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Peter W.
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Chuck <chuck445@yahoonospam.com> wrote:
You are clearly confused. NE555 that you mention clearly shows
that chinese unit is a switching one. Quality switching unit
would contan inductor, but with slow elements like LM358 and
NE555 switching freqency must be rather low, so one would
need big (read expensive) inductor. So you probably have
plain PWM unit. Such unit is fit only for driving LED-s
that have integrated resistors (most consumer lights seem be
like that) and which are (at least approxiatly) matched to
your power supply. That is 12V LED still needs 12V supply.
If you used 12V LED with 13.8V supply, then you get much
higher current than expected.
Concerning design, with 2 mosfets in parallel switched
fully on you expect resonably current sharing, so
nominally 15A per mosfet. With nominal Rdson of 0.006 Ohm,
we get 0.006*15^2 = 1.35 W per mosfet. However, at that
power mosfet will get hot and Rdson will grow, say double
(when junction temperature is 175C). Then we get 2.7W,
which is too much for TO-220 without a heatsink. Howewer,
at 12A per mosfet we get 1.78W, which gives some margin
for unequal current sharing. So, 30A is short term
rating, 24A (that is 12*2) is reasonable max. For reliable
long term operation I would prefer better margin. I would say
that 10A is quite conservative. Above was assuming unit
fully on. When duty cycle is less than 1, power loss is
proportionally reduced, so we have less heat.
Concerning voltage, LM358 and LM7805 are rated for
30V, so no problem here. Mosftes have 70V rating,
also OK. NE555 has lower rating, but presumably it
is supplied by LM7805 so no problem with overvoltage
here.
In info you gave the only suspiciuos part is 7805
regulator. Namely, we need some voltage to control
gates of mosfets. 5V is too low (Rdson would increase).
8V would be enough and 7808 could easily make it
from 12V input so 7808 would be more natural. There
are other possible sources of gate control voltage,
but if circuit uses 5V then maximal current would be
reduced (10A still should be OK).
When LED is for lower voltage then your supplay, then
you can use PWM circuit as long as peak current
trough PWM circuit is limited (by LED and supply)
to safe value (say 24A), but clearly this lowers
possible power of LED much more than decrease in
voltage would suggest. Current trough LED grows
quickly with voltage, so nominal current of LED
at its nominal voltage must be much smaller than
24A to make sure that it will not increase 24A
at increased supply voltage.
Now, what went wrong? Some possibilies are:
1) units were defective, possible but IME quite rare
with chinese selers
2) design error like too low gate voltage or too
limited heat dissipation trough case
3) your 13.8V coused much larger peak current,
4) your LED has some electronics inside including
nontrivial filtering capactior(s), in such case
you can get fairly large peak current
5) very high ambient temperature
6) other like surge, short in load etc...
Now, except for 1, 4 and 6 it looks unlikely that other
reasons alone would cause failure because at nominal
8.5 there is considerable safety margin, but combination
of them together may be enough.
Anyway, I would expect good seller to have clear instruction
with warning that power supply should be matched to
LED-s.
--
Waldek Hebisch
On 11/25/20 3:30 PM, Phil Allison wrote:
Chuck wrote:
============
I am considering it and it would be the first time, in the 25 years I
have been registered on eBay, that I left a negative feedback. Really,
this seller should have not haggled on this and refunded immediately
especially after I submitted the pictures.
** Pics of the damage you caused by stupidity ?
You have been less than honest about what you were *really* doing all along.
What PSU was used ??
What LED lights ?
Did you make a wiring error ?
You sound like an expert at blame shifting.
..... Phil
Uh, excuse me, but I have already made myself quite clear and won\'t
elaborate further. If you doubt, then order one and have a load drawing
13.8 VDC at 7 A from a linear supply and see what happens in a few
hours. You said before that the 12-24 V was missing information. Who
would ever know that it is only a 12V device because that\'s not what
they advertise. In addition to no heatsinking. I think I\'ve been
clear. I thought you were helping, but now I question YOUR motives.
You are clearly confused. NE555 that you mention clearly shows
that chinese unit is a switching one. Quality switching unit
would contan inductor, but with slow elements like LM358 and
NE555 switching freqency must be rather low, so one would
need big (read expensive) inductor. So you probably have
plain PWM unit. Such unit is fit only for driving LED-s
that have integrated resistors (most consumer lights seem be
like that) and which are (at least approxiatly) matched to
your power supply. That is 12V LED still needs 12V supply.
If you used 12V LED with 13.8V supply, then you get much
higher current than expected.
Concerning design, with 2 mosfets in parallel switched
fully on you expect resonably current sharing, so
nominally 15A per mosfet. With nominal Rdson of 0.006 Ohm,
we get 0.006*15^2 = 1.35 W per mosfet. However, at that
power mosfet will get hot and Rdson will grow, say double
(when junction temperature is 175C). Then we get 2.7W,
which is too much for TO-220 without a heatsink. Howewer,
at 12A per mosfet we get 1.78W, which gives some margin
for unequal current sharing. So, 30A is short term
rating, 24A (that is 12*2) is reasonable max. For reliable
long term operation I would prefer better margin. I would say
that 10A is quite conservative. Above was assuming unit
fully on. When duty cycle is less than 1, power loss is
proportionally reduced, so we have less heat.
Concerning voltage, LM358 and LM7805 are rated for
30V, so no problem here. Mosftes have 70V rating,
also OK. NE555 has lower rating, but presumably it
is supplied by LM7805 so no problem with overvoltage
here.
In info you gave the only suspiciuos part is 7805
regulator. Namely, we need some voltage to control
gates of mosfets. 5V is too low (Rdson would increase).
8V would be enough and 7808 could easily make it
from 12V input so 7808 would be more natural. There
are other possible sources of gate control voltage,
but if circuit uses 5V then maximal current would be
reduced (10A still should be OK).
When LED is for lower voltage then your supplay, then
you can use PWM circuit as long as peak current
trough PWM circuit is limited (by LED and supply)
to safe value (say 24A), but clearly this lowers
possible power of LED much more than decrease in
voltage would suggest. Current trough LED grows
quickly with voltage, so nominal current of LED
at its nominal voltage must be much smaller than
24A to make sure that it will not increase 24A
at increased supply voltage.
Now, what went wrong? Some possibilies are:
1) units were defective, possible but IME quite rare
with chinese selers
2) design error like too low gate voltage or too
limited heat dissipation trough case
3) your 13.8V coused much larger peak current,
4) your LED has some electronics inside including
nontrivial filtering capactior(s), in such case
you can get fairly large peak current
5) very high ambient temperature
6) other like surge, short in load etc...
Now, except for 1, 4 and 6 it looks unlikely that other
reasons alone would cause failure because at nominal
8.5 there is considerable safety margin, but combination
of them together may be enough.
Anyway, I would expect good seller to have clear instruction
with warning that power supply should be matched to
LED-s.
--
Waldek Hebisch