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Need for inductive kickback diode

M

Martin

Jan 1, 1970
0
If you are using PWM to control the brightness of an incandescent
light, should you use an inductive kickback diode to protect the
transistor?

It is of course recommended to use when you are driving coils, relays,
motors, etc.

A light is a coil, althought I would guess the inductance is pretty
low.

Would good practice be to put the diode in "just in case", or its
absolutely unneeded?

Does the answer change depending whether you use a bipolar or mosfet.

TIA

Martin
 
D

default

Jan 1, 1970
0
If you are using PWM to control the brightness of an incandescent
light, should you use an inductive kickback diode to protect the
transistor?

NO. not necessary - can't hurt, but can't help
It is of course recommended to use when you are driving coils, relays,
motors, etc.

A light is a coil, althought I would guess the inductance is pretty
low.

Yeah, it is resonant in the MHZ range and very low Q - ignore it.
Would good practice be to put the diode in "just in case", or its
absolutely unneeded?
Waste of time and money, unless you might stick a coil in there.

Relays with snubber diodes across the coils will open more slowly and
arc longer - and shorten the relay life in some applications - one of
those compromise things again, but only if the relay has to interrupt
power on its contacts with some inductance present.
Does the answer change depending whether you use a bipolar or mosfet.

Yes

Mosfets are great devices (they save power and can do what bipolars
can't)

Mosfets do require special treatment. A spike on a mosfet may damage
it, if the intrinsic diode isn't fast enough. Generally it takes a
fast switching diode (reverse biased) between Drain and Source to
protect it from a load transient.

You might use a 30 amp mosfet protected by a 15 amp fuse - but the
transient caused by the fuse interrupting the 15+ amp short circuit
current may still eat the mosfet.

Mosfet gates are very fussy to transients. A long wire run to the
gate can have enough inductance to cause the mosfet to go belly-up.

Generally you'd use a resistor in series with the gate (which slows
turn-on) to protect it, by dropping the transient across the intrinsic
gate capacitance, and may also want diodes to snub transients in
extreme cases. And a mosfet once turned on - will stay on if there
isn't some discharge path for the gate - only a few pico farads as a
rule but still requires a discharge path. (high power mosfets usually
have larger gate capacitance but we are still only talking a 100K gate
to source (or so depending on transistor, and speed needed - relays
are slow in comparison so speed shouldn't be a concern)

I love mosfets. They can really make a lot of otherwise tricky
projects go smoothly (like replacing relays with SS devices and no
maintenance). But they have some quirks before you can apply them for
every application.

Not trying to dissuade you - mosfets are better than bipolars for most
things that I do, but they take a little understanding.
 
J

Jamie

Jan 1, 1970
0
Martin said:
If you are using PWM to control the brightness of an incandescent
light, should you use an inductive kickback diode to protect the
transistor?
No, incandescent is not inductive. It's resistive.
It is of course recommended to use when you are driving coils, relays,
motors, etc. Because those are inductive.

A light is a coil, althought I would guess the inductance is pretty
low.
The resistance in the element creates a very low Q, any inductance that
may be there isn't going to be much.
Would good practice be to put the diode in "just in case", or its
absolutely unneeded?
The only time you may get inductive fly back is when making and breaking
electrical connections. This becomes more prevalent if the control wire
is bundled tightly with other control wires that are already passing
current. The eddy currents that exist may be caught at the correct time
as you're making and breaking. that could generate a transient.
if it makes you feel better, go ahead.

Does the answer change depending whether you use a bipolar or mosfet.


As far as i'm concerned, it does not change the answer in your case..

Circuit configuration has a lot to do with what type of protection you
require.
 
E

Eeyore

Jan 1, 1970
0
Martin said:
If you are using PWM to control the brightness of an incandescent
light, should you use an inductive kickback diode to protect the
transistor?

It is of course recommended to use when you are driving coils, relays,
motors, etc.

A light is a coil, althought I would guess the inductance is pretty
low.

Would good practice be to put the diode in "just in case", or its
absolutely unneeded?

You might want to do that. *All* real circuits have at least some inductance
from the loop area of the wiring..

Does the answer change depending whether you use a bipolar or mosfet.

No.

Graham
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
The only time you may get inductive fly back is when making and breaking
electrical connections.

What do you think PWM does ? As in many thousandds of times a second typically.

Graham
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
The only time you may get inductive fly back is when making and breaking
electrical connections. This becomes more prevalent if the control wire
is bundled tightly with other control wires that are already passing
current. The eddy currents that exist may be caught at the correct time
as you're making and breaking. that could generate a transient.

You really need to learn some basic electronics instead of relying on 'folksy
ideas' to get you by.

LOL @ 'eddy currents'.

Graham
 
J

Jamie

Jan 1, 1970
0
Eeyore said:
Jamie wrote:




EVERYTING has some inductance.

Graham
Yes, including you obviously

Message stripping again are we?
 
J

Jamie

Jan 1, 1970
0
Eeyore said:
Jamie wrote:




What do you think PWM does ? As in many thousandds of times a second typically.

Graham
Yeah, I guess your version of a PWM circuit would be a mechanical
timing wheel with a set of points on it.

Stop flaunting and try to impress some one for a change. You're not
winning any popularity contests over here. That i'm sure of.

Have a good day sucker.
 
J

Jamie

Jan 1, 1970
0
Eeyore said:
Jamie wrote:




You really need to learn some basic electronics instead of relying on 'folksy
ideas' to get you by.

LOL @ 'eddy currents'.

Graham

This only concludes my original assessment of you, you are nothing but an
arm chair bull shit artist.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This proves beyond the doubt, that you have absolutely no idea of
basic construction of electrical/electronic requirements of signal
control and path ways.

I won't go into to it because it's beyond you. (obviously).
 
E

ehsjr

Jan 1, 1970
0
Martin said:
If you are using PWM to control the brightness of an incandescent
light, should you use an inductive kickback diode to protect the
transistor?

It is of course recommended to use when you are driving coils, relays,
motors, etc.

A light is a coil, althought I would guess the inductance is pretty
low.

Would good practice be to put the diode in "just in case", or its
absolutely unneeded?

Does the answer change depending whether you use a bipolar or mosfet.

TIA

Martin

It is not needed, and would just waste space and
increase cost.

Ed
 
R

Rich Grise

Jan 1, 1970
0
What do you think PWM does ? As in many thousandds of times a second typically.

But with an ordinary incandescent bulb, and just ordinary wires,
the inductance is negligible.

I'd be more concerned about the thermal response of the filament,
and even be inclined to use a parallel capacitor to smooth out
the spikes.

Cheers!
Rich
 
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