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salo2k4
Posts: 9
Hello. Can anyone explain that thing... i made radio stack panel on one mega2560 with connected max 7219 modules 4 in series and 3 in series. When i use only USB connection for powering all led modules lit correctly (flicker a bit but all modules on). But if i connect external power (9 V DC) to mega2560 power connector at start moment all led modules (without flickering) on, but after 2 seconds some modules completely destinguished. Whats wrong?
... and one more thing. With ext power connected when i use Test buttons in Config Wizard of MF App all led modules separately tested OK. No destinguish in 2 sec. Previous behavior in-sim only




[Last edited by salo2k4, 2021-04-18 10:05]
2021-04-18 09:17
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StephanHo
Moderator
From: EDDG, Germany
Posts: 1867
Supporter
Hi salo,

first check whether your external voltage source can supply enough current. The power supply should be able to deliver at least 500mA.

If possible, set your external power supply to 5V output voltage. If this is not possible because the voltage is fixed at 9V, use a 5V voltage regulator so that only 5V is present at the output.

Basically, the Mega does not need an external voltage source for itself, because the current via USB is completely sufficient.

To supply the MAX modules with their own voltage source, proceed as follows:

- disconnect the external power supply from the Mega.
- disconnect the 5V voltage connection from the MAX modules and the Mega (ideally you only have to separate one line).
- connect the GND of the external power supply to the GND of the Mega
- make sure that the MAX modules are only supplied with 5V from the external power supply!
- connect the 5V of the external power pack to the voltage supply of the MAX modules
- turn both on.
- if the MAX modules do not display properly, press the reset button on the Mega

What will these measures achieve?

You now have two independent circuits for your panel. One 5V for the Mega via USB and one 5V for the MAX modules from the external power supply. By connecting the GND of the Mega and the GND of the external power supply unit, both circuits have a common reference potential (GND) but separate power supplies.
The power consumption of the MAX modules no longer burdens the Mega.

If the problems persist, it may be that the supply voltage of your external power supply unit is not stabilized or the output voltage is not smoothed enough and the MAX chips are disturbed as a result. Try an electrolyte capacitor of 2200uF,/15V which you connect to the output of the external power supply. It is essential to observe the polarity!

Please report the result
Grüße,
Stephan (Time: UTC+2)
2021-04-18 12:44
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salo2k4
Posts: 9
StephanHo thanx for helping. Power supply 3A, i think enough. It is not helpful to powering mega with 5V external power. Ext power source for mega must be within 7...12V. But I will try to use 5V separate power to each MAX.
But firstly i'll try use capacitor. Thank you! I'll report as soon as made
2021-04-18 15:52
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pizman82
Moderator
From: ETSI, Germany
Posts: 6010
Supporter
Hi

My personal experience is..... There is fully no need to Power a Mega external.
500mA USB Power is fine.
Just get sure there are 500mA USB Power..... So if you use a HUB then get sure its a powerd Hub that again support 500mA to each Port via USB.

If there are devices that need lots of Power ( Steppers e.g.) then a power via external device ( and a shared GRD) is a good way as Stephan recommendet.

Whatever..... A Setup with just a couple of 7Seg pretty sure not need one or the other. Here 500mA from USB are way enough to handle it !
Good Luck !
2021-04-18 18:19
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salo2k4
Posts: 9
Hi :) it's not a "couple"... seven MAXs on one Mega :)
p.s. i see that my pictures not loaded on first post, which host i can use to upload pictures on forum?
2021-04-18 18:53
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PaulH
Posts: 15
Hi

I can confirm that powering a chain of MAX display drivers may require an external power supply.

A single segment of a 7-segment digit may draw up to 6 mA (measured value). For my 22 digits displays (3 MAX chained) I calculated an average current of ~400 mA. Depending on what you display (blanks or all eights) the power consumption can vary widely. So it would by wise to supply from an external power supply. I use a 7.5V 2A supply with a L7805 voltage regulator.

The raw supply voltage (7.5V) can also be provided to the MEGA via pin VIN where the board creates it's own regulated 5V supply. This way the MEGA does not draw power from USB.

Paul
2021-04-18 22:05
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PaulH
Posts: 15
DO NOT power the MEGA externally AND draw power from the MEGA for your display drivers.

The 9 Volts are regulated down to 5V on the MEAG board. 4 Volts are burned in the on board voltage regulator. If you draw 500 mA, that is 2 Watts of thermal power burned by the regulator. Just measure the temp on the regulator. It must get pretty hot there

Paul
2021-04-18 22:18
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salo2k4
Posts: 9
Thank you Paul! I don't know Mega very vell, is there some overcurrent protection? My case looks like in-sim all led modules become active and then off due to overcurrent
2021-04-18 22:42
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PaulH
Posts: 15
The board has a resettable polyfuse. This protects the USB ports from shorts and overcurrent. If more than 500 mA is drawn the fuse will automatically break the connection until the situation is removed.

But what I mentioned above is an overlaod of the regulator. Even if you supply external power via the jack, the regulator must also be able to handle the current drawn. The regulators are not equipped with heat sink. I assume they are not designed for high current draw. The regulator may not be able to maintain the 5 Volts and the voltage drop is causing the drop outs in your displays.

Either way supply extra power (regulated 5 Volts) directly to your MAX drivers, instead of drawing from the MEGA board.

Paul
2021-04-18 23:26
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salo2k4
Posts: 9
Hi! problem solved with dividing 4 MAXs in series on two parts with adding capacitors to each MAXs pair power inputs. Thank for all for helping!
2021-04-24 16:13
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apachebcn
Posts: 3
Hello.
I see that you were talking about voltage and consumption problems of the screens with the max chip.
I have a very strange matter on my hands.
I am using 2 linear potentiometers for the throttle.
The values and their behavior are very stable.
But when only one display is turned on, the potentiometers start to oscillate constantly.
I have 6 displays connected in series, but I have already tried disconnecting them all and only 1, to rule out interference.
Even with that 1 display and minimum brightness, the strange problem happens to me.
It would be strange if it were interference, but I also find it strange that it is due to voltage because just one display at minimum brightness I don't think it will make any consumption.
2024-03-03 11:52
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kjoussen
From: Hamburg, Germany
Posts: 178
Just my 5 cents on this and similar "probs" from what i've learned with experience and during my university time(many many years ago).

External power for the Mega is senseless, as the "bad consumer" are the leds and the Mega can't serve what they need.

So, when chaining several MAX and displays:
1) Capacitors on the MAXes are a good idea at any time to cover any hick-ups.
2) Use external Power for all "power hungry" modules but connect the ground from the Mega-Poser-Supply and the power supply for the consumers.
With this you'll have the same "ground" and everything will be fine.

I had same problems when driving a servo motor (10 turns), which i had to supply with 12V.
After i connected the grounds of the Mega's powersupply and the motor power supply...tada...all probs where gone.
2024-03-06 23:03
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