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How hot is too hot? #ubitx

Bo Barry <bobarr@...>
 

Anyone put their finger on the 7805 regulator on the randuino board?  13v in, 5v out isn't optimum for it. I attached a huge heatsink and now I can at least count to 10 with my finger on it. :)
Also I cut the red line in the USB cable so the power off switch worked on everything. As is, it was providing 5v from the PC, along with the 7805 regulator in the rig.
The tiny raspberry pi 5v fan seems to be working fine.
Bo W4GHV

MVS Sarma
 

With 13.8V , perhaps the temperature goes high as 70 Celsius. It is always a good idea to sink the heat,.
it was manageable w/o heatsink at 12V but not 13.5 or 13.8.

Regards
MVS Sarma
 

On Thu, Jun 14, 2018 at 8:59 AM, Bo Barry <bobarr@...> wrote:
Anyone put their finger on the 7805 regulator on the randuino board?  13v in, 5v out isn't optimum for it. I attached a huge heatsink and now I can at least count to 10 with my finger on it. :)
Also I cut the red line in the USB cable so the power off switch worked on everything. As is, it was providing 5v from the PC, along with the 7805 regulator in the rig.
The tiny raspberry pi 5v fan seems to be working fine.
Bo W4GHV


Skip Davis
 

Bo I found the same thing what I did was to add a 47 ohm resistor in the 12vdc into the regulator. I added it on the Raduino board and now I can hold my finger on the regulator without burning my finger. I just cut the trace from pin 15&16 on the Raduino and wired in a 1/4 watt resistor.

Skip Davis, NC9O 

On Jun 13, 2018, at 23:29, Bo Barry <bobarr@...> wrote:

Anyone put their finger on the 7805 regulator on the randuino board?  13v in, 5v out isn't optimum for it. I attached a huge heatsink and now I can at least count to 10 with my finger on it. :)
Also I cut the red line in the USB cable so the power off switch worked on everything. As is, it was providing 5v from the PC, along with the 7805 regulator in the rig.
The tiny raspberry pi 5v fan seems to be working fine.
Bo W4GHV

Jerry Gaffke
 

My Raduino with display draws 0.100 Amps.
Voltage drop across your 47 ohm resistor would be 47ohms * 0.1amps = 4.7 volts.
Assuming a 12v supply, that leaves 12-4.7 = 7.3 volts at the LM7805,
which gives it the required headroom of 2 volts over the 5 volt output of the LM7805.
Power consumed by the resistor would be 0.1amps * 4.7volts = 0.47 Watt.

How hot is your 1/4 Watt resistor?
I'd suggest at least a 1/2 Watt resistor if sticking with 47 ohms.
Or a 25 ohm 1/4 Watt (better, 1/2 Watt) resistor, then the LM7805 will stay in regulation
down to a power supply of  5.0v+7.0v + 25ohms*0.1amps  = 9.5 volts.

The LM7805 can get hot enough to sizzle spit and still not be breathing hard:
    https://groups.io/g/BITX20/message/47859

But the IRF510's while also rated for a 150 degrees C junction temp, should never
have tabs so hot that you can't keep your finger on them:
    https://groups.io/g/BITX20/message/47871

Jerry


On Wed, Jun 13, 2018 at 08:54 pm, Skip Davis wrote:
Bo I found the same thing what I did was to add a 47 ohm resistor in the 12vdc into the regulator. I added it on the Raduino board and now I can hold my finger on the regulator without burning my finger. I just cut the trace from pin 15&16 on the Raduino and wired in a 1/4 watt resistor.

Bo Barry <wn4ghv@...>
 

Thanks for all the inputs! I've done lots of work with the regulators over the years. One comment I can add to this excellent discussion is that the output ripple increases badly as the voltage difference approaches 2 volts.  Zener diodes are horrible at their 'knee' to. 
Being old fashioned I always thought if I couldn't count to 10 touching something (electronic only) it needed a heat sink. 🤔.    When I converted 50 degrees C, which is OK I stopped counting to 10.  The 150C idea scares me. 
The big Radio Shack heat sink works great.
I had extensive college coverage (with the math) of heat sinks, mica and grease, etc. Loved the math.
BTW the TINY 5v fan does great for WSPR & FT8 duty cycles. I even put my tongue on the finals. 😂.  Thanks for discussion. 

Allen Merrell
 

Mine in the oem state with 16x2 display never got too hot to hold. went to 20x4 display and it got HOT too hot to hold added a Nextion 3.2 enhanced and it got even hotter so I am considering building an auxiliary 5v supply off the 13.8v input line to run what addon's I add to it to get this load completely away from the raduino board.
73's  kn4ud
--
Allen  Merrell

Bo Barry <wn4ghv@...>
 

I put a handy buck converter that has a voltage display on it and dropped the voltage to 8 volts with it, AND LABELLED THE PLUG.  
Currently using it with the addon arduino/display, but might incorporate it to the ubitx built in display.

Jack, W8TEE
 

Our 5" TFT color display can draw as much as 0.7A @3.3V, so we're working with a pretty hefty buck converter.

