On Thu, 9 Aug 2012 09:31:11 -0700 (PDT), fungus <tooby@artlum.com> wrote:>On Thursday, August 9, 2012 3:19:09 PM UTC+2, k...@att.bizzzzzzzzzzzz wrote: >> >> Boost regulators will cost power, twice >> (higher voltage for the ballast resistor >> to dump and the inefficiency of the boost >> regulator). > >I've measured them at about 90% efficient >on 3x 1.5V batteries.I can believe that but even at 90% efficiency, you're throwing away >10% of your batteries' capacity *and* (Vboost-Vbatt)*Iled.>> It's better to use a current source, >> if "constant" brightness is needed. >> > >I don't think a textbook current driver >will really be more efficient than one >of those little boards. Plus you more >of them if there's multiple LEDs.It certainly is. You're both boosting to a higher voltage *AND* dissipating more in the ballast resistor because of it. To light an LED, current is the only thing that matters. You're throwing away everything above battery voltage, plus the 11% of the total for the boost regulator. The current source is nothing more than a voltage variable resistor so it dissipates *less* power than a resistor at high voltage (assuming the current is set to light the LED at the minimum voltage).>Having a known, regulated voltage to >work with makes life sooo much easier.Different discussion. Wall warts are even simpler.
How to power my circuit.
Started by ●August 7, 2012
Reply by ●August 9, 20122012-08-09
Reply by ●August 9, 20122012-08-09
On 8/9/12 9:31 AM, fungus wrote:> On Thursday, August 9, 2012 3:19:09 PM UTC+2, k...@att.bizzzzzzzzzzzz wrote: >> >> Boost regulators will cost power, twice >> (higher voltage for the ballast resistor >> to dump and the inefficiency of the boost >> regulator). > > I've measured them at about 90% efficient > on 3x 1.5V batteries. > >> It's better to use a current source, >> if "constant" brightness is needed. >> > > I don't think a textbook current driver > will really be more efficient than one > of those little boards. Plus you more > of them if there's multiple LEDs.Actually, I'm planning on using the TLC5916 from TI, which is a constant-current sink 8bit shift register, for exactly this purpose. That way brightness will be consistent.
Reply by ●August 10, 20122012-08-10
On Thursday, August 9, 2012 9:11:55 PM UTC+2, k...@att.bizzzzzzzzzzzz wrote:> > >I don't think a textbook current driver > >will really be more efficient than one > >of those little boards. > > It certainly is. You're both boosting to a higher voltage *AND* dissipating > more in the ballast resistor because of it.Yes...but current sources also need a higher voltage to allow the circuit to do its thing. eg. In the context of this project you'll need 4xAA for it to work. Even 4xAA is marginal for a 3.6V LED. With a cheapo current source (eg. twin-BJT) you'll run the risk of not being able to sustain the LED current until the battery is fully drained, making efficiency moot. ie. In most practical contexts the extra efficiency probably cancels out.
Reply by ●August 10, 20122012-08-10
On Friday, August 10, 2012 2:01:47 AM UTC+2, Daniel Pitts wrote:> > Actually, I'm planning on using the TLC5916 from TI, which is a > constant-current sink 8bit shift register, for exactly this purpose. > That way brightness will be consistent.Looks like the ideal chip for this... you get a shift register *and* current source all in one.
Reply by ●August 10, 20122012-08-10
On Friday, August 10, 2012 2:01:47 AM UTC+2, Daniel Pitts wrote:> > Actually, I'm planning on using the TLC5916 from TI, which is a > > constant-current sink 8bit shift register, for exactly this purpose. > > That way brightness will be consistent.PS: Have you seen the MAX7219 LED driver chip? It's similar to the TLC5916 but has built in multiplexing for 8 rows of LEDs.
