How to... make a peak volt­age me­ter

To get a pre­cise fix on the AC volt­ages in your bike’s elec­tri­cal sys­tem. It’s not that tricky


Make a sim­ple plug-in to help your mul­ti­me­ter

IF YOU HAVE a mul­ti­me­ter you will no doubt be smit­ten by its in­dis­pens­abil­ity. How­ever un­less you’ve opted for a top-of-the-range, fully func­tional mul­ti­me­ter with a three-fig­ure rather than two-fig­ure price tag, you might find that it has the odd lim­i­ta­tion here and there. One such is the in­abil­ity of the av­er­age mul­ti­me­ter to record peak AC volt­age from the likes of gen­er­a­tor and ig­ni­tion wind­ings. The sim­ple fact is that the volt­age

fluc­tu­ates so wildly and quickly that the me­ter can’t keep up – blink and you’ll miss the key fig­ure.

A peak volt­age adap­tor is the so­lu­tion. In a nut­shell it uses a diode to rec­tify the AC to DC, charg­ing up a ca­pac­i­tor which acts like a lit­tle bat­tery. Your mul­ti­me­ter can then mea­sure the AC volt­age as a DC volt­age stored in the ca­pac­i­tor. You can also use one on in­ter­mit­tent DC feeds such as in­di­ca­tors. Peak volt­age adap­tors are ex­pen­sive to buy – about £78 for a Fluke one – but cheap to make. We went on the in­ter­net and looked at some di­a­grams be­fore amal­ga­mat­ing a cou­ple to make our own ver­sion.

A re­sis­tor across the ca­pac­i­tor al­lows it to dis­charge slowly so you don’t get an electric shock off of the leads should you in­ad­ver­tently pick them up soon af­ter test­ing. We also fit­ted a push-to-make switch to al­low the ca­pac­i­tor to be in­stantly dis­charged for quick retest­ing.

All you need are the tools listed above fol­lowed by a quick trip to Maplin for a hand­ful of com­mon and cheap com­po­nents.

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