Frequency response of a (Metex)M3800 DMM?

[Old Sparky]

My M3800 is almost certainly a Chinese Copy, but it works well.  However, when checking an ATX PSU primary circuit, the indicated voltages on the two power/switching transistors  dropped rapidly to almost zero, yet the HT+ supplying these devices via a tap on a transformer primary winding, was about what I expected.  Having mislaid the instruction booklets which came with my M3800, I don't know if the readings I obtained were affected by either the frequency response of the meter or the waveform involved (probably both)  Can't find the M3800 data on the web, so can anyone provide the relevant info.,please?

[ppppenguin]

Not sure I'd trust many DMMs in that sort of nasty pulse environment.

Please also note that the primary side of a switchmode PSU is effectively mains. While Old Sparky knows what he's doing, when doing this sort of work you really want an isolating transformer plus Class III rated meter and probes.

[pwdrive]

According to this it's an average sine measuring job from 40Hz to 1KHz:

http://wrack.ped.muni.cz/datasheet/soubory/3800.pdf

Lawrence.

[Old Sparky]

Thanks, Lawrence. I have the booklet somewhere, but cannot find it. I suspect that both the frequency and the waveform may affect the readings, although there is a fault somewhere in the PSU under investigation.
Jeffrey, for the benefit of others less familiar than you or I with electronics, that warning is very helpful,hanks for posting it on the forum. I must admit I don't use an isolating transformer (perhaps I should!). The power to the bench is protected by RCDs, and my test leads should be up to Cat III standards, but of course there is no such thing as being 'too careful' when dealing vith high voltages. It is certainly possible to become complacent, adopting the attitude 'It'll never happen to me'

[Mark Hennessy]

That supply will be running at upwards of 50kHz.

If it's a flyback type, the voltage at the collector of the switching transistor can reach many hundreds of volts when it turns off. Enough to damage many a multimeter, especially when you consider that is a very narrow spike with lots of HF energy.

Most ATX power supplies are bridge types, so the switching voltage is constrained to the unregulated DC input voltage (325V). Still not a job for a multimeter though.

Isolation transformer, high voltage divider probes, oscilloscope.

All that said, most faults are capacitors or start-up problems. Anything more involved than this is simply not worth investigating, as ATX PSUs are so cheap to replace (£20 or less!).

And when on the bench, another trap is to forget to include a load on the output - this causes the main rail to fire its crowbar, shutting down the PSU with a whine.

Don't forget that an ATX PSU has two separate power supplies within. And the main supply won't come to life unless you short pin 16 to ground.

The standby PSU is usually a flyback type. This needs to be working - if not, the main PSU won't ever come up.

For power-supply geeks like me, this site is a joy: http://danyk.cz/s_atx_en.html - there are a whole load of PC power supply schematics on there, and while it's unlikely that the exact model in question will be covered, it's still interesting to look at the range of circuit techniques used.

[Mark Hennessy]

I must admit I don't use an isolating transformer (perhaps I should!).  The power to the bench is protected by RCDs, and my test leads should be up to Cat III standards, but of course there is no such thing as being 'too careful' when dealing vith high voltages.

And to add to this, I should say that an isolation transformer is only to provide a means of safely connecting earthed test equipment to the live part of the circuitry - nothing more. There are a lot of misconceptions about isolating transformers. We've covered them in the past: http://golbornevintageradio.co.uk/forum/...p?tid=4593 - for anyone reading who is any doubt, that thread covers the ground fairly well - and gives some examples of the sorts of misconceptions that exist.

[pwdrive]

And to add to this, I should say that an isolation transformer is only to provide a means of safely connecting earthed test equipment to the live part of the circuitry - nothing more.

Not to mention eliminating the shock risk between a chassis supplied by the transformer and earth whether test gear is being used or not.

Lawrence.



{Mod edit: quoting sorted}

[ppppenguin]

Not to mention eliminating the shock risk between a chassis supplied by the transformer and earth whether test gear is being used or not.

When working with this sort of circuitry I'm really undecided about using an isolating TX or RCD. I understand the arguments for and against, we've covered them in detail in the past. If I touch something I shouldn't and enough current flows to be nasty an RCD will trip. But as Lawrence says, the isolating T will eliminate certain kinds of shock altogether.

Neither will really protect you well against the RF burn you'll get from touching a high frequency, high voltage point. I once touched the collector of a line output transistor and got a small burn on my finger. Not serious, but I only brushed past. It could have been a lot worse.

[Terry]

... that thread ... gives some examples of the sorts of misconceptions that exist.

This one is a cracker - or might have been ...

[Mark Hennessy]

When you boil it down, the downside of isolation transformers - like many other safety devices (and not just restricted to the world of electronics) - is that they can give a false sense of security.

I personally think that it is much clearer to not include "safety" as a selling point of such a device. Rather, it is a device to enable certain activities - activities that wouldn't be possible in any other way in a normal professional environment today.

Obviously, there are other uses for isolation transformers - such as the types with grounded centre-tapped secondary windings - but that's a different application altogether...