Britbike forum Forums British motorcycles in general British motorcycles in general Testing zeners

I have been thinking about how to test your zener at home (or zenor if you advertise on eBay <s>).

For a start you need a multimeter and ideally you need a resistor as well (about 1 thousand ohms or 1K ohm).

Ok so you have a meter switched to diode test (the red meter in the picture below.) Attach your leads to your zener, you should see around 600 on your meter. If you don't swap your leads over.

If your meter doesn't have a diode test just select resistance (see the yellow meter below). Measure the resistance of your zener then swap the leads over and measure again.

If your zener is ok you will see a high resistance with the leads one way and a lower resistance with the leads in the other.

If you see a high resistance with the leads both way round then your zener is broken.

Now if you want to measure what voltage your zener works at you will need about 20 volts and a 1k resistor.

You can buy a 1k resistor quite readily online or at your local component shop. Any power rating will do.

To generate around 20 volts you can wire a couple of batteries in series. Your bike battery will give you 12v for a start. If you have two bikes you have two batteries and 24v.

If not do you have any 9v batteries at home? If you do then connected together you will have 21v for testing.

With 21v and 4 Lucas zeners I measured the following voltages. 13.9, 14.2, 14,27, 14.4.

I hope this helps.

With a 1K resistor and 20 volt source the zener current will be roughly 6-8 milliamps, which is not realistic for a shunt regulator in a BI application. Zeners will have higher voltage at higher current (the E/I curve is not perfect).

If you have the equipment, the best test is with a current limited variable power supply... but for the typical BI owner your test is adequate for pass/fail criteria. Thanks for posting that.

So what's wrong with using the Zener test procedure given in the factory manuals and Lucas service manuals where the motorcycle is used as the variable power source?

<which is not realistic>
True, but I was trying to be realistic and use what bits and pieces the average owner would have to hand.

<So what's wrong>
Nothing LAB but have you tried that test, it is kludgey.

If you have a test of zeners that you prefer then please publish it here so we may all benefit. Our bikes may be from the past but we don't have to live in the past.

Please could you explain (preferably in plain English) why you think that the Lucas/factory Zener testing method is "kludgey"?

I must admit I had to look up "kludgey".

The definition being;- "a system and especially a computer system made up of poorly matched components". But admit to being none the wiser as to which components are poorly matched? And I'm not trying to trip you up, I'm just trying to understand what it is that you actually mean?

The comapatively high internal resistance of the Lucas selenium rectifiers would also act as a significant series resistor in the generated DC voltage.
Anyone notice an overall improvement when they changed out the Lucas rectifier for the newer silicon "bridge" rectifiers? Possibly this would lead to cooked up zeners due to less forward resistance in the bridge. How about making a bridge up from schottky diodes?



(from wiki)

As will the impedance of the stator itself.



I replaced the Lucas Selenium bridge with a Silicon pack long ago, and yes it was an improvement. With the stock alternator the zener was not unhappy (for 30 years).



I don't think they make any in a package that's easily adaptable to a bike. If you're willing to make up a custom assembly with stud rectifiers, you might gain a half a volt or so.

The whole concept of a shunt regulator is one of compromises. Zero insertion losses make it ideal for a design where the source sometimes dips below the regulation point. But at higher input voltages the effieciency drops drastically as the shunted current goes up in wasted heat.

Put a bulb in series with the zener, that way the waste energy is being utilised AND you can see when your system is charging

There isn't anything much simpler than the zener diode system, I found these on Farnell, should be OK with 2 of them in parallel on a heatsink, zeners

They look similar what Lucas referred to as a "Clipper Diode" used for regulating direct lighting? http://www.britcycle.com/Products/391/391_83137.jpg

<I'm just trying to understand what it is that you actually mean>
I was looking for a zener test that I could do on my bench, that didn't need my bike running. A test that I could do when perhaps I wasn't sure if my alternator and rectifier were ok.

If you prefer the methods listed in the Triumph or Lucas manuals then please use them they will work. I was just giving an alternative.

Derry, how timely of you to post these tests just now! I'm currently in the middle of kludging together a 12V system on my RE 250!! I have one of the newer Radio Shack bridge rectifiers, and a used Zener from Ebay on the way!

thanks!

I am sure it will go ok,let us know how you get on.

Derry

Paralleling zeners is not good design practice. If it must be done, as Lucas did, they should be very carefully matched for zener voltage. The above referenced zeners are 5% at best, which isn't good enough. The wattage rating of 50W could be exceeded by one of them that zenered while the other didn't, if that maximum tolerance were at its worst case of 10%.

So if you wanted to do what Andrew porposed HH how would you do it?

Derry.

Buy a handful of the 5% tolerance zeners, then set up a test bed with a high current (10A or so) variable voltage supply with voltmeter and ammeter. Insert each zener diode into the test rig (with an attached heat sink) and crank up the voltage until a reasonably realistic working current is displayed, somewhere between 3A and 5A or so, and then note the voltage at which that current flows. Test every diode at the chosen current and try to find a pair that runs within about a hundred millivolts of each other.

With a second ammeter, you could run them in parallel and verify that current is shared reasonably equally between them. Calculate the maximum required dissipation by subtracting your minimum vehicle electrical load from your alternator maximum output and ensure that one diode won't be carrying more than that when they are shunting that load to ground.