Power

A "new fashioned" televisor, using an Arduino to drive the motor and display.

Moderators: Steve Anderson, Dave Moll, Andrew Davie

Re: Power

Postby Andrew Davie » Tue Jul 04, 2017 12:08 am

Steve Anderson wrote:Using a MOSFET is a good move, though I doubt you notice any real difference in LED brightness just because the voltage loss has gone from 1.2V down to less than half that.


True, but it might be the difference between being able to run 3 LEDs in series and only 2. In any case I'm moving to a higher voltage source for rev 2.

Steve Anderson wrote:I assume you have a series resistor in the base lead for the TIP device, with a MOSFET that can be removed. i.e. zero Ohms.


Yes, 16 ohms at the moment, tailored to my original 26V LED matrix - I will/have remove/d in the design for rev 2.
Correction: I have a 16 ohm resistor in series with the emitter of the TIP122. The base is connected directly to the Arduino pin. I'm pretty sure we worked out in the earlier discussions on the LED matrix circuit that this was OK.

Steve Anderson wrote:I would investigate why this happened, not just hope it doesn't return.


How "what" happened? ............;) lol. OK, so I don't really have much of an idea where to even start with this. I replaced the IRL540 same issue seen. There's the component, and then it's connected to the Arduino - with a 10K resistor from the PWM input to ground (this is optional and to cure the 'floating' state of the pin possibly causing motor spin. I now think this is way overkill anyway, but it's there nonetheless). I can't desolder/replace the Arduino - way too much work. So, not really sure I want to tackle this issue. I'm moving on with a completely new/revamped circuitboard so .. mmmh....
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Re: Power

Postby Andrew Davie » Tue Jul 04, 2017 12:51 pm

10 of these DC-DC converter boards arrived today. They're truly tiny.

ps.jpg
ps.jpg (221.39 KiB) Viewed 1768 times


What I'm now thinking is using these on my rev 2 board instead of the LM317+resistors - I may even have room for 3 of, so one for the Arduino, one for the motor, one for the display. Then the board would be adjustable for different scenarios (motor/LEDs) without any soldering required. Input voltage: 4.75V-23V, Output voltage: 1.0V-17V, Current output: peak 3A, 1.8A sustained
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Re: Power

Postby Steve Anderson » Tue Jul 04, 2017 2:27 pm

Andrew Davie wrote:Yes, 16 ohms at the moment, tailored to my original 26V LED matrix - I will/have remove/d in the design for rev 2.


When I said the 'base', I meant the electrical base, not the mechanical 'base' which on a TIP122 is the electrical collector. If you DO have a 16 ohm in series with the electrical base of the TIP122 you probably are overloading the output of the Micro. To avoid confusion the mounting arrangement for TO220 cased devices, and others, is usually called a 'tab'.

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Re: Power

Postby Andrew Davie » Wed Jul 05, 2017 12:34 am

Steve Anderson wrote:
Andrew Davie wrote:Yes, 16 ohms at the moment, tailored to my original 26V LED matrix - I will/have remove/d in the design for rev 2.


When I said the 'base', I meant the electrical base, not the mechanical 'base' which on a TIP122 is the electrical collector. If you DO have a 16 ohm in series with the electrical base of the TIP122 you probably are overloading the output of the Micro. To avoid confusion the mounting arrangement for TO220 cased devices, and others, is usually called a 'tab'.

Steve A.


The resistor is in series with the emitter of the TIP122. I should have been clear.
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Re: Power

Postby Steve Anderson » Wed Jul 05, 2017 5:07 pm

Ah! Now all is clear. With the MOSFET that arrangement won't work as you'll be dropping volts across the 16 ohm resistor which is sorely need for the gate drive. The source should go directly to ground/0V, the gate driven directly by the Micro and the drain to the LEDs via a current limiting resistor(s) - probably not of 16 ohms.

This will give you around an additional 4V or so of LED drive voltage.

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Re: Power

Postby Andrew Davie » Wed Jul 05, 2017 5:09 pm

Steve Anderson wrote:Ah! Now all is clear. With the MOSFET that arrangement won't work as you'll be dropping volts across the 16 ohm resistor which is sorely need for the gate drive. The source should go directly to ground/0V, the gate driven directly by the Micro and the drain to the LEDs via a current limiting resistor - probably not of 16 ohms.

Steve A.



