fusing both the positive and negative lead of devices, especially those that may be wired directly to the vehicle battery terminals, is extremely important. If the vehicle battery chassis ground return strap fails or develops resistance, that equipment negative wire could become a current-passing path to ground for the rest of the vehicle's electrical system, causing wires to overheat, or other dangerous failure. That negative-wire fuse protects you from high DC current from sources other than the fused radio power taking the path of the chassis via antenna coax braid or metal mounting bolts back to the battery.

A non-vehicular ham shack (assuming you don't live in a metal house which you are using as your antenna ground plane) is a different situation. There may be other shack DC-powered devices that don't have fuses in their thinner DC return wires, such as external antenna tuners or powered antenna switches. If that Flex radio ground fuse opened up but not the positive fuse, the next time you transmit you could end up with high DC current taking paths such as antenna braid to these other devices with the thinner DC wires to your power supply return. Fuses introduce low but non-zero resistance, and sometimes open up unexpectedly due to poor connections or internal failure. I suggest giving careful consideration to how your specific shack equipment is powered and grounded. There is a reason why even fused AC appliances do not have fuses in their safety-ground or neutral (in the US and similar) wire, and I believe this is a comparable situation.

I'm not saying it is never a good idea to fuse both positive and negative in non-vehicular setups, but often there may be unexpected consequences. Also, if your power distribution is already fused at reasonable per-circuit values, adding additional fuses in positive power wires doesn't buy further safety but does introduce more voltage drop and possible points of failure.

-Mark KC3DRE
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Tim - W4TME, Customer Experience Manager

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Thank you for your insightful reply.  It makes a lot of sense.
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Ria - N2RJ, Elmer

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In vehicular setups it’s not really recommended either. The Motorola way is to run a fuse on the positive lead and then run the negative lead to the chassis near the battery.

See Tom, W8JI’s article:


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That is one thing I could not believe that Flex did not provide when I unboxed the 6400M in Mid March. No fuses in the powerline, every major manufacturer provides them. Not knowing if it was protected or not I placed 25A fuses inline on both leads going to my MFJ hash free power supply.

I couldn't believe that protection would be sacrificed for cost of 2 inline fuses.

I guess my concerns were not valid since there was protection there inside the rig but no where in the installation manual or setup was this ever made known to the user.

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Tim - W4TME, Customer Experience Manager

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The reason we did this is that since the shift to using the Anderson PowerPole power connector, the feedback we received was that customers did not primarily direct connect the power supply to the radio.  The use fused power distribution block to power other peripherals and adding in-line fuses would result in redundancy and creates additional resistance that lowers the supply voltage to the radio causing it not to put out full RF power.

And if need to change an internal fuse, you just pull the cover off.  The fuse is very prominent (it's tall and boldly colored) and easy to locate.
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Greg - N8GD

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Many of the "Major" manufacturers absolutely need fuses in the power leads because the transmitter PA section is fed directly from the power leads with NO internal fusing.  There may be a lower current fuse internally for the receive and control circuits, but the major manufacturers would have a fire hazard if they didn't have in-line fuses with such an arrangement.  Flex does it right: internal fuse(s) to handle everything, including the PA !
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Rick l

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Great response, fusing my new 6400 is handled in the design of the radio. Thank you all for you detailed and logical explanations.  This is a great group.

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What are the amps of the flex supplied dc power cord? My hot (red) fuse failed but I cannot read the amperage. I have the flex 6300 if that makes a difference.

OK, so now with a magnifying glass was able to read the amperage on the hot (red wire) of the flex supplies dc anderson power pole cord and it indicates "15A 250V". Is that correct? I thought it should be higher but for dc that does sound about right. Could someone confirm this for me?

Thanks Alan KZ6B
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Bill -VA3WTB

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Did that cable come with your radio. I thought the Flex supplied cables do not have inline fuses? Because there is no need for them. But i think It should be 30Amp
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John KB4DU

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Remember, the fuse is there primarily for the wiring. They take way too long to blow to protect the electronics, esp. microprocessor/fpga etc.