Prev: 5 Ghz Routers Cause Nausea & Dizziness - To You Too?
Next: A question regarding voltage controlled oscillators
From: ehsjr on 23 May 2010 22:44
Ade wrote:
> don(a)manx.misty.com did gone and wrote:
>
>>In article <MPG.26634d95671d95009896f8(a)news.enta.net>, Ade wrote:
>>
>
> [diode in series]
>
>>>That, as it happens, fixed the problem completely (no more afterglow).
>>>By using green LEDs on "High" and "Medium", I only needed to add 1
>>>diode, in series with the red LED.
>>
>> Congratulations! Good!
>>
>
>
> Ta :) It was a nice moment, for sure...
>
>
>>>The one downside is it's increased the overall current draw to ~12.5mA
>>>when the red LED is lit (vs. 5.6mA when the mid LED is on, and ~9.5mA
>>>for the "High" LED).
>>
>> Are you saying that current draw while the red LED is on increased as a
>>result of adding a diode in series with the red LED? (If so, can yiou
>>retry your measurements?)
>
>
> On the breadboard, it definitely affected consumption. Total draw (when
> the red LED was lit) rose from ~9.5mA to ~13mA. By touching one side of
> the diode with my pliers, I could reduce the current to ~7.9mA, by
> touching the other side it would go back to 13mA. I decided this was
> ludicrous, and so now I've built the circuit on stripboard. Max current
> draw @ 5v is now ~8mA with the red LED lit - little different to when
> the high green LED is on. The middle LED still uses less current,
> presumably for the reasons you explain below.
>
> Part of the new lower-consumption may be because I've made the voltage
> divider with 3M3ohm resistors instead of the 1M ohm specified; only
> because I don't have any 1M ohm here, and I didn't want to muck about
> with 3x 3M3s on each side like I did on the breadboard. The circuit
> still works perfectly - so I'm going to stick with the 3M3's I think.
>
>
>> Or that the red LED is using more current than either green one?
>>
>> The mid LED may be using less if the 74LS02 has higher output resistance
>>or higher voltage drop in its output than the op-amps have.
>
>
> Quite possible. The '02 chip is around 30 years old, so it probably had
> a higher resistance/voltage drop from new.
>
>
>> The green LED may have a little more voltage drop than the red one
>
> plus
>
>>a diode.
>
>
> The output is very similar now with the two op-amp driven LEDs. Only the
> logic-driven LED is lower. The overall brightness of each LED is pretty
> similar, however, which makes no sense to me (unless the difference is
> too small to make much odds.
>
>
>>>You're right that there's no decoupling caps - and there probably
>
> ought
>
>>>to be. Is this likely to be an issue if the circuit is powered from a
>>>steady-state DC source (i.e. a battery)?
>>
>> Sometimes these caps prevent oscillations and absorb power supply noise
>>caused by things switching.
>
>
> It's no bother to add some to the stripboard version.
>
>
>>>OK, another question to add to the mix:
>>>
>>>When in place, this circuit will be powered from a 24v battery (2x truck
>>>batteries), stepped down to 5v via a regulator.
>>>
>>>http://www.dimensionengineering.com/switchingregulators.htm contains a
>>>handy (assuming it's right) formula for estimating the waste current
>>>that a linear regulator will have to dump as heat. Assuming my circuit
>>>draws a maximum of 19mA:
>>>
>>>(24v-5v) * 13mA = 247mW
>>
>> That is correct.
>>
>> Will this have to work if the vehicle is running? (battery voltage more
>>like 28V - 299 mW to dissipate)
>
>
> It's possible that the circuit could be accidentally left on when the
> vehicle is running (it wouldn't ordinarily be used in that
> circumstance). However, now that consumption is less than 9mA in all
> conditions, I reckon that regulator will have an even easier job: 171mW
> @ 24V or 207mW @ 28V.
>
>
>>>(or, I could hook it to just one of the batteries, and reduce the waste
>>>to 91mW, at the risk of unbalancing the battery bank)
>>>
>>>Does that sound about right, and will a standard 7805 regulator
>>>dissipate 247mW without an added heatsink?
>>
>> This does sound OK to me. I would let things run for a while (at least
>>10 minutes) and see if a damp tissue sizzles when touched to the 7805's
>>heatsink tab. In the unlikely eent that occurs, a small heatsink should
>>easily be sufficient for 299 mW.
>
>
> I lightly burned myself the other day on a 7805 after it had
> accidentally tried to feed a short circuit for a few seconds... I like
> the sound of the wet tissue test...
>

You can move some of the heat out of the 7805, by putting
a resistor in series between the battery and Vin of the chip.
e.g. At 9 mA, a 1K resistor will drop 9 volts, so the 7805
would dissipate (24-9-5)*.009 or ~ 90mW. The resistor would
dissipate ~ 81 mW. That said, the 7805 will handle the 171 mW
you'd get at 24V Vin and 9mA current, without the added resistor
and without a heatsink.

Ed
 | 
Pages: 1
Prev: 5 Ghz Routers Cause Nausea & Dizziness - To You Too?
Next: A question regarding voltage controlled oscillators