Topics

Nextion with KA78T05

Herman Scheper
 

I am using the Nextion 5"display. When building my Ubitx V5 I used a small step down converter to generate the 5V for the Nextion display. It all worked 100%
Now the building is almost finished i want to replace the 5V stepdown converter. ( It is giving a little amountt of noise…)

I ordered ( Aliex..) 5 pcs of KA78T05 TO-220 stabilisors. ( is the 3A version-  )

Then I replaced the 7805 mounted on the Arduine and replaced it for the KA78T05 version…. No working Arduino and no working display.

I find input=12V output=0V.   Checked before and after the pinning  1=input / 2 = earth/ 3 =output   Find a datasheet on Internet and can not find what I am doiing wrong.

After that make a test print only for the KA78T05 and I can not get the 5V output. Output also on a 2nd KA78T05 is still 0Volt.   Frustrating.  ( I used a 7809 on the same test and output was 9.00 Volt!)

What I am doiing wrong...…….?     ( ordered today local few 78S05   ( = 1.5A version  )

So who knows what I am doiing wrong...?                       Tnx.  Herman

MVS Sarma
 

I suffered with 10pcs of  bad 78t05 from ali seller and demanded refund and got it.


On Mon, 16 Sep 2019, 9:37 pm Herman Scheper, <Hsrscheper@...> wrote:
I am using the Nextion 5"display. When building my Ubitx V5 I used a small step down converter to generate the 5V for the Nextion display. It all worked 100%
Now the building is almost finished i want to replace the 5V stepdown converter. ( It is giving a little amountt of noise…)

I ordered ( Aliex..) 5 pcs of KA78T05 TO-220 stabilisors. ( is the 3A version-  )

Then I replaced the 7805 mounted on the Arduine and replaced it for the KA78T05 version…. No working Arduino and no working display.

I find input=12V output=0V.   Checked before and after the pinning  1=input / 2 = earth/ 3 =output   Find a datasheet on Internet and can not find what I am doiing wrong.

After that make a test print only for the KA78T05 and I can not get the 5V output. Output also on a 2nd KA78T05 is still 0Volt.   Frustrating.  ( I used a 7809 on the same test and output was 9.00 Volt!)

What I am doiing wrong...…….?     ( ordered today local few 78S05   ( = 1.5A version  )

So who knows what I am doiing wrong...?                       Tnx.  Herman

Werner de Vos
 

Probably a bad batch, I stopped order parts from AliExpress. Had the same with a batch of IRF510. Complete rubbish. 

Murray Wills (ZL2IQ)
 

Hi I did the same only the KA78T05 I got (bought 10 cause they were cheap) went red hot 🥵 so I shut it all down and went back to the step down to power the Nextion. Luckily it didn’t seem to introduce any noise for me.

Something wrong with the KA78T05’s as they should have been able to handle the current.


From: BITX20@groups.io <BITX20@groups.io> on behalf of MVS Sarma via Groups.Io <mvssarma@...>
Sent: Tuesday, September 17, 2019 4:12:57 AM
To: BITX20@groups.io <BITX20@groups.io>
Subject: Re: [BITX20] Nextion with KA78T05
 
I suffered with 10pcs of  bad 78t05 from ali seller and demanded refund and got it.

On Mon, 16 Sep 2019, 9:37 pm Herman Scheper, <Hsrscheper@...> wrote:
I am using the Nextion 5"display. When building my Ubitx V5 I used a small step down converter to generate the 5V for the Nextion display. It all worked 100%
Now the building is almost finished i want to replace the 5V stepdown converter. ( It is giving a little amountt of noise…)

I ordered ( Aliex..) 5 pcs of KA78T05 TO-220 stabilisors. ( is the 3A version-  )

Then I replaced the 7805 mounted on the Arduine and replaced it for the KA78T05 version…. No working Arduino and no working display.

