Isolation between transformer secondaries.

[Hero999]

I'm feadup with replacing the batteries on my multimeters so I'm thinking about building a miniture mains powered multimeter power supply to power two meters with. I'm planning on using a small twin secondary 9V transformer and adding two separate rectifiers, smoother caps, and linear regualtors to each secondary.

The only question is, what's the minimum withstand voltage between the transformer's secondaries?

I know it's rated for double insulation and therefore can take 2kV between the primary and secondary but it doesn't say anything about the secondaries.

I'm concerned about this because the situation might arise when I'm using both meters to measure voltages on different parts of a mains circuit, so one might be floating at 340VDC and the other at 0V.

 

[Nigel Goodwin]

Well, apart from the fact it's a really silly idea mains powering your multimeters, if you do go ahead and do so - use seperate transformers.

But better still, keep buying the batteries - they last a good long time, and running them off the mains reduces their usefulness a huge amount.

 

[Hero999]

Why do you think it's a silly idea?

The batteryies don't seem to last for long on two of my meters and I've tried rechargables but recharging them is a pain.

I can see your point about it reducing their usefullness but that's only in a few situations when you want to use them where a mains supply isn't available (like wireing a house).

I don't plan to do this to all of my meters, just two of them. I was hoping to be able to use just one transformer because I have a small plastic box I want to use for a case and there isn't room for two.

Perhaps I'll just use them for voltages below 60V. Unless anyone here is certain that the secondaries are sufficiently isolated of course.

 

[Nigel Goodwin]

Why do you think it's a silly idea?

Because it's restricting the portability of the meter, for very little real gain - meter batteries should last a fairly long time, switch it off when you're not using it!.

The batteryies don't seem to last for long on two of my meters and I've tried rechargables but recharging them is a pain.

Rechargables aren't a good choice either, they self discharge far too fast.

 

[audioguru]

My Fluke multimeter has an LCD display and has a lot of features. It shuts itself off if it isn't being used so its 9V battery lasts a very long time.

Get name-brand fresh (they have best-before dates now) alkaline batteries.
Energizer has a new 9V lithium battery that lasts only twice as long as an alkaline battery at about 4 times the price.

Chinese alkaline batteries are leaking all over the shelves at The Dollar Store.

 

[Hero999]

Well, I see your point and I'll take your advice. I'll buy some decent batteries for my two expensive meters.

I'm still interested in building this, but I'll only use it with my cheap meters.

Anyway I very rarely use any meter when I don't have access to the mains supply.