XPost: 24hoursupport.helpdesk
From: ianREMOVETHISjackson@g3ohx.demon.co.uk
In message , Zootal
writes
>
>"Ian Jackson" wrote in message
>news:PVRIUMAqWhcKFwia@g3ohx.demon.co.uk...
>> In message , ian field
>> writes
>>>
>>>"Ian Jackson" wrote in message
>>>news:4Lvlz7KQMdcKFwkC@g3ohx.demon.co.uk...
>>>> In message , Zootal
>>>> writes
>>>>>
>>>>>"Ian Jackson" wrote in message
>>>>>news:GvAqKUAuZbcKFwgH@g3ohx.demon.co.uk...
>>>>>> In message , ian field
>>>>>> writes
>>>>>>>
>>>>>>>"Ian Jackson" wrote in
>>>>>>>message
>>>>>>>news:oQqRlSBFmUcKFw3i@g3ohx.demon.co.uk...
>>>>>>>> In message , ian field
>>>>>>>> writes
>>>>>>>>>
>>>>>>>>>"Ian Jackson" wrote in
>>>>>>>>>message
>>>>>>>>>news:RK8$DzIdHMcKFwRq@g3ohx.demon.co.uk...
>>>>>>>>>> In message , rf
>>>>>>>>>> writes
>>>>>>>>>>>Roger Dewhurst wrote:
>>>>>>>>>>>>> You can get simple to use regulator chips that drop the voltage
>>>>>>>>>>>>> down, you only need a couple of components to make a working
>>>>>>>>>>>>> voltage
>>>>>>>>>>>>> regulator. Ask on News:sci.electronics.basic - the folk on
>>>>>>>>>>>>> there
>>>>>>>>>>>>> are
>>>>>>>>>>>>> usually very helpful and should explain all you need.
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>> Why not just drop the voltage through a few diodes? Very
>>>>>>>>>>>> simple.
>>>>>>>>>>>> Very
>>>>>>>>>>>> cheap.
>>>>>>>>>>>
>>>>>>>>>>>A *few* diodes at a couple of ten cents per each. A single 7809
>>>>>>>>>>>for
>>>>>>>>>>>less
>>>>>>>>>>>that a $.
>>>>>>>>>>>
>>>>>>>>>> More like "a *few* diodes at a couple of cents per each".
>>>>>>>>>>
>>>>>>>>>> 3V to drop = 5 diodes @ 0.6V per diode. No other circuitry
>>>>>>>>>> required.
>>>>>>>>>> More
>>>>>>>>>> than good enough for the job.
>>>>>>>>>> --
>>>>>>>>>> Ian
>>>>>>>>>
>>>>>>>>>The forward conduction knee curve on diodes isn't *that* sharp,
>>>>>>>>>depending
>>>>>>>>>on
>>>>>>>>>current draw and rating of the diode the drop can be as low as 0.55V
>>>>>>>>>and
>>>>>>>>>as
>>>>>>>>>high as 1.1V.
>>>>>>>>>
>>>>>>>> For most 'normal' Si diodes, that isn't really the case. The actual
>>>>>>>> voltage drop does, of course, increase with current, but at
>>>>>>>> 'sensible'
>>>>>>>> currents, you can reckon on around 0.65V per diode. How much current
>>>>>>>> is
>>>>>>>> the Tardis toy going to take? 1A max? 4 or 5 1N4000-series diodes
>>>>>>>> should
>>>>>>>> work fine in this application. I've used this non-elegant 'KISS'
>>>>>>>> technique
>>>>>>>> on several occasions, and haven't found any problems.
>>>>>>>> --
>>>>>>>> Ian
>>>>>>>
>>>>>>>A potential danger with a cassette recorder is the difference in
>>>>>>>current
>>>>>>>draw between motor on and motor off. In the condition of low current
>>>>>>>draw
>>>>>>>(and low diode drop) supply decoupling electrolytic capacitors can
>>>>>>>charge
>>>>>>>to
>>>>>>>a higher voltage which is then dumped into the circuit when switched
>>>>>>>to
>>>>>>>play.
>>>>>>>
>>>>>> True, true. But I reckon that a momentary short burst of a
>>>>>> rapidly-decaying additional 3V won't hurt too much.
>>>>>> --
>>>>>> Ian
>>>>>
>>>>>It won't even be noticeable. The capacitors won't charge up that high to
>>>>>start with, and they don't "dump" into the circuit, they just quickly
>>>>>discharge down to the lower voltage that is present at the output of the
>>>>>last diode - how fast this happens depends on the size of the caps and
>>>>>the
>>>>>load. I wouldn't even call it a surge. A resistor from the last diode to
>>>>>ground will prevent them from charging more then a few tenths of a volt
>>>>>and
>>>>>is a good idea.
>>>>
>>>> Indeed. A bleed of a few mA will prevent the off-load voltage rising to
>>>> 12V. 9V at 10mA would require 900 ohms (say 1k), 90mW (so even a 1/4W
>>>> will
>>>> do).
>>>>
>>>>>And the capacitor doesn't need to be that big.
>>>>>
>>>> Which capacitor do you mean?
>>>>
>>>>>I would *not* use 1n400x diodes. ~1 amp will make them hot and
>>>>>susceptible
>>>>>to failure. Use 2 or 3 amp diodes, they are cheap and readily available.
>>>>>They are bigger and will run cooler and won't fail as easily.
>>>>
>>>> 1A through diodes dropping 0.6V means 600mW per diode. 1N4000-type will
>>>> run a bit warm, so maybe a physically larger (higher current) diode
>>>> might
>>>> be better. But it depends on how much current the Tardis takes!
>>>>
>>>>>Or use two
>>>>>strings of 1n400x in series, that is good enough for an app like this.
>>>>>
>>>>>So - 12v -> diode -> diode -> diode -> diode -> ~9.2v Output
>>>>>
>>>> That's only one string of diodes in series. Did you mean parallel? If
>>>> so,
>>>> no, you shouldn't parallel diodes. As you suggest, use higher current
>>>> diodes.
>>>
>>>Actually if you have two strings each having the same number of diodes and
>>>put the two strings in parallel it doesn't matter, when you have a few or
>>>more diodes in series as the variation in Vf for each diode averages out.
>>>
>> Possibly. Possibly not. Murphy and his Laws move in mysterious ways. You
>> don't see many circuits with paralleled diodes (stringed or otherwise).
>> --
>> Ian
>
>Larger power supplies will have diodes in parallel, and they will even put
>the output trannies in parallel. You can get away with it with diodes but
>you have to be carefull to use the same type, preferably from the same
>batch. Otherwise you end up with some saturated and others barely turned on,
>and you get a high failure rate.
>
During 50 years 'in electronics', I can't immediately recall seeing any
power supplies with diodes in parallel. I'm not saying it's never done
but, if it is, there should also be some current-balancing resistance in
each path. This could be low-value resistors, or even the resistance of
the secondary windings of paralleled transformers which feed each set of
diodes. I don't think that having to use diodes from the same batch is a
very good design criterion!
--
Ian
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