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Fuses, diodes and switches #ubitx

DD5ET
 

I recently wired up my uBITX. I've attached a photo that shows how I wired up the DC socket with some fuses. I used two fuses: one 0.5 A fuse for the main board (red wire) and one 2.0 A fuse for the PA power (brown wire). Before I started, I was only able to find one photo on the Internet showing a uBITX wired up with two fuses and, due to the way the picture was cropped, it wasn't entirely clear to me just exactly what wires went where and how the diode was wired with the fuses. 

As you can see in my photo (hopefully), starting at the P1 connector, the black wire is soldered to the (-) pin on the DC connector. The brown wire goes to one pin of a fuse holder (with the 2.0 A fuse), and then another brownish wire goes from the other pin on that fuse holder to the (+) pin on the DC connector. The red wire from the P1 connector goes to one of the "on/off" pins on the back of the volume control pot (not shown). Another red wire goes from the other "on/off" pin on the back of the volume control pot (not shown) back to one pin of another fuse holder (with the 0.5 A fuse), and then another red wire goes from the other pin on that fuse holder to the (+) pin on the DC connector (with the brown wire). I have soldered the diode across the (+) and (-) pins on the DC connector, as shown and described in the wire up instructions -- which puts the diode *before* the fuses. This is how the latest "Wiring Diagram Ubitx V5 V2.0.pdf" shows it, and it's also shown this way on the uBITX Assembly wiki.

However, in several messages in this forum, I have seen folks say the diode should be *after* the fuses. Would someone please explain why the diode should go behind the fuses (and is it "should go behind" or "must go behind")? Also, could someone please describe or show specifically where, in relation to the fuses, the diode would be soldered so that it's *after* the fuses? Like I said, the latest wiring diagram shows the diode wired to the pins on the DC connector *before* the fuses.

Also, the way I've wired the fuse for the PA power seems to bypass the on/off switch and, thus, regardless of whether the on/off switch is on or off, power is supplied to the PA. Is this how others see this? Should I also have the brown wires going to the volume control pot on/off pins, too, just like the red wires? Or, is there another way I can wire this so that no power will be supplied to the board or the PA while the on/off switch is off?

Btw, I've successfully powered on the uBITX and everything seems to be working properly (although, there is so much noise around here I haven't actually been able to hear much of anything).

Anyway, any suggestions/comments regarding the fuses and diode are appreciated.

Jerry Gaffke
 

Nothing terribly wrong with what you have done, seems better than most I've seen described here.
But I'd have a switch in line with the PA-PWR, as you suggest.  
When the power switch for the rig is off, the rig should be completely off (in my opinion).

The reason to have a fuse before the switch instead of after is so you will blow the fuse
if you accidentally short out the switch with a screwdriver or something.
Not a big deal to have the fuse after the switch, such an accident should not blow up the uBitx.
And if the power supply is properly designed to deal with over-current, it should not be damaged.
Though if the supply is big enough, you might melt a perfectly good screwdriver.

On my rig, I have a 3A fuse in line with main power coming into the rig, it then goes to the main power switch.
A series combination of a 0.5A fuse and a diode goes from that switch to the main 12v into the uBitx.
Also from that first switch, a wire to a second switch to power the PA-12v rail into the IRF510's.

Reverse protection for the IRF510's is not needed, they have their own intrinsic reverse diode,
it will conduct if the power supply leads are reversed and blow the 3A fuse up front.
Hopefully that fuse will blow before the traces into the IRF510's do.

The second switch is very handy for testing the transmitter.
With that switch off I don't need to worry about having the dummy load
while working on everything up through the driver stages.

Actually, instead of a diode, I use an LM2940CT-12 low dropout regulator.
It gives reverse polarity protection with about the same dropout voltage loss,
and allows me to safely feed the rig more than 12v.  

If you do choose to use a diode, a relatively wimpy one like a 1n4148 
may work well enough as a roughly 0.5 Amp fuse, then you only have
one fuse of 2 or 3 Amps up front.  I have not tried this.
 
There are probably hundreds of different ways to do this.
Especially if you plan to sometimes have an entirely different power supply
of higher voltage to feed PA-PWR.

Jerry


On Thu, Sep 12, 2019 at 09:12 AM, DD5ET wrote:
I recently wired up my uBITX. I've attached a photo that shows how I wired up the DC socket with some fuses. I used two fuses: one 0.5 A fuse for the main board (red wire) and one 2.0 A fuse for the PA power (brown wire). Before I started, I was only able to find one photo on the Internet showing a uBITX wired up with two fuses and, due to the way the picture was cropped, it wasn't entirely clear to me just exactly what wires went where and how the diode was wired with the fuses. 

As you can see in my photo (hopefully), starting at the P1 connector, the black wire is soldered to the (-) pin on the DC connector. The brown wire goes to one pin of a fuse holder (with the 2.0 A fuse), and then another brownish wire goes from the other pin on that fuse holder to the (+) pin on the DC connector. The red wire from the P1 connector goes to one of the "on/off" pins on the back of the volume control pot (not shown). Another red wire goes from the other "on/off" pin on the back of the volume control pot (not shown) back to one pin of another fuse holder (with the 0.5 A fuse), and then another red wire goes from the other pin on that fuse holder to the (+) pin on the DC connector (with the brown wire). I have soldered the diode across the (+) and (-) pins on the DC connector, as shown and described in the wire up instructions -- which puts the diode *before* the fuses. This is how the latest "Wiring Diagram Ubitx V5 V2.0.pdf" shows it, and it's also shown this way on the uBITX Assembly wiki.

