zener diode protection

On 24 Jul 2004 13:20:04 -0700, [email hidden] (james) wrote in
message <[email hidden]>:

Quoted message said:

I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
B&M headlight has a zener diode. I am doing the dunwich dynamo next
weekend. Does anyone think I it woul be unwise to directly wire the
lights to the dynamo.

Should be fine. Not that I've tried it with mine :-)

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

88% of helmet statistics are made up, 65% of them at Washington University

james said:

I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
B&M headlight has a zener diode. I am doing the dunwich dynamo next
weekend. Does anyone think I it woul be unwise to directly wire the
lights to the dynamo. Should I consider limiting top speeds (if so,
to what?) or does nobody forsee any problems (barring the
inconvenience of no switch)

I've not used B&M stuff but I used to frequently blow unprotected bulbs on
bog standard dynamo systems. At a guess, you'll be pushing your luck at
30 mph.

~PB

(james) said:

I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
B&M headlight has a zener diode. I am doing the dunwich dynamo next
weekend. Does anyone think I it woul be unwise to directly wire the
lights to the dynamo. Should I consider limiting top speeds (if so,
to what?) or does nobody forsee any problems (barring the
inconvenience of no switch)

tia
james

Will there be a rear light in the circuit? If not, make sure the front
has a 3W bulb.
If there's a rear in the circuit, it shouldn't be any problem at all
as the zener is designed to soak up bulb-damaging voltages.
We've four zenered B&M front lights in the family, (3 of which are
wired to Shimano nx-30 dyno hubs) and yet to blow a bulb from excess
speed - and like to hit the downhills! If still unsure take a spare
bulb.

Good luck on the dynamo - I'd love to do it myself, but on student
budget, buying a train ticket from manchester to london would mean not
eating for two weeks :~(

Chris

james said:

I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
B&M headlight has a zener diode.

Why wouldn't it be? Does the dynamo switch have a regulator in it?

Quoted message said:

I am doing the dunwich dynamo next
weekend. Does anyone think I it woul be unwise to directly wire the
lights to the dynamo. Should I consider limiting top speeds (if so,
to what?) or does nobody forsee any problems (barring the
inconvenience of no switch)

I have a Schmidt SON with a B&M 2.4W bulb. The SON is designed to work
without a zener diode, but with a 3W bulb. I haven't yet got round to
wiring in a switch, so it's lasted me for 1000 miles no problem.

AC

james said:

I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
B&M headlight has a zener diode. I am doing the dunwich dynamo next
weekend. Does anyone think I it woul be unwise to directly wire the
lights to the dynamo. Should I consider limiting top speeds (if so,
to what?) or does nobody forsee any problems (barring the
inconvenience of no switch)

I've done over 40mph with no problens (except that the light isn't quite
enough too see where you're going at that speed!)

anonymous coward said:

I have a Schmidt SON with a B&M 2.4W bulb. The SON is designed to work
without a zener diode, but with a 3W bulb. I haven't yet got round to
wiring in a switch, so it's lasted me for 1000 miles no problem.

Presumably the zener diode is wired in parallel, and effectively dumps
excess current, relying on the internal resistance of the dynamo to keep
the voltage down. What stops this excess current melting the dynamo?

- Richard

--
_/_/_/ _/_/_/ _/_/_/ Richard dot Corfield at ntlworld dot com
_/ _/ _/ _/
_/_/ _/ _/ Time is a one way street,
_/ _/ _/_/ _/_/_/ Except in the Twilight Zone.

Richard Corfield said:

anonymous coward said:

I have a Schmidt SON with a B&M 2.4W bulb. The SON is designed to work
without a zener diode, but with a 3W bulb. I haven't yet got round to
wiring in a switch, so it's lasted me for 1000 miles no problem.

Presumably the zener diode is wired in parallel, and effectively dumps
excess current,

Pretty much - the diode only starts conducting above a certain voltage so
some power will be dissipated in the bulb and some in the diode.

Quoted message said:

relying on the internal resistance of the dynamo to keep
the voltage down.

In practice, dynamos use several methods of regulation. They produce AC
power at a frequency proportional to their speed, normally in the range of
50-1000 Hz (or thereabouts).

1) As speed and frequency increase, energy losses due to eddy currents in
metal parts of the dynamo increase.

2) As speed and frequency increase, inductive reactance in the windings
increases, and this tends to oppose increases in current

The resistance of the dynamo windings will be pretty much constant with
frequency / voltage / power output (and very low) so it will not play much
of a role in regulating current.

What does increase with speed / frequency is the maximum voltage the
dynamo can source. This is why it's possible to wire two 6V 0.5A lights in
series at high speeds.

