Advice on Schottky Diodes, please

Hello all you electronics wizards

I am using a Schottky diode in place of the cut-out on my dynamo system, and the one I have used for two years has now failed*. So, what spec should I choose from the long list at RS to replace it?

12 volt system, so 14 V on a good day.
Max output 10 Amps
Ignition is Lucas electronic coil, so ignition spikes are a factor.

The diode will be attached to an aluminium heat sink under the bonnet of the car.

Anything else you need to know?

* it was a gift on the basis 'try this, it might do what you want' - and it has.

Thanks, Tim

PS a Schottky diode has a low voltage threshold, so it allows current to pass at a low voltage, and this is preferred for this use (I think) as it ensures rapid build up of the current in the field coils.

Schottky diodes have a low forward threshold and a very fast recovery when the voltage across them reverses. These are separate characteristics. The first may help you when the voltage to the coil from the battery system is low, but the difference from an ordinary diode is only about 0.2 volts so not very significant. The second is probably irrelevant in your application, though it does help in rectifiers for high frequency inverters. Unless you are very concerned about operation at low voltage I would select an ordinary rectifier diode specified for a high current and high reverse voltage.

Tim

I have some diodes left over from when made some batches of 12 V electronic regulators for converting 6 V Lucas / Miller motorcycle electrical systems. If one of those would do I'll have a root round the stores. So far as I know never had a failure. No one has complained and none of the ones sold to folk I still know broke. Heck should be some circuit boards and component kits too, maybe even a couple of complete ones in the box too. But it was about a decade back last time I looked in it.

As John says there is, in practice, no particular advantage in using a Schottky diode in this application. The ones I used were proper alternator rectifier diodes so desinged for that sort of duty.

Clive.

Very helpful, Clive, and I would like to relieve you of your surplus stock. Just give me an estimated price to include postage.

Tim Stevens
Knighton Powys

John - very useful information. I was clearly advised to use a Schottky but now I know that is not necessary.

And for the record, when you have a car which only offers 8 amps on alternate good Fridays, all driving especially in the dark and in the rain, you are always concerned about operation at low voltage.

Thanks, Tim

John & Clive, does this mean that the excess voltage produced by a fast running two brush dynamo is only restrained by the battery being in circuit? I am thinking about a 50's motorbike I'm restoring. Colin

Tim

Found the stash. Diodes I have are 25 Amp alternator diodes, says Autocar Electrical on the box, but they were actually made by Motorola. Type number 1N 3491 R. Body pushes about 1/4 inch deep into a nominal 1/2 inch diameter hole in the heat sink. Made for negative earth system so body is cathode.

If you can use a diode of this style PM me your address and I'll send one up. Does £2.00 sound OK for one. Last batch I got came out at £4.00 for box of 3 maybe 1980 or thereabouts.

If you are still running an electromagnetic relay type voltage regulator an electronic one will release another amp or maybe even two as it uses much less power to drive itself than the old type vibrating contact style. Way back got involved with trying to squeeze the last milliamp out of a similarly marginal system and, as I recall things, we found the most efficient set-up was an electronic regulator with low current relay to do the cut out job. I think we tweaked the voltage up a bit too so the system was delivering 14.5 volts at the battery.

Clive.

Colin

Voltage regulation is by modulating the current in the dynamo field coil. Old style regulator used a set of vibrating points controlled by an electromagnet, pretty much same as a relay. When the dynamo output isn't enough to cover the load the output voltage is pulled down. If this output voltage is too low the points close putting full voltage across the field coil pushing up the dynamo output. When the dynamo output is too high the voltage is also too high so the contact points open cutting off the current in the field coil so the dynamo no longer generates and the output falls. Assuming you need less than the maximum output of the dynamo to support the electrical loads the points vibrate between open and closed and field current averages out at the right level to give sufficient output from the dynamo to cover the load and keep the battery charged.

Electronic regulators like they one I used to build do pretty much the same thing only lots faster and need much less power to operate. The standard Lucas 6V electro mechanical relay gobbles up nearly half the dynamo output power. Miller regulators are bit better when correctly adjusted. Unfortunately correct setting and servicing of old style regulators (and dynamos) was completely beyond the British standard motorcycle mechanic. I never had trouble because I followed the book and did it right. I never saw one that was set correctly either. Started doing the 12V converters in self defence to stop people turning up with "fix it requests". Especially as half of them couldn't keep their fingers out afterwards!

Objectively the Lucas and Miller 6V dynamos are excellent little electrical machines. Better than any motor vehicle alternators which pay a heavy price in lower efficiency for their mechanical robustness. Output is basically defined by safe operating current. You can take 10 amps out of a short Lucas and 12 out of a long Lucas or Miller on top of the power needed to drive an electronic controller. So a 12 volt conversion more than doubles the power available.

Clive

Edited By Clive Foster on 23/10/2016 20:03:39

Clive, thank you for your excellent expert explanation, it has helped me a lot and I never realised how inefficient the old regulator/cut out units were. Colin

Should anyone have any interest in the MCR1 etc. and EL Lucas dynamos the BSA service sheets are here:

**LINK**

Have to agree with Clive re: the average motor cycle mechanic, in general they just used to faff with them until they sort-of worked .. no real idea of what was required. But then most of their employers just wanted to sell new or B90 (?) replacement units.

D.

From bitter and expensive experience don't try to run head lamp bulbs greater than 36W with short Lucas dynamos. I have done this a number of times over the last 30 years with 12V solid state regulator and have destroyed the commutator every time. Never had a problem with long dynamos though.

JA

Hello again

Clive's explanation is very helpful, but it does not apply in my case. I have no regulator, only a cut-out (which does, none the less, 'use' some of the output). My system is a three brush system (out of date by about 1935) in which a feed from the winding is used to energise the field coils, but these are not earthed directly. Instead they are earthed with a third brush running between the main ones, so the voltage across the field varies with output and speed. Varying the position of this brush varies the strength of the output, and this system levels out the output voltage. Usually. Somewhat.

So, my field winding, in order to work properly with the third brush, has a lower resistance than a later two-brush system, and this makes running it with two brushes and a regulator rather difficult. But, instead of the original charging scheme, which could be switched on or off only (ie too much or not enough, take your pick) I have a four-position switch and a series of low resistances in the field circuit.

Anyone wishing to know more is asked to get in touch, as I guess this is a bit arcane for most of you.

Cheers, Tim

And Clive, I'll send a PM when I have had breakfast.

Tim