Jack, W8TEE

On Thursday, June 14, 2018, 1:18:25 PM EDT, Allen Merrell via Groups.Io <kn4ud@...> wrote:


Mine in the oem state with 16x2 display never got too hot to hold. went to 20x4 display and it got HOT too hot to hold added a Nextion 3.2 enhanced and it got even hotter so I am considering building an auxiliary 5v supply off the 13.8v input line to run what addon's I add to it to get this load completely away from the raduino board.
73's  kn4ud
--
Allen  Merrell

ajparent1/KB1GMX
 

One of the big power consumers in the Raduino is the backlight for the display.
Before I tamed mine it was over 80ma!  At 20ma it was easier on the eyes.


Allison

Bo Barry <wn4ghv@...>
 

Yep, i got the 5A LM2596 version on eBay $8.00

Allen Merrell
 

Nextion  recommends using at least 500 ma supply for their display. 
--
Allen  Merrell

ajparent1/KB1GMX
 

Recommends or maximum of 500ma?  That seems like a very high
number and I'd expect it to be seriously bright.

The reason I ask as my 7" touch screen all total draws less than that with a
HDMI interface(video)  and USB( touch screen). 

As to 7805 heat... Keep it under 70C (168F) as the device has a thermal shutdown
and it lives longer.  

Allison

Kees T
 

I would sure (and have) reduced the intensity of the backlight with a pot for any of those type displays. Then add a buck converter to reduce the 13.4V to about 7V for the poor ol' 5V regulator. 

73 Kees K5BCQ

Lawrence Galea
 

Simply add a 7808 or 7809 before the 7805
Regards
Lawrence

On Fri, Jun 15, 2018 at 7:17 PM, ajparent1/KB1GMX <kb1gmx@...> wrote:
Recommends or maximum of 500ma?  That seems like a very high
number and I'd expect it to be seriously bright.

The reason I ask as my 7" touch screen all total draws less than that with a
HDMI interface(video)  and USB( touch screen). 

As to 7805 heat... Keep it under 70C (168F) as the device has a thermal shutdown
and it lives longer.  

Allison


Jack, W8TEE
 

Allison:

Which 7" touchscreen are you using that has that low current draw? Is that when the screen is "populated" or blank?

Jack, W8TEE

On Friday, June 15, 2018, 1:17:37 PM EDT, ajparent1/KB1GMX <kb1gmx@...> wrote:


Recommends or maximum of 500ma?  That seems like a very high
number and I'd expect it to be seriously bright.

The reason I ask as my 7" touch screen all total draws less than that with a
HDMI interface(video)  and USB( touch screen). 

As to 7805 heat... Keep it under 70C (168F) as the device has a thermal shutdown
and it lives longer.  

Allison

Bo Barry <wn4ghv@...>
 

Thanks for the temp limits. I had forgotten about its thermal shutdown.   So @ 168 how long can I hold my finger on it? I stopped at 10.  I have a nice big Radio Shack sink on it.

Reminds me of advice I once got. My buddy said I shouldn't use the finger touch method. That I should use the upper lip method.

Yep. When I asked how to get my head/lip down into my equipment he threw something at me. 🤔. I figured it out the next day and won't reveal how to do it!

MVS Sarma
 

I would prefer to use unregulated 12 or 13.5V for back lit by cutting it from 5V regulated and manage to change the series resistor to maintain needed current.

Regards
MVS Sarma
 

On Sat, Jun 16, 2018 at 2:59 AM, Bo Barry <wn4ghv@...> wrote:
Thanks for the temp limits. I had forgotten about its thermal shutdown.   So @ 168 how long can I hold my finger on it? I stopped at 10.  I have a nice big Radio Shack sink on it.

Reminds me of advice I once got. My buddy said I shouldn't use the finger touch method. That I should use the upper lip method.

Yep. When I asked how to get my head/lip down into my equipment he threw something at me. 🤔. I figured it out the next day and won't reveal how to do it!


ajparent1/KB1GMX
 

Picked it up over a year ago on Amazon no recognizable name.
I'd get two more save for I've not seen another.

That a RPI-3b will run on a 6AH 5V "charger" for at least 6 hours and still
going before I shut down.  

My laptop is a PI3b with a 10.1" hdmi that requires 12V and eats 4.5W at full
brightness without a touchscreen.  Not bad for 1024x720.  Went with a touch
pad and a real keyboard (apple mini) at a few milliamps as screens that size
with touch screen hardware eat wallets for breakfast.

THe big issues many of the displays there is no provision for dimming the
led backlight and that's where most of the current goes.  Even the little 16x2
runs high current until you change the resistor on the back or add one to 
the LCD 5V line (or go with a switch mode dimmer).

I tend to go if I need a screen on the radio I use a cheap 10" Android pad with 
as the screen as I can see it without classes and at low power.  With USB as
IO it can run everything with a Arduino as the other end of the USB. 


Allison

Kees T
 

What you can always do to reduce the load and heat on the poor ol' 5V Regulator is to just add a series resistor of adequate wattage to drop the regulator input voltage to 7V or so. Simple, but not as efficient as using a buck regulator for the 5V in the first place....important if you are off battery power.    

73 Kees K5BCQ