I find input=12V output=0V.   Checked before and after the pinning  1=input / 2 = earth/ 3 =output   Find a datasheet on Internet and can not find what I am doiing wrong.

After that make a test print only for the KA78T05 and I can not get the 5V output. Output also on a 2nd KA78T05 is still 0Volt.   Frustrating.  ( I used a 7809 on the same test and output was 9.00 Volt!)

What I am doiing wrong...…….?     ( ordered today local few 78S05   ( = 1.5A version  )

So who knows what I am doiing wrong...?                       Tnx.  Herman

Razvan (M0HZH)
 

Why would you need another regulator ? The LM7805 is designed for 1A output and the 5" Nextion display + Raduino uses about 500mA (per specs).

You only need a heatsink for the LM7805.

This will be true for any other linear replacement you use (KA78T05 / 78S05 etc) as the dissipated power is the same.

Jerry Gaffke
 

Could be that you got bad parts.

Or could be that the part is getting too hot, and going into thermal shutdown.
So you need a better heatsink.


On page 2 of this Fairchild datasheet for the LM78T05, it says that
the "thermal resistance junction to air" is 65 degrees C per Watt.
The "junction" is on the silicon die, heat must travel from die to the metal TO220 tab,
and from there get carried off into the air.
    https://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/LM78T05.pdf
Thermal resistance junction to case", that is from die to the TO220 tab 
is 2.5 degrees C per Watt.  Useful to know if you have a good heatsink for it.

Assume we are supplying the LM78T05 with 12 Volts and drawing 1 Amp at 5 Volts from it.
The LM7805 is then dissipating  (12v-5v) * 1a = 7 Watts,  and if being used without
a heatsink then the tab will be 7*65= 455 degrees C (819 F) above the ambient air temperature.
If the ambient air is 23 degrees C (73 F), then the tab will be around 23+455 = 478 C (892 F).

Actually, it will never get anything like that hot. 
The device goes into thermal shutdown (and the output goes to zero Volts) when it gets too hot,.
Not explicitly stated in the datasheet but I assume somewhere around 125 C (257 F).
(It also has a separate mechanism to limit the maximum current
in case you manage to short the output with a screwdriver.)

So if the device seems to work for a few seconds and then quits, touch the tab with a wet finger. 
If your finger sizzles, then the LM78T05 (or LM7805) is too hot.

I can believe that some devices out there sold for cheap are factory seconds
that don't meet spec, or rejects that don't work at all, or some totally different device
that has been painted over with an "LM78T05" logo.

A test for basic function would be to put a 50 ohm resistor (able to dissipate at least 1/2 Watt)
across the 5 Volt output, so it is only being asked to give  5v/50 = 0.1 Amp.
Then measure the output voltage, see if it really is 5 Volts.

The Fairchild "thermal resistance junction to air" figure of 65C/Watt
is much higher than TI's LM7805 figure of 19C/Watt.
  https://www.sparkfun.com/datasheets/Components/LM7805.pdf
Perhaps due to different assumptions about available air flow?
The "thermal resistance junction to case" figures are fairly close,
2.5C/W for the LM78T05 vs 3.0C/W for the LM7805.
Both are TO220 devices, so the additional thermal resistance from case to air
should also be about the same.

Jerry, KE7ER


On Tue, Sep 17, 2019 at 04:05 AM, Razvan (M0HZH) wrote:

Why would you need another regulator ? The LM7805 is designed for 1A output and the 5" Nextion display + Raduino uses about 500mA (per specs).

You only need a heatsink for the LM7805.

This will be true for any other linear replacement you use (KA78T05 / 78S05 etc) as the dissipated power is the same.

Herman Scheper
 

You are 100% right!! tnx

 

Van: BITX20@groups.io <BITX20@groups.io> Namens Razvan (M0HZH)
Verzonden: dinsdag 17 september 2019 13:06
Aan: BITX20@groups.io
Onderwerp: Re: [BITX20] Nextion with KA78T05

 

Why would you need another regulator ? The LM7805 is designed for 1A output and the 5" Nextion display + Raduino uses about 500mA (per specs).

