Measuring DC Volts on a 4.5V AC device killed it!?

T

tom61

[H]ard|Gawd
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Feb 3, 2001
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I recently got a Tapwave Zodiac 2 retail demo unit, and I've been hacking around with it. The unit plugs into AC adapter without needing batteries, which could come in handy as a regular Zodiacs are nearing their end-of-life on the Li-Ion cells. I didn't look at the adapter closly enough, and thought it was DC. Which would make sense, as it plugged directly into the battery harness, but it turns out that it was AC. Since I needed to know what poliarity it was, I was trying to measure DC Volts on the power supply. All I got was zero... Figuring it might need load to work, and fearing that I had somehow killed the power supply, I plugged it into the unit, the unit fired up just fine. I moved to the rear of the molex on the device side, where there was far less chance of touching the probes together, and put the probes in. As soon as both were touching the metal, the unit shuts off! I immediately remove the probes, and the unit turns back on.

However, now it can't find files on the SD card! IT gets worse, as I pull the power supply, wait a few seconds, and try again, now it can't find the signature for an app in flash rom!

What the heck could've happened? The probes did not short together, and I don't see any signs of shorting on the cable itself. Not only that, how could the unit itself have been damaged instead of the power supply? Is measuring AC with a cheapy DMM set to DCV really that dangerous?
 
That's odd. Measuring DC on an AC line shouldn't do anything nasty--you should just get 0VDC. I don't mean to insult your intelligence, but are you sure that you were measuring VDC and not Amps DC? It's easy to make mistakes sometimes...
 
I was measuring DC volts. I had already double checked when I had gotten a measurement of 0 Volts on the power supply itself, and triple checked after the incident with the unit.

I there a good way of checking a multi-meter for faults? Right now, that's the only thing I can think of that could cause the unit to be damaged. It's been a good meter up until now, but being cheap, it's entirely possible to have fragged itself.
 
Mohonri said:
That's odd. Measuring DC on an AC line shouldn't do anything nasty--you should just get 0VDC.
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That depends; few meters filter their inputs on a DC setting, though most scopes do. I'd expect a varying reading, depending on the AC frequency.

It sounds like your problem, tom61, has got to do with