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From: Ken James (no email)
Date: Tue Apr 01 2008 - 03:52:13 EDT
Robb Triton wrote:
> Hey Ken,
>
> Great article and thanks for the info!! I'm a bit puzzled though,
> I've always been told that one should use a small resister inline with
> the LED, both to drop the voltage down to the appropriate amount but
> also to act as a sort of surge protector during this spikes. Do you
> happen to know if any of this is true?
A current limiting resistor will work for a crude, inefficient
protection and power limiting device in an led driver circuit supplying
older style leds, but it is not used much if at all with modern high
power leds because these devices, which are from 30-150 times as
powerful as leds of just ten years ago (which were themselves 15-50
times as powerful as the typical indicator light led), some of them
running well over an amp of current draw per led (and as much as 240
lumens per watt or about 2.5 times as efficient as a good fluorescents
light), are much more critical and finicky about the power regulation
suppled to them.
Even so, for realistic lifetimes in such an electrical environment as a
sailboat, a diode or other protection device would often be pared with
the current limiting series resistor for added protection in such an
application. The reason you seldom see that for indicator lights is
because most of them were originally not designed for the boating market.
The two most common types of led power regulation circuit designs today
are the linear type (basically a transistor controlled rheostat with
some extra protection functions), which is simple, compact, robust and
cheap, and although it is a lot more efficient than a simple resistor it
is still relatively wasteful of power and can make a lot of heat, or the
second method, the DC switch mode type, which is much more
sophisticated, very efficient, and so also cool running, and although
more expensive it is also more reliable in the long run if properly
designed.
Another type is pulse width modulation, which has its own advantages
(and dis-advantages).
These methods of supplying power to a led make the device much more
efficient, reliable, and brighter with longer led lifetimes than a
simple resistor, and can actually be a lot more cost effective also,
since with a good power regulator circuit you can utilize much more of
the leds expensive potential and drastically reduce the size of the
light as well since far less leds are needed in the led array to get the
required amount of light, today ONE led can make as much light as a car
headlight! With proper heat sinking and a good driver, that is.
See the tech pages at my web site at www.firststarled.com or look at the
candelpower forum for much more information. (Please do not think I am
being 'tricky' here, I DO sell led light on my site but I am NOT pushing
them, just trying to get the technology out for all our benefit.)
> I've used 12V LED indicators everywhere, and haven't seen any problems
> yet, but if there are simple ways of protecting the circuit, it would
> be great to know.
> Also, would placing a cap inline in the circuit before the LED add
> additional protection?
A cap of the correct value would help a bit but a much better way is to
use a zener diode wired in parallel across the led so that the zener
diode will freely conduct voltage of a reverse polarity through the
resistor and the zener and thereby throughly clamp reverse polarity
spikes. The resistor must of course be of a value which will keep the Vr
current of the zener to a safe level and the voltage then at a safe
level also.
This zener diode in parallel across the led should have the zeners Vf
voltage selected so that a 'forward' or normal polarity voltage spike or
transient that would be a value above what the led could safely stand
would be conducted through the resistor and the zener and thus around
the led, keeping it safe. (keep in mind that the correct way to use a
zener diode is with the cathode mark, or bar, toward normal POSITIVE not
NEGATIVE as with a normal diode).
Again, for this to work the resistor in series with the zener must limit
the current through the zener during its Vf conduction to safe levels
and must be able to dissipate the power the resistor will burn up during
the zeners conduction, but this is not a big deal as long as the
resistor can keep the power level low enough at normal voltages, because
when the transients occur causing the large increase in power through
the resistor when the zener conducts they will not last long.
It should also be noted that the zener should be selected so that it
will NOT conduct during normal led voltages (or current flow that
correlates with a given voltage), but so that it will fully conduct when
the led would see unsafe voltages. This can mean that the led will need
to be under-driven under most normal operating conditions to obtain the
correct zener Vf levels when a transient occurs.
There are other similar ways to accomplish this.
A somewhat better method is to put a diode in series with the resistor
and led so that it is able to conduct in forward polarity but so that
the led is protected from reverse spikes and so that the proper polarity
of voltage is provided for the next part of the circuit, which consists
of a zener or zeners (use no more than two in parallel per circuit for
more power handling capacity) in a 'crowbar' configuration across the
input power (IE the zeners are connected with their bar toward positive,
wired with one end of the zeners connected to the wire going to the
series limiting resistor and then the led, and the other end of the
zeners connected to the wire coming from the other side of the led and
going back to the source).
The zeners MUST be connected AFTER and in series with the 'blocking'
polarity diode, then across/in parallel with the series limiting
resistor and the led, so that any voltage above that which would cause
an unsafe voltage and current at/through the led would be first flow
through the blocking diode and then be 'clamped' by these zeners.
For example, say for a nominal 12VDC circuit about 15V 'breakdown
voltage' rated zeners, and rate the zeners when selecting for a large
value transient voltage...
It is a given that you will need a fuse in series with the 'crowbar'
zener(s) or an overvoltage will seem as a dead short and could damage
the boats wiring.
A good device to use in the fuse application is something known as a
Positive Thermal Coefficient resistor, (PTC), which is in either a very
low resistance state (one half ohm or less typically) or a high
resistance state (many thousands of ohms) depending on the current flow
through it. These devices can 'trip' in a fraction of a sec and
automatically 're-set' once the current is lowered below a given
threshold value.
The fuse or PTC should be wired in BEFORE the 'blocking' diode *or any
other component* (!), and as mentioned before, the follow-on zener
diode(s) (only two zeners in parr max) should be wired (as a group if
two zeners) in parallel across the series limiting resistor and the led
right AFTER the 'blocking' diode.
