STK500 Current sourcing capability

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Hi I'm new.

I wanted to know the current sourcing capability of the VTG pin for external connections.

I want to multiplex 7 segment leds. Each segment will take 10ma and the decoder (74ls47) will take about 30ma. So a total of about 100ma is required.

My question is: Can i use the VTG pin as Vcc of the external circuitry for the 7 segment led multiplexing? Also, how much maximum current can the pin provide?

I have connected a 12V 1200ma adapter to the STK500.

Thanks in advance.

Aakash

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As the Vtarget regulator already gets pretty warm with the
default load current (it draws something like 130 mA from
the supply then), I pretty much doubt it can source that much
additional current.

Also, consider replacing the 74LS47, and drive the segments
directly. Decoding can be done in software. (Only downside
is you need 7 pins rather than 4.)

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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Page 3-14 of the AVR STK500 (version 1925D-AVR-10/04) Users Guide says:

Quote:
Using the on-board supply voltage, approximately 0.5 A can be delivered to the target section. See "Appendix A" in section 10.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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The voltage drop on most linear regulators is in the 2 volt range, so just use a voltage setting on the power supply that is 2-3 volts above the VCC you plan on using, like Steve said it will keep the heat down on the regulator, and if you're not using high voltage programming there's no reason to use a 12 volt power supply on the STK500

-Curiosity may have killed the cat
-But that's why they have nine lives

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Thanks for the replies.

The adapter can be adjusted to 9, 7.5, 6, 4.5 and 3 volts. So I'll give it 7.5V since i plan to use 5V as Vcc. Since I have got a 8515L I'll experiment with giving the STK500 6V and the VTG as 3V later. Is this allright?

I just looked up the datasheet and ATmega8515 consumes 5ma (with all pins as inputs) at 4MHz (?). Is that right?

As transistor base currents for multiplexing the 7 segment leds will be negligible and if I configure it to sink current while low to drive the segment on, the current consumpion should be about the same as 5ma.

Does that mean i have 495ma for the external circuit?

But dl8dtl said that it consumes 130ma at default. Where am I going wrong?

Aakash

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Quote:
Using the on-board supply voltage, approximately 0.5 A can be delivered to the target section. See "Appendix A" in section 10.

Edit:
Of course, this assumes that your power supply is capable of delivering the current to the components of which it is supplying.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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> But dl8dtl said that it consumes 130ma at default. Where am I going wrong?

That's the value our power supplies in the lab are displaying. The
STK itself already draws a lot of power for its various components.
While the 500 mA spec'ed (which I feel a bit too much) is certainly
meant to be the current to be fed into the target circuitry, the
remaining 120 mA (or so) will also cause power dissipation in the
voltage regulator.

Give it a try, but don't press your finger for too long onto the heat
sink copper area of the STK when you try it the first time...
Lowering the input voltage will certainly help (though unless you've
got a switchmode power supply for the input power, it will only move
the location of where the heat is generated from the STK into the
power supply). When going down to 3 V supply, keep the following
things in mind:

  • you cannot run the AVR at full speed
  • the AVR's source and sink capabilities will decrease quite a bit
  • if you still plan on interfacing to a 74LS47, think of the level conversion issue between both

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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Thanks dl8dtl and thanks to everyone else in this thread :)

I am thinking of not using 74LS47 and drive the segments directly. Also, I'm thinking of removing the transistors for selecting the 7 segment display, and select it by connecting the common anode directly to the pin as it can source 70mA (though it would be stretching it and also considerable voltage drop on the pin).

I'll try this and keep you updated. :P

Aakash

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Quote:

While the 500 mA spec'ed (which I feel a bit too much) is certainly
meant to be the current to be fed into the target circuitry, the
remaining 120 mA (or so) will also cause power dissipation in the
voltage regulator.

Hello, Jorg--

It is very common for us to use our STK500 to power prototype apps, and also as a quick-and-dirty power supply for one-off test rigs and the like.

Yes, dropping the input supply V lessens the heat dissipation. But a balance must be struck with lower-voltage supplies--weird things can happen when a heavy load kicks in and the supply V drops momentarily and the regulator drops out and ... . I find that for everyday work (no HV programming) a 9.6V supply from an old cordless phone works fine.

If you will regularly use your STK500 as a bench supply, simply grab a small chunk of heatsink (or two) and cement to the regulators. It works a treat; you should always be able to hold your finger on the heatsink at a few 100mA in my experience, so the regulators are actually running nicely cool.

I think your 100mA recommendation is very conservative. We regularly power apps with LCD backlights (150-200mA by itself) all day long off the STK500 without problems or excessive heating of the regulators.

[edit] We've been using our STK500 more and more as a programmable bench supply in the past year or two. We've had a series of battery-powered and other lower-supply-voltage (than good old 5V Vcc) apps; the battery apps include battery monitoring. With AVRStudio or similar tool, programming the supply V up and down the full range is very helpful for checking the app's response at different levels. The digital settings come quite close to actual--within a few hundredths of a volt.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.