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ravi00bkp
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TTL source current

03/02/2008 2:23 AM

Can a ttl gate source 10mA to drive an LED if its output is connected directly to the led with appropriate resistance ?

Will the high current drawn damage the emitter follower in the totem pole?

pls help me ,am confused!!!
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user-deleted-1105
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#1

Re: TTL source current

03/02/2008 4:13 AM

you need a current limiting resistor in series with the LED.

Check the data sheet for the device... some logic families can source more current than they can sink (or vice versa).
E.G If you connect the led from 5v down to the output pin then the pin is 'sinking' current .
If you connect the Led from the output pin down to ground then the pin is 'sourcing' current.
(Of course the two situation above give opposite logic for 'ON')

Del

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JohnDG
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#2

Re: TTL source current

03/02/2008 6:10 AM

Further to Del's comments, a 'genuine' SN7400 (from the TI databook) is spec'd to sink 16mA @ 0.4V max.- so it would happily give you 10mA (with a suitable series resistor). It'll only source less than ½mA.

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Electroman
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#3
In reply to #2

Re: TTL source current

03/02/2008 9:45 AM

Correct John!

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uweka
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#4

Re: TTL source current

03/03/2008 4:17 AM

You should also observe the whole IC`s power as every connected output plus the internal power determine the case`s temperature.

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Randall
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#5

Re: TTL source current

03/03/2008 10:31 AM

There are loads of ttl devices (both varieties of gates/functions, and, different technologies) you need to look at the data sheet of the device you wish to use, or, choose the device to do the job.

Plenty will do it easily.

Here's just a few early technologies: IOH won't do for any of them; IOL reads 7400:16 mA, 74LS00:4mA, 74S00:20 mA and 74F00: 20 mA

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vikas_303
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#6

Re: TTL source current

03/07/2008 9:45 PM

it will glow properly but use 5V supply with a small valued resistance
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