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Adam Munich

Mon Jan 03 2011, 04:18AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Battery powered DSO.

Good idea or no?

I used the guts from one of those plug in your car outlet inverters (a good one, I think it was made by DeWalt), and a rechargeable LiPo.

Arcstarter

Mon Jan 03 2011, 04:30AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, a problem i can think of is grounding, which is in turn a shielding issue. It's probably best to keep your scope at ground potential so that it doesn't build up a charge, ya know?

Adam Munich

Mon Jan 03 2011, 04:36AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I suppose, but that's what a metal pipe in the ground is for.

As for noise, you can see a square wave below 10mv/div. Grounded by my finger that goes away. I expected it to be a lot noisier but no.

This is more or less just an experiment, if you ever need to use an oscilloscope in a wheat field for some reason.

Sulaiman

Mon Jan 03 2011, 12:11PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

My first thought was about safety, but next, do you 'trust' the inverter and battery to not damage the 'scope?
Because I have access to differential probes etc. I'm a strong believer in well grounded 'scopes for safety reasons. I have colleagues who 'float' their bench 'scope citing their interpretation of safety which is valid for them. We each ultimately take responsibility for our own lives.
(Ive had multiple accidental electric shocks in my youth, some later, I'd rather not get any accidental shocks anymore as I don't feel quite as 'Immortal' as I used to)

P.S. In the '70s I used to carry a battery powered portable oscilloscope and there's not really been that much progress at the entry level - ...... comming soon from China I suppose.

Steve Conner

Mon Jan 03 2011, 12:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I can think of a bunch of situations where a battery powered DSO would be handy. I actually was going to do this to a TDS210 myself.

The safety concerns are valid, but the TDS210's case and knobs are all plastic, so not that serious. The power supply should accept 90 to 260V AC or 120 to 300V DC, so you'd have to try hard to fry it. Your inverter might even be the kind that supplies DC.

I have an old Tek 222a. Not only is it battery powered, but the inputs are isolated from the case and from each other. That comes in really handy. Too bad it only samples at 1MS/s. Tek have always had a scope like this in their range, aimed at power electronics troubleshooting. The 222 is from the days when power electronics meant SCRs and Darlington bricks, and 1MS was all you needed.

ScotchTapeLord

Mon Jan 03 2011, 03:57PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

Wouldn't grounding it to a metal pipe defeat the purpose of using batteries to isolate it?

IntraWinding

Mon Jan 03 2011, 04:01PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Having a floating oscilloscope is handy. Suppose you want to measure current by measuring the voltage across a small value resistor in the live line of a mains powered circuit. You can connect the scope probe 'earth' clip and probe tip across the resistor if the scope is floating. Where I worked they would just undo the earth connection in the plug and bend it back so it stuck out of the plug so you could easily see it had been disconnected. That relied on the isolating properties of the scopes internal PSU transformer to float the scopes circuitry and so was ok up to mains voltages. The supervisor banned this practice (which makes the whole scope live) however and told perpetrators to instead use the scope in differential mode with two scope probes.

A battery powered rather than un-earthed oscilloscope would reduce dangers, but you wouldn't want to use it at very high voltages. Ultimately you'd get sparks jumping at you when you tried to operate controls, but before reaching those voltages I think electrostatics would wreck your IC's.

I did this many times to a small power supply with optional output earthing when I forgot to use the earthing.
Just by using it to power an EHT generator from a photocopier, enough charge leaked from the EHT generator output to it's primary to raise my isolated low voltage power supply output circuitry high enough above ground to blow a CMOS analogue switch chip. I did it about 3 times over the years! The effect was that the digital output voltage display stopped working - very annoying. For a modern switching supply it would likely be terminal!

Avalanche

Mon Jan 03 2011, 05:15PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I've done this with the exact same model scope at work except I didn't take the inverter apart I just used it off the shelf with a lead acid battery. I've done it where the only power available was 600VDC, and it's also handy where no mains power is available at all.

It's safer than lifting the earth wire out of the plug (and not telling anyone!) but you still need to be wary of the other scope connectors and your ground clips! I recently got a large 300VDC shock across the chest holding onto the chassis of some equipment with one hand whilst adjusting the scope, and brushing the unused probe socket with the other hand. My beer went everywhere!

GeordieBoy

Mon Jan 03 2011, 06:08PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Just wanted to say that it is generally safer to float the equipment under test than to float the instrument your using to measure it with!

In other words power up your equipment under test from a mains isolation transformer and keep the oscilloscope that you need to operate firmly connected to mains earth. Since the equipment is powered through an isolating transformer you can now clip your scope probe ground lead wherever you need to with no fear of unwanted explosions. (Just remember that all of the scope grounds are connected together! So you need to be careful and only connect the scope ground to one place at a time.)

If the equipment under test is powered by an isolation transformer you have the added safety that if you should touch a single part of it's innards, there won't be a path back to ground to complete the circuit until you connect a scope probe ground to it somewhere.

I do appreciate however, that the thing Andy was working on was probably some massive power electronics thing that would have probably required a huge 3-phase multi kVA isolating transformer.

-Richie,

Arcstarter

Mon Jan 03 2011, 11:00PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

ScotchTapeLord wrote ...

Wouldn't grounding it to a metal pipe defeat the purpose of using batteries to isolate it?

Well i would love to have a battery operated scope, moving a ground stake might not be fun though, but it should be doable.

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