Yes, that's almost what I have. I don't have the current-limiting resistor, but instead rely on the balancing resistors to do that job by making them all the correct values.
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Re: Power

Postby Andrew Davie » Mon Jul 10, 2017 1:25 pm

I tested a LM2596 DC converter today. I used a bench-top power supply and provided a load via my LED display matrix (which draws about 250mA at 12V). Firstly, I confirmed the working over a wide range of output voltages - achieved by a screw adjustment on the board. Below about 4V it's useless, but above that and up to the limit I tested (just the 12V I need) it's reasonably good. I took some images of the output waveform on my oscilloscope, and recorded temperatures using a heat sensor imager.

lm2596@1.jpg
lm2596@1.jpg (76.06 KiB) Viewed 1733 times


Firstly, the temperatures: I tested from input voltage 14V to 23V, all with output 12V. The temperatures at the hottest part of the LM2596 board were as follows...

Code: Select all
Input V   Temperature
14.8        68
16          63
17          65
18          77
19          71
20          75
21          78
22          85 and climbing rapidly


Timing and measurement issues contributed to the non-linear readings, but I conclude that it will run about 70C at the voltages I anticipate, and anything above 21V input is inadvisable as the temperature skyrockets.

I also measured the temperature of the LED matrix after it had been running for about 10 minutes - it was a steady 70C or thereabouts. Much different to my earlier measurement which must have been faulty. Ambient temperature in the office was about 21C.

Now to the oscilloscope images...


1) 14V input, 12V output

lm2596@14Vto12V.jpg
lm2596@14Vto12V.jpg (73.59 KiB) Viewed 1733 times


Quite noisy, with +/- 0.75V. A low input voltage relative to output doesn't look desirable.

2) 17V input, 12V output - looking at the ripple... about +/- 0.5v

lm2596@17Vto12V.jpg
lm2596@17Vto12V.jpg (74.03 KiB) Viewed 1733 times


3) 20V input, 12V output

lm2596@20Vto12V.jpg
lm2596@20Vto12V.jpg (80.32 KiB) Viewed 1733 times



So based on the ripple reducing in magnitude and becoming more regular as input voltage increases, it looks like a higher input voltage (balanced by the issues related to temperature) is better, at least for converting to 12V.

I also did some measurements of ripple at various output voltages - specifically, 7v, 12V and 5V that I use in my circuit... all with 17V input

1) 5V output (for PAM8302 amplifier)

lm2596@5Vwith17Vinput.jpg
lm2596@5Vwith17Vinput.jpg (71.73 KiB) Viewed 1733 times


2) 7V output (for Arduino)

lm2596@7Vwith17Vinput.jpg
lm2596@7Vwith17Vinput.jpg (77.52 KiB) Viewed 1733 times


3) 12V output (for LED matrix and motor)

lm2596@12Vwith17Vinput.jpg
lm2596@12Vwith17Vinput.jpg (73.28 KiB) Viewed 1733 times




Conclusions: The DC regulator works. It has fairly hefty ripple when input voltage is low and not much above the output. However, the ripple appears to be fairly high frequency (I guessed 120KHz, datasheet suggests 150KHz). At higher voltages deltas the ripple becomes smaller. The final configuration of 17V input seems reasonably good, with all the desired output voltages showing what I consider acceptable ripple. When tested under constant load, the device seemed to work OK, though heat becomes an issue at input voltages over 21V. I have not tested under varying load (e.g., a motor) but at this point I think the device looks suitable for intended use.
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Re: Power

Postby Andrew Davie » Mon Jul 10, 2017 3:39 pm

Question: A smallish capacitor across the output would reduce the ripple, right?
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Re: Power

Postby Steve Anderson » Mon Jul 10, 2017 6:42 pm

This is why when used with analogue circuits I don't use switched-mode supplies, they are efficient, yes, but just too damn noisy. In this case you have a mixture. I can't really make any suggestion here except starting with a known clean battery supply. A 12V sealed lead-acid battery would be ideal, say of 7Ah. Then gradually swap the supplies over to the switched-mode supplies and see what suffers (if anything).

With these SLABs as I call them, be careful, they can deliver 100s of amps during a short, molten copper everywhere. Fuses are essential!

But you can't really get a cleaner supply than a battery. Bench power supplies can be crap however many digits they have on the front - think China. If they're made by Fluke, you're insured but probably deeply out of pocket.

Steve A.

I bought two variable bench power supplies sometime in the early 90's in Hong Kong. Made in China, unknown brand, crap. I gutted them, used only the case, transformer and panel meter - far better. I still use them on an almost daily basis some 25 years later.
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