You only need a heatsink for the LM7805.

This will be true for any other linear replacement you use (KA78T05 / 78S05 etc) as the dissipated power is the same.

Herman Scheper
 

Hello Jerry,

 

I ordered 5 pcs. of them. After it did not work-from the 1th second!! )- i tested 2 of them on a small breadboard. Without load and with a load of a few 100 Ohms.

The did never work. Always output  zero Volt.   Some other members on this forum had the same experience with them.

So never buy this parts form Aliex.

 

Tnx fort he explanation.

 

Rgds,  Herman  PA0BAB

 

 

Van: BITX20@groups.io <BITX20@groups.io> Namens Jerry Gaffke via Groups.Io
Verzonden: dinsdag 17 september 2019 16:13
Aan: BITX20@groups.io
Onderwerp: Re: [BITX20] Nextion with KA78T05

 

Could be that you got bad parts.

Or could be that the part is getting too hot, and going into thermal shutdown.
So you need a better heatsink.


On page 2 of this Fairchild datasheet for the LM78T05, it says that
the "thermal resistance junction to air" is 65 degrees C per Watt.
The "junction" is on the silicon die, heat must travel from die to the metal TO220 tab,
and from there get carried off into the air.
    https://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/LM78T05.pdf
Thermal resistance junction to case", that is from die to the TO220 tab 
is 2.5 degrees C per Watt.  Useful to know if you have a good heatsink for it.

Assume we are supplying the LM78T05 with 12 Volts and drawing 1 Amp at 5 Volts from it.
The LM7805 is then dissipating  (12v-5v) * 1a = 7 Watts,  and if being used without
a heatsink then the tab will be 7*65= 455 degrees C (819 F) above the ambient air temperature.
If the ambient air is 23 degrees C (73 F), then the tab will be around 23+455 = 478 C (892 F).

Actually, it will never get anything like that hot. 
The device goes into thermal shutdown (and the output goes to zero Volts) when it gets too hot,.
Not explicitly stated in the datasheet but I assume somewhere around 125 C (257 F).
(It also has a separate mechanism to limit the maximum current
in case you manage to short the output with a screwdriver.)

So if the device seems to work for a few seconds and then quits, touch the tab with a wet finger. 
If your finger sizzles, then the LM78T05 (or LM7805) is too hot.

I can believe that some devices out there sold for cheap are factory seconds
that don't meet spec, or rejects that don't work at all, or some totally different device
that has been painted over with an "LM78T05" logo.

A test for basic function would be to put a 50 ohm resistor (able to dissipate at least 1/2 Watt)
across the 5 Volt output, so it is only being asked to give  5v/50 = 0.1 Amp.
Then measure the output voltage, see if it really is 5 Volts.

The Fairchild "thermal resistance junction to air" figure of 65C/Watt
is much higher than TI's LM7805 figure of 19C/Watt.
  https://www.sparkfun.com/datasheets/Components/LM7805.pdf
Perhaps due to different assumptions about available air flow?
The "thermal resistance junction to case" figures are fairly close,
2.5C/W for the LM78T05 vs 3.0C/W for the LM7805.
Both are TO220 devices, so the additional thermal resistance from case to air
should also be about the same.

Jerry, KE7ER


On Tue, Sep 17, 2019 at 04:05 AM, Razvan (M0HZH) wrote:

Why would you need another regulator ? The LM7805 is designed for 1A output and the 5" Nextion display + Raduino uses about 500mA (per specs).

You only need a heatsink for the LM7805.

This will be true for any other linear replacement you use (KA78T05 / 78S05 etc) as the dissipated power is the same.

Mark Hatch
 

Perhaps thermal overload, but I was having problems with the 7805 until I replaced it when I put in a 5" Nextion. I did not try the heat sink as it required more cutting to fit then I wanted to try. I also planned on additional boards (stand alone signal analyzer and JACKAL, so a higher rated 7805 mounted on the back panel (using the case as the heat sink) worked well for me. 

