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Brightspark Magnetos

NEW ... Take a look at some of the equipment we use in our workshop for magneto servicing and overhauls.

 

 

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FAQ sections:

FAQs about the EasyCap
FAQs about magneto condensers
FAQs about magnetism and remagnetising
FAQs about ATDs and timing
FAQs specific to Lucas magnetos
FAQs about magnetos generally

In this section:

Do I really need to disconnect the old condenser?
Is the EasyCap covered by a guarantee?
How can a capacitor so small do its job properly?
What is the capacitance of the EasyCap?
What is the voltage rating of the EasyCap?
What is the dV/dt rating of the EasyCap?
Over what temperature range can the EasyCap be used?
Is the EasyCap susceptible to high humidity and dampness?
What quality control is the EasyCap subjected to?
How long will an EasyCap last?
How can I check my EasyCap?
Can I make my own version of the EasyCap?

FAQs

How can a capacitor so small do its job properly?

In short, because of modern technology and our careful selection of a capacitor employing that technology that does do the job.


In more detail, a simple capacitor consists of a pair of conductive plates separated by an insulator called a "dielectric." Two of the main parameters of a capacitor are:

  • its "capacitance" - namely how much electrical charge does it store for each Volt applied between its plates, and

  • its "breakdown voltage" - namely how many Volts can be applied between its plates before it stops acting like a capacitor, for example because the dielectric breaks down and a spark jumps between the plates and in all likelihood damages the dielectric.

The capacitance is related to the area of the plates, multiplied by the "dielectric constant" of the dielectric, and divided by the distance between the plates.

The breakdown voltage is related to the "dielectric strength" of the dielectric multiplied by the distance between the plates.

If you start with one design of capacitor and want to make it smaller, but with the same capacitance and the same breakdown voltage, then:

  • Each time you halve the area of the plates, you need to halve the distance between the plates, or double the dielectric constant of the dielectric (or a combination of the two) so as to maintain the capacitance, BUT

  • Each time you halve the distance between the plates, you need to double the dielectric strength of the dielectric so as to maintain the breakdown voltage.

So, you need a dielectric whose dielectric constant multiplied by its dielectric strength is higher.

The original Lucas capacitors has a dielectric of wax-impregnated paper, whereas BTH used mica. By contrast, the EasyCap capacitor has one of the many ceramics that have been developed for use in capacitors today. A guide to the dielectric constant and dielectric strength of these materials and air, together with the all-important product of the two, is:

Material

Dielectric
constant

Dielectric strength
(V/µm)

Dielectric constant
x dielectric strength
(mid range)

Air

1

3

3

Waxed paper

4

40 - 60

200

Mica

5

120

600

X7R ceramics

1,000 - 4,000

75 - 150

280,000

The discussion here is necessarily simplified, because this is a very complicated subject. We are always ready, in fact we are keen, to enter into technical discussion with those whose views may diverge from ours, or who can otherwise add to the debate constructively.

We include the discussion because we all have a natural tendency to doubt claims made for new products which are strikingly different from what they replace, unless legitimate queries are addressed and explained. We have the same tendency ourselves, in abundance.

In the days when Lucas and BTH and Bosch and others were designing their magnetos, the sorts of materials used to make the capacitors used in EasyCaps hadn't been invented. The constructors were therefore forced to use larger items than they would have wished, to obtain satisfactory performance. This led to sizeable condensers having to be buried deep in the armatures of all rotating coil magnetos. Had the manufacturers had the option, they may well have ended up mounting their condensers where they could be accessed easily as a routine service item, which they were of course able to do with magnetos with rotating magnets like the Lucas SR series, which are much loved by their owners for this and for other reasons.

As an addendum, one reason why many owners regard the BTH rotating coil magneto as superior to the Lucas equivalent is because of the use of mica as the dielectric in the BTH condenser. As the table shows, it has superior dielectric strength to waxed paper, and it also has higher resistance to humidity. Thus an original BTH item is more likely to have survived in working order than a Lucas item.

A very good paper on the volumetric efficiency of capacitors can be read by clicking here.

 

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