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AID: RE: Re: News from AID Development

From: robc   robccitytv.com
Date: Tue, 25 Jan 2005 13:31:22 -0800

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I played around a bit, (after re-reading the previous post, and realizing
that you had done this already) and made up a test with 8 270 ohm resistors,
and found that yes, the regulator does get uncomfortably hot. I made a small
heatsink from a chunk of aluminum, and bolted it on, this seemed to help
considerably, so maybe it should be standard to spec this, at least until a
solution for load supplies is sorted out.
As for switching supplies, integrated DC-DC converters tend to be about 20
to 60 bucks from digikey- this increases the mainboard cost by a fair bit. 
I've noticed lately that generic laptop power supplies are available for
reasonable prices, it might be one solution to suggest using one of these if
higher currents are required- you could split the power into logic and load
power, and run any load off the 12v from the supply. If people are just
running small motors and LEDs a simple dollar-store plug adapter would be
fine. 
 

Rob Cruickshank 
Engineering Projects Group   
CHUM Television
Voice: 416.591.7400 x2175
Fax: 416.591.2321
Email: robc@citytv.com                            


> -----Original Message-----
> From: Rob Cruickshank [mailto:robcruic@sympatico.ca]
> Sent: Wednesday, February 23, 2005 3:29 PM
> To: mike2005@acc.ocad.on.ca; AID Mailing List
> Subject: AID: Re: News from AID Development
> 
> 
> : Sent via the Art Interface Device mailing list: aid@interaccess.org
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> private response
> 
> 
> 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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