However, in several messages in this forum, I have seen folks say the diode should be *after* the fuses. Would someone please explain why the diode should go behind the fuses (and is it "should go behind" or "must go behind")? Also, could someone please describe or show specifically where, in relation to the fuses, the diode would be soldered so that it's *after* the fuses? Like I said, the latest wiring diagram shows the diode wired to the pins on the DC connector *before* the fuses.

Also, the way I've wired the fuse for the PA power seems to bypass the on/off switch and, thus, regardless of whether the on/off switch is on or off, power is supplied to the PA. Is this how others see this? Should I also have the brown wires going to the volume control pot on/off pins, too, just like the red wires? Or, is there another way I can wire this so that no power will be supplied to the board or the PA while the on/off switch is off?

Btw, I've successfully powered on the uBITX and everything seems to be working properly (although, there is so much noise around here I haven't actually been able to hear much of anything).

Anyway, any suggestions/comments regarding the fuses and diode are appreciated.

Ian Reeve
 

The diode is there to "protect" against reverse polarity and as it is wired directly across the supply and reverse biased,the circuit will effectively short circuit the supply if the supply is connected the wrong way round.Problem with this are that too large a current will destroy the diode thus removing any protection. A simple and more safe approach is to put a diode in series with each supply line allowing current to pass if connected correctly and block it if reversed.The diodes need to be rated at say 2amps and 4 amps if using separate supplies for the PA and the remainder of the circuit.The diode may drop up to 0.5v so a slight rise in power supply is to be considered.Only use appropriate sized fuses which should be the fast blow type.  Ian M0IDR


From: BITX20@groups.io <BITX20@groups.io> on behalf of DD5ET <dd5et.scott@...>
Sent: Thursday, September 12, 2019 5:12:15 PM
To: BITX20@groups.io <BITX20@groups.io>
Subject: [BITX20] Fuses, diodes and switches #ubitx
 
I recently wired up my uBITX. I've attached a photo that shows how I wired up the DC socket with some fuses. I used two fuses: one 0.5 A fuse for the main board (red wire) and one 2.0 A fuse for the PA power (brown wire). Before I started, I was only able to find one photo on the Internet showing a uBITX wired up with two fuses and, due to the way the picture was cropped, it wasn't entirely clear to me just exactly what wires went where and how the diode was wired with the fuses. 

As you can see in my photo (hopefully), starting at the P1 connector, the black wire is soldered to the (-) pin on the DC connector. The brown wire goes to one pin of a fuse holder (with the 2.0 A fuse), and then another brownish wire goes from the other pin on that fuse holder to the (+) pin on the DC connector. The red wire from the P1 connector goes to one of the "on/off" pins on the back of the volume control pot (not shown). Another red wire goes from the other "on/off" pin on the back of the volume control pot (not shown) back to one pin of another fuse holder (with the 0.5 A fuse), and then another red wire goes from the other pin on that fuse holder to the (+) pin on the DC connector (with the brown wire). I have soldered the diode across the (+) and (-) pins on the DC connector, as shown and described in the wire up instructions -- which puts the diode *before* the fuses. This is how the latest "Wiring Diagram Ubitx V5 V2.0.pdf" shows it, and it's also shown this way on the uBITX Assembly wiki.

However, in several messages in this forum, I have seen folks say the diode should be *after* the fuses. Would someone please explain why the diode should go behind the fuses (and is it "should go behind" or "must go behind")? Also, could someone please describe or show specifically where, in relation to the fuses, the diode would be soldered so that it's *after* the fuses? Like I said, the latest wiring diagram shows the diode wired to the pins on the DC connector *before* the fuses.

Also, the way I've wired the fuse for the PA power seems to bypass the on/off switch and, thus, regardless of whether the on/off switch is on or off, power is supplied to the PA. Is this how others see this? Should I also have the brown wires going to the volume control pot on/off pins, too, just like the red wires? Or, is there another way I can wire this so that no power will be supplied to the board or the PA while the on/off switch is off?

Btw, I've successfully powered on the uBITX and everything seems to be working properly (although, there is so much noise around here I haven't actually been able to hear much of anything).

Anyway, any suggestions/comments regarding the fuses and diode are appreciated.

Jerry Gaffke
 

Like Ian, I think the uBitx main rail is a good candidate for a series diode.
We don't care much if we lose a half volt there.

The hfsignals.com wireup diagram shows a shunt diode that conducts
if the power supply is hooked up backwards, but doesn't bother to show a fuse.
This is definitely a case where you absolutely need a fuse in front of the diode.
And the diode should be Schottky (as supplied with the kit) so it has a 
very low voltage drop when it does conduct.

As I said previously, there's hundreds of ways to do this.

Jerry



On Thu, Sep 12, 2019 at 09:44 AM, Jerry Gaffke wrote:

A series combination of a 0.5A fuse and a diode goes from that switch to the main 12v into the uBitx.