Mark

ajparent1/KB1GMX
 

Very simple the 7805 and kin are linear regulators and at 500ma the max voltage that 
can be tolerated before the power dissipated as heat is about 1.5W which places the
input voltage at not more than 8V, you need to be at more like 7.5 due to heating.

Now add a decent chunk of metal, a heat sink, and that goes up depending on
the size of the heat sink.

How to calculate all that is trivial, the series current though the device
(current into pin1) times the voltage across it (voltage from pin 1 to pin3)
and that in watts of heat dissipated.  *Note the 78xx series of regulators
must have about 2V minimum across it to regulate (for 5V part that's
about 7V).  Even at 100-150ma a heatsink (not less than 2" square exposed
air) is advised.  The maximum temperature before shutdown or damage is
about 125C (hot enough to burn skin).

Max wattage is dependent on quality of heatsink but the 2.5degreesC/Watt
is the limit or about 40W but max current is usually reached before that
and in some cases max voltage.  However to get that out of the nominal
7805 that means a heat sink of about 2x2 inches with .5" fins with some airflow.

For those making up their own circuits insure there are a 1uf (or larger) at 
Pin 1(input), and .1uf on pin 3 (output) to ground close to the regulator as
it can oscillate and that will cause failure like behavior.

I started using those regulators when the first came out in the early 70s for
microcomputers (8008 and on) and I have a bag of them from the early
80s I still use.  Good reliable part, cheap too but often abused and
misused.

Allison

_Dave_ K0MBT
 

Have had success with the stock 7805 mounted on the raduino. Have 4 of the critters with up to 3.5" nextions I use a stock power transistor heat sinks with an additional hole. To mount it.

Have found mounting the 7805 out of the front raduino offers more room if the board is mounted properly in the case.
--
73
Dave
k0mbt
Ham_Made_Keys

MVS Sarma
 

Check , raduino with nextion miight need 1amp odd. You cant work at 100% with input 12 or 13.5v.

If the 7805 input is just 3v above what you said is understandable.

Better try a genuine 78s05 it might manage. 
All the best

Sarma vu3zmv

On Tue, 17 Sep 2019, 4:35 pm Razvan (M0HZH), <razvanfatu@...> wrote:
Why would you need another regulator ? The LM7805 is designed for 1A output and the 5" Nextion display + Raduino uses about 500mA (per specs).

You only need a heatsink for the LM7805.

This will be true for any other linear replacement you use (KA78T05 / 78S05 etc) as the dissipated power is the same.

Razvan (M0HZH)
 

Maximum current draw for the 5" Nextion (backlight @ 100%) is 410mA 
Arduino Nano ~20mA
Si5351 (all outputs enabled) ~22mA

Assuming 13.8V input, there's over 4W of thermal power to be dissipated. As Allison already explained, whatever IC you'll use it'll need a heatsink.

Also, try to keep the heat away from the Si5351 oscillator crystal as it would generate frequency drift.

Lawrence Galea
 

When dropping down voltage I always use more than one regulator in series to split the dissipation.
E.g. 7809 or 7808 from the nominal 12V then to the 7805 and bolt them to the aluminium case or to a heat sink with thermal compound.
They are cheap enough not to bankrupt you.
Take care when buying from Ali etc
Regards
Lawrence

On Tue, Sep 17, 2019 at 11:50 PM Razvan (M0HZH) <razvanfatu@...> wrote:
Maximum current draw for the 5" Nextion (backlight @ 100%) is 410mA 
Arduino Nano ~20mA
Si5351 (all outputs enabled) ~22mA

Assuming 13.8V input, there's over 4W of thermal power to be dissipated. As Allison already explained, whatever IC you'll use it'll need a heatsink.

Also, try to keep the heat away from the Si5351 oscillator crystal as it would generate frequency drift.