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Shorting Tx diode

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Old 08-04-2003, 01:12 AM
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Jim13704
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Default Shorting Tx diode

The diode in my Futaba 8U tx prevents being able to charge the battery while in the tx by use of a peak charger. I realize that the function of the diode is to limit current flow to only one direction. But, I questlion why this is needed.

I am considering sordering a jumper around this diode in order to be able to use a peak charger and wonder what potential problems this might cause. I assume Futaba had some reasons for placing the diode in the tx in the first place but don't know what those reasons are.

Anyone know of any practical reason why I might not want to short the diode? Thanks for any insights.

Jim Parker
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Old 08-04-2003, 01:37 AM
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Tim_Indy
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Default Shorting Tx diode

Jim, I'm assuming you do know where the diode in question is. I have an 8U Futaba and have jumped the diode with zero problems. I used a single strand from a servo wire and found this to be ideal in that it accomplishes the purpose allowing peak charging, while acting as a quick blow fuse case of a problem. If it DID blow, no problem, I'd just be back where I started with the diode now again functioning.

BTW, I've never blown a strand after 3 years of use.
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Old 08-04-2003, 04:53 AM
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Default Re: Shorting Tx diode

Originally posted by Jim13704
Anyone know of any practical reason why I might not want to short the diode? Thanks for any insights.
Hi Jim,

I can answer this question, and your previous question about the need for the diode in the first place, very easily with another question... Do you have any JR chargers lying around? JR chargers have reversed polarity relative to the rest of the world with the same plug size or shape. That's why there is a diode is in the charging circuit of your 8U -- to protect your transmitter from JR chargers... So the practical reason for not jumping the diode is if you cannot absolutely guarantee you'll never accidentally plug a JR charger into your 8U. If you can make that guarantee then it's not a problem to jump it. I would, however, suggest using a poly switch. Same protection as the diode, and the ability to fast charge... I had my jumper replaced when I last had my transmitter tuned... It was cheap and is a better solution. Also -- it is a good idea to get your transmitter tuned every couple of years...
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Old 08-04-2003, 05:10 AM
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Default Re: Re: Shorting Tx diode

Originally posted by GalenB
Hi Jim,

I can answer this question, and your previous question about the need for the diode in the first place, very easily with another question... Do you have any JR chargers lying around? JR chargers have reversed polarity relative to the rest of the world with the same plug size or shape. That's why there is a diode is in the charging circuit of your 8U -- to protect your transmitter from JR chargers... So the practical reason for not jumping the diode is if you cannot absolutely guarantee you'll never accidentally plug a JR charger into your 8U. If you can make that guarantee then it's not a problem to jump it. I would, however, suggest using a poly switch. Same protection as the diode, and the ability to fast charge... I had my jumper replaced when I last had my transmitter tuned... It was cheap and is a better solution. Also -- it is a good idea to get your transmitter tuned every couple of years...
Be easier for Futaba/Hitec etc.. to just change the plug outputs of the TX to not fit the chargers from a JR TX. Just like they do on servos.

I'm kinda upset at the whole deal myself. I have a new Futaba which no one has diagramed where the diode is and I really hate the idea of poking around to find it.

I really shouldn't have to take apart a brand new radio to use a charger that is becoming more and more popular.

By the same token why does JR not have to put a diode in it to protect it from a Futaba charger?
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Old 08-04-2003, 03:22 PM
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Jim13704
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Default Shorting Tx diode

Tim and Galen,

Thanks much for your insights.


Borzak,

Here is a web site that should help you find the diode in your Tx:

http://www.siriuselectronics.com/fut8uapd.htm


Jim
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Old 08-04-2003, 04:08 PM
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Default Shorting Tx diode

Thanks for the link, Jim. I wanted to do this for some time, but didn't know where the darn thing was!

... and thanks to Sirius.

- George
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Old 08-04-2003, 07:13 PM
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Default Shorting Tx diode

What's really stupid is all they have to do is add a few more diodes and they can rectify the input so it doesn't matter which line is positive.
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Old 08-04-2003, 07:37 PM
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Default Shorting Tx diode

By the same token why does JR not have to put a diode in it to protect it from a Futaba charger?
North American JR systems uses a fuse. It blows (most of the time) when you try to charge in reverse, use excess charge currents, or if you short the charge leads. These are all good reasons to pop a fuse. In Europe I have heard that they incorporate a diode on some Tx's too (I have not confirmed this).

If you want to defeat the diode protection then "short" it out with a 3A fast blow fuse. That way you have protection from nasty faults. Those little Pico fuses are nice (look like 1/4 W resistors). Or, use a RayChem Polyfuse (solid state resetting fuse). Futaba Service will even install the Polyfuse for you for a few dollars.

What's really stupid is all they have to do is add a few more diodes and they can rectify the input so it doesn't matter which line is positive.
The reverse protection diode does more than just protect against reversed charge voltages. It also protects against shorts on charge leads (for those folks that have banana jacks on them).

If a bridge diode was used then the short protection is lost. A fuse would fix that, but then you are back to the JR complaint that the fuses can be blown.

Frankly, the protection is good when you need it. At all other times it is often considered a hassle. Just like seat belts.
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Old 08-05-2003, 02:32 AM
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Default Shorting Tx diode

Originally posted by Jim13704
Tim and Galen,

Thanks much for your insights.


Borzak,

Here is a web site that should help you find the diode in your Tx:

http://www.siriuselectronics.com/fut8uapd.htm


Jim
Thanks but it's the new 6EXA and no one has it listed/shown yet. I looked around from the battery cover and couldn't see one or the symbol for one on the circuit board...
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