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AID: Re: News from AID Development

From: Rob Cruickshank   robcruicsympatico.ca
Date: Sun, 23 Jan 2005 09:23:27 -0800

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8 LEDs at 20Ma each is only 160 ma- the 7805 is capable of up to 1 amp.
Of course, as you noted, input voltage makes a big difference, remember that
the power dissipated in the regulator
is the product of the current through it (which is the total current in the
circuit) times the voltage across it, ie the input voltage minus the output
voltage.  So for 9 volts in, the power (assuming no other loads in the
circuit, is 4*160mA=.64W.  It would be informative to measure the actual
current the circuit is drawing, and compare it to what should be expected.
This can be done for sections of the circuit too, just cut the trace and put
the ammeter in circuit. Remember that all pull-up/down resistors are
connected in parallel, and contribute to the load- this can add p
suprisingly quickly. (simply multiply the current through each resistor by
the number of resistors- if you have 10 10k ohm pulldown resistors, that
would be 10*.5ma=5ma.
.

Perhaps the solution would be a regulated and unregulated power bus? Often I
put two or more regulators in a circuit,-one for the logic and one for the
loads. Adding a slot for a second regulator would alow the user to drop in a
7812 for 12 volt loads, or a 7805 for 5 volt loads.
My gut feeling is that a switching supply is overkill for this and that
there's still a problem, if the regulator is too hot to touch.  I often
build circuits with a microcontroller, and motors drawing a few hundred
milliamps, running off 3 terminal regulators with no problems whatsoever.



----- Original Message ----- 
From: <mike2005@acc.ocad.on.ca>
To: "AID Mailing List" <aid@interaccess.org>
Sent: Saturday, January 22, 2005 10:03 AM
Subject: AID: News from AID Development


> : Sent via the Art Interface Device mailing list: aid@interaccess.org
> : Use your "Reply All" to  reply to the list, "Reply" for private response
>
> We have good news!
>
> The Art Interface Device is bug free as of last weekends meeting! In the
> previous meeting Mike and Mel squashed the startup bug and last week we
> confirmed that the mainboard is working properly.
> By far the most significant factor proved to be the input voltage to the
> regulator. The 7805 can take an input voltage of 7 to 35 volts but the
> heat generated and the ability to dissipate it compounds as the voltage
> increases. We found that 12v was too hot to hold, 9v could just barely
> be held, and 7v was almost skin temperature with the circuit as detailed
> above. Of course this heat means that power is being wasted and battery
> life will be diminished so we are now considering a switching power
> supply which would be very efficient with no heat problems. An ideal
> solution would be a switching supply with the same pin outs as the 7805
> so that users could choose which to use. Please make suggestion for your
> favorite switching power supply.
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