This PTC auto fuse device can protect the circuit 'downstream' as well
as 'upstream' and will automatically reset after the overvoltage or
transient is removed.
The operation is as follows; in normal operation with normal voltages,
normal current flows through the PTC on one side of the circuit and does
little to heat it so it remains in a very low resistance state. The
power then goes through the blocking diode, which insures the correct
polarity in the rest of the circuit and prevents reverse polarity
spikes. This diode is relatively over-sized to allow for a much greater
transient current flow when the zeners 'fire' as will be seen.
The power does nothing with the zeners if the voltage is below their
zener threshold value, since zeners are designed to conduct in one way
with little resistance (but this is NOT the way they are in the
circuit!) and with the opposite polarity voltage they will NOT conduct
until the zener voltage is reached, and then they conduct in a manner
similar to a turned-on transistor, developing the voltage they are
rated at across themselves with a current flow that is not ohmic, that
is a very small change in applied voltage across the zeners results in a
very large change in current flow through the zeners.
What this means is that the zeners will try to keep the voltage across
them constant once they 'fire', and they will conduct any amount of
current it takes to achieve that, within limits of course.
But initially in our consideration, the voltage is below this threshold
so the power does not flow through the zeners but instead it all goes
first through the PTC then through the blocking diode then not through
the zeners at all, but instead it all goes through the series current
limiting resistor which drops the voltage enough for the led (you must
of course know what voltage the led 'wants'), then the power flows
through the led and returns to the source.
In this normal operation mode the PTC does little to resist power flow
and so little voltage is dropped across it and the crowbar circuit is
very efficient, feeding almost all the applied power to the series
limiting resistor and the led.
But, if the voltage IS above the threshold needed to 'fire' the zeners,
they instantly go into massive conduction, pulling a relatively large
amount of current through the PTC (one or two amps typically) and also
this same current goes through the blocking diode and then flows flows
through the zeners causing the clamping of the applied voltage across
the circuit at the zeners to their rated voltage so that the series
resistor and led that are in parallel with the zeners never see an
unsafe voltage across them or an unsafe current flow through them and
the led is therefore never over driven.
The relatively massive zener current, if it continues more than a very,
very short time, will quickly cause the PTC to go into a high resistance
state and then any voltage above what the zeners are rated at will be
dropped across the PTC only (plus a bit for the blocking diode), which
will limit current flow through the circuit to levels that the blocking
diode and zeners can sustain indefinitely without overheating.
In this mode the PTC acts as a current regulator causing a voltage drop
in excess of the zeners clamping voltage, which keep the voltage across
the series limiting resistor and led in a safe range, with the PTC
safely dissipating the extra power of overvoltages before they can harm
the rest of the circuit.
Before the PTC trips, the resistor and led see normal voltages even as
the input voltage is undergoing an over voltage or transient condition,
after the PTC trips the resistor and led may see slightly lower than
normal voltages (depending on how much current the led array is trying
to pull through the PTC) but they will still likely be normal voltages
more or less unless a huge input voltage surge has heated up the PTC a
lot and it must cool off a lot.
Once the input voltage is reduced to normal levels, the PTC will cool
and re-set and the operation returns to before overvoltage conduction mode.
Transients and spikes will not appear long enough to trip the PTC but
will nonetheless be clamped completely by the zeners, which safely
dissipate them as heat and keep voltage levels safe to the series
limiting resistor and the led.
The very fast functioning of of the 'crowbar' circuit renders it
practically 'transparent to ground' for spikes and transients while
clamping overvoltages totally and so throughly protecting the leds, and
the PTC in the circuit also provides a overcurrent or 'device short
circuited' protection for the boats wiring.
Again keep in mind to rate both the 'crowbar' zener diodes and the
'blocking' diode for the max transient current flow they will see, (a
zener diode and a normal silicone diode can safely and repetitively, if
allowed to cool in between cycles, take a lot more current than they are
normally rated for in continuous operation for a few seconds, access the
data sheets online) the zener(s) will see a huge amount of current until
the PTC can 'trip' and will need to be oversized to dump this surge of
heat safely. The 'blocking' diode will also see the same surge of
current, and will get hot too, so it must also be larger than otherwise
dictates.
The thing to be concerned about with the PTC , besides its 'trip'
current rating which must of course be below what the zeners and
blocking diode can safely handle, and the 'hold' current rating which
must be about half what the device will use in normal operation, is its
operating voltage rating, it must be at least as large as the zeners
plus 25-30 % greater than it will ever be expected to see in normal
continuous operation.
As you can see, the 'crowbar' method has the disadvantage that its
'protection' parts must be well over-rated to withstand the surge of
current/heat they will see before the PTC goes into a high resistance
state and the zeners can safely clamp the voltage, but the big advantage
is the drop dead simplicity and reliability, and the very, very fast
reaction speed, and the fact it is a cheap method and fairly compact.
One other dis-advantage is that if it is used 'upstream' of a digital
driver, if it is not integrated properly, it can cause control IC
'brownouts' when overvoltages occur.
But, if it is properly designed it is 'bulletproof'.
BTW this 'crowbar' design can be used with innumerable devices such as
fans, pumps, etc.
But the best way to solve the 'led hot spike' problem is with a led
driver that has the transient, spike and overvoltage protection built
in, either as a dc-dc switch mode 'soft start' scheme as the app note
recommends (link in my previous email), or some other method of
absolutely guaranteeing that spikes and transients cannot harm the led
arrays.
Again, this may seem too troublesome or too expensive until you factor
in total cost in terms of lumens/watt and reliability in dollars/hour
run time probability. -Ken
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