Reply by ●August 10, 20122012-08-10
On Fri, 10 Aug 2012 00:28:23 -0700 (PDT), fungus <tooby@artlum.com> wrote:>On Thursday, August 9, 2012 9:11:55 PM UTC+2, k...@att.bizzzzzzzzzzzz wrote: >> >> >I don't think a textbook current driver >> >will really be more efficient than one >> >of those little boards. >> >> It certainly is. You're both boosting to a higher voltage *AND* dissipating >> more in the ballast resistor because of it. > >Yes...but current sources also need a higher >voltage to allow the circuit to do its thing.Geez. With the current source the current *does* its thing, without burning the extra power.>eg. In the context of this project you'll >need 4xAA for it to work..6V overhead is all that's needed.>Even 4xAA is marginal for a 3.6V LED.What LED? We used blues down to 3V and they were plenty bright to be seen outside during full sunlight (the product is used outside).> With >a cheapo current source (eg. twin-BJT) you'll >run the risk of not being able to sustain >the LED current until the battery is fully >drained, making efficiency moot.Nonsense. It works. BJT + FET>ie. In most practical contexts the extra >efficiency probably cancels out.Simply wrong, as shown.
Reply by ●August 10, 20122012-08-10
On Fri, 10 Aug 2012 00:37:06 -0700 (PDT), fungus <tooby@artlum.com> wrote:>On Friday, August 10, 2012 2:01:47 AM UTC+2, Daniel Pitts wrote: >> >> Actually, I'm planning on using the TLC5916 from TI, which is a >> constant-current sink 8bit shift register, for exactly this purpose. >> That way brightness will be consistent. > >Looks like the ideal chip for this... >you get a shift register *and* current >source all in one....and you just said it couldn't be done.
Reply by ●August 10, 20122012-08-10
On Fri, 10 Aug 2012 03:13:51 -0700 (PDT), fungus <tooby@artlum.com> wrote:>On Friday, August 10, 2012 2:01:47 AM UTC+2, Daniel Pitts wrote: >> >> Actually, I'm planning on using the TLC5916 from TI, which is a >> >> constant-current sink 8bit shift register, for exactly this purpose. >> >> That way brightness will be consistent. > >PS: Have you seen the MAX7219 LED driver chip? >It's similar to the TLC5916 but has built in >multiplexing for 8 rows of LEDs.But it's Maxim. Good luck actually getting them.
Reply by ●August 10, 20122012-08-10
On Fri, 10 Aug 2012 08:56:15 -0400 "krw@att.bizzzzzzzzzzzz" <krw@att.bizzzzzzzzzzzz> wrote in Message id: <i61a28dbkjfnu729kccufca1d1df6qhjb9@4ax.com>:>On Fri, 10 Aug 2012 03:13:51 -0700 (PDT), fungus <tooby@artlum.com> wrote: > >>On Friday, August 10, 2012 2:01:47 AM UTC+2, Daniel Pitts wrote: >>> >>> Actually, I'm planning on using the TLC5916 from TI, which is a >>> >>> constant-current sink 8bit shift register, for exactly this purpose. >>> >>> That way brightness will be consistent. >> >>PS: Have you seen the MAX7219 LED driver chip? >>It's similar to the TLC5916 but has built in >>multiplexing for 8 rows of LEDs. > >But it's Maxim. Good luck actually getting them.Well, in that particular chips case, not a problem. http://www.findchips.com/avail?part=MAX7219
Reply by ●August 10, 20122012-08-10
"krw@att.bizzzzzzzzzzzz" wrote:> > On Fri, 10 Aug 2012 03:13:51 -0700 (PDT), fungus <tooby@artlum.com> wrote: > > >On Friday, August 10, 2012 2:01:47 AM UTC+2, Daniel Pitts wrote: > >> > >> Actually, I'm planning on using the TLC5916 from TI, which is a > >> > >> constant-current sink 8bit shift register, for exactly this purpose. > >> > >> That way brightness will be consistent. > > > >PS: Have you seen the MAX7219 LED driver chip? > >It's similar to the TLC5916 but has built in > >multiplexing for 8 rows of LEDs. > > But it's Maxim. Good luck actually getting them.There are plenty of them on Ebay, and in several packages. <http://www.ebay.com/sch/i.html?_trksid=p4340.m570.l1313&_nkw=MAX7219&_sacat=0>
