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Topic: Power supply troubles (Read 2026 times) previous topic - next topic

SimbaSpirit

Retrolefty:

When I got everything working 100% I wasn't using the the input/output caps or bypass caps, so I didn't include them here.
I'll look into a higher rated transistor, I'm kinda limited by what I have in 2 stores near me.

Robcam:

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With the 9V 650mA hooked to the Uno and the nte:
The 9v 650mA was loaded just enough to keep the nte and your crt. happy (you got lucky).

You may be right. I ordered a 5V 1A power supply from sparkfun to swap out with this one.

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With the 9V 650mA hooked to the nte only or the 12V 7Ah battery to nte:
Nte not happy, too much input voltage to output 5V @ >500mA

The spec sheet rates this regulator at 35V max input voltage and "Output Current in Excess of of 1.0 Ampere".

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To get the rated 1Amp from the nte you need:
A heat sink, not much more than 7V on its input and perhaps a small sacrifice to the electron god.
The more voltage you input to the nte the less current it can provide.

Don't need the whole amp, just .7 or so. And would the the 2 extra volts over its minimum operating voltage cause that much of a problem?
I didn't heatsink the regulator because I figured I was operating pretty well under tolerance... didn't notice anything heating up, but maybe it would have under a longer test run.

Thanks for bouncing all these ideas off me guys. So far I'm going to look into a sturdier transistor to solder in this one's place.

James C4S

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The spec sheet rates this regulator at 35V max input voltage and "Output Current in Excess of of 1.0 Ampere".

It also states with proper heat sinking.

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Don't need the whole amp, just .7 or so. And would the the 2 extra volts over its minimum operating voltage cause that much of a problem?

Use some math to see:

Linear amps have the same amount of current In and they do Out.  So using system current and the voltage drop across the amp, you can calculate how much power the amp is dissipating.

With 9V in:
Vdrop = 9V - 5V = 4V
Power = Voltage * Current = 4V * 700mA = 2.8Watts

With 12Vin:
Vdrop = 12V - 5V = 7V
Power = Voltage * Current = 7V * 700mA = 4.9Watts

That's 1.75 times the power which, in this case, is just heat.  The regulator is likely going into thermal shutdown in both cases.
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retrolefty

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When I got everything working 100% I wasn't using the the input/output caps or bypass caps, so I didn't include them here.


That doesn't eliminate the need for the regulator input and output caps. A circuit not using them is bound to be unstable and the regulator likely to break into oscillation at any instant. Find a good datasheet for a 7805 voltage regulator and it will show and explain the need for the caps. The NTE device is just a 're-branded' 7805.

Lefty


SimbaSpirit

Looked at the fairchild datasheet and it agreed that one was needed on both sides if the regulator is an appreciable distance from the power supply filter.
I'll do some more research into regulator oscillation.

I looked at the datasheet for the thermal issues and I see my error there.
If I read it correctly it gains 65C/watt and has a maximum operating temperature of 150C.
So anything 2.3+ watts needs a heatsink, though logically anything over 1.5 should have one...

MarkT

If you are switching 0.5A of LEDs at logic speeds you need _substantial_ decoupling on your 5V, start with 0.22uF ceramic close to 328 + 47uF electrolytic for the board as a whole.  A ground plane is wise.
[ I won't respond to messages, use the forum please ]

SimbaSpirit

Ok I placed a 22 pF ceramic capacitor on both sides of the dip socket across +5v and gnd. Both capacitors are close enough to touch the socket.
I also placed a 47uF electrolytic between the power source and the rest of the circuitry.

I'm looking into a local source for a more powerful npn transistor, but it's looking like I'll just have to use what's there.

Any other suggestions?

SimbaSpirit

Hooked it up to 5 volts after adding the cap to test it out.
The first time the lights came on then dimmed, which was weird.
Every subsequent test worked perfectly.

Thanks for the help guys :)
I feel better having those caps in there.

MarkT


Ok I placed a 22 pF ceramic capacitor on both sides of the dip socket across +5v and gnd. Both capacitors are close enough to touch the socket.



Do you mean 22pF?  A pF is a millionth of a uF, it won't do anything for a regulator (or is this for a crystal?
[ I won't respond to messages, use the forum please ]

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