Quoted message said:

What stops this excess current melting the dynamo?

The resistance of the zener diode will always be much greater than that of
the dynamo. For example a 6V zener diode dissipating 3W will have a
resistance of 12 ohms but the dynamo will probably have a resistance of
less than an ohm. So the bulk of the energy it produces will be dissipated
in the zener diode.

AC

On Sun, 25 Jul 2004 03:52:45 +0100, anonymous coward
<[email hidden]>

was popularly supposed to have said:

james said:

I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
B&M headlight has a zener diode.

Why wouldn't it be? Does the dynamo switch have a regulator in it?

Quoted message said:

I am doing the dunwich dynamo next
weekend. Does anyone think I it woul be unwise to directly wire the
lights to the dynamo. Should I consider limiting top speeds (if so,
to what?) or does nobody forsee any problems (barring the
inconvenience of no switch)

I have a Schmidt SON with a B&M 2.4W bulb. The SON is designed to work
without a zener diode, but with a 3W bulb. I haven't yet got round to
wiring in a switch, so it's lasted me for 1000 miles no problem.

Buy in a couple of zener diodes from Maplins, and wire them in so they're
in parallel, one pointing one way, one the other. Bike dynamos aren't
dynamos, they're alternators and produce alternating current, so unless
you only want to see half the phase, you need a couple of zeners.

This page has a couple of 12-volt zeners on it, if you need 'em:
http://www.maplin.co.uk/products/module.asp?CartID=040725181932463&moduleno=19091&Products=2

--
Dan Holdsworth PhD [email hidden]
By caffeine alone I set my mind in motion, By the beans of Java
do thoughts acquire speed, hands acquire shaking, the shaking
becomes a warning, By caffeine alone do I set my mind in motion

Dan Holdsworth said:

On Sun, 25 Jul 2004 03:52:45 +0100, anonymous coward
<[email hidden]>

was popularly supposed to have said:

james said:

I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
B&M headlight has a zener diode.

Why wouldn't it be? Does the dynamo switch have a regulator in it?

Quoted message said:

I am doing the dunwich dynamo next
weekend. Does anyone think I it woul be unwise to directly wire the
lights to the dynamo. Should I consider limiting top speeds (if so,
to what?) or does nobody forsee any problems (barring the
inconvenience of no switch)

I have a Schmidt SON with a B&M 2.4W bulb. The SON is designed to work
without a zener diode, but with a 3W bulb. I haven't yet got round to
wiring in a switch, so it's lasted me for 1000 miles no problem.

Buy in a couple of zener diodes from Maplins, and wire them in so they're
in parallel,

ITYM in series. In parallel, they start conducting at 0.7V (or
thereabouts) in either direction. My B&M headlight has a zener diode in it
already, like the OP's.

Quoted message said:

one pointing one way, one the other. Bike dynamos aren't
dynamos, they're alternators and produce alternating current, so unless
you only want to see half the phase, you need a couple of zeners.

This page has a couple of 12-volt zeners on it, if you need 'em:
http://www.maplin.co.uk/products/module.asp?CartID=040725181932463&moduleno=19091&Products=2

I forget what value is best for a 6V bulb? I thought it was somewhere
between 6 & 8 volts...

AC

anonymous coward said:

Dan Holdsworth said:

On Sun, 25 Jul 2004 03:52:45 +0100, anonymous coward
<[email hidden]>

was popularly supposed to have said:

On Sat, 24 Jul 2004 13:20:04 -0700, james wrote:

> I am awaiting a warranty swapout for my sw-nx30 dynamo switch. My
> B&M headlight has a zener diode.

Why wouldn't it be? Does the dynamo switch have a regulator in it?

> I am doing the dunwich dynamo next
> weekend. Does anyone think I it woul be unwise to directly wire the
> lights to the dynamo. Should I consider limiting top speeds (if so,
> to what?) or does nobody forsee any problems (barring the
> inconvenience of no switch)

I have a Schmidt SON with a B&M 2.4W bulb. The SON is designed to work
without a zener diode, but with a 3W bulb. I haven't yet got round to
wiring in a switch, so it's lasted me for 1000 miles no problem.

Buy in a couple of zener diodes from Maplins, and wire them in so they're
in parallel,

ITYM in series. In parallel, they start conducting at 0.7V (or
thereabouts) in either direction. My B&M headlight has a zener diode in it
already, like the OP's.

To clarify: in series with each other; in parallel with the bulb; each
Zener pointing in a different direction.

~6V AC (approx)
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Zener1 | |
| Bulb Dynamo
| | |
Zener2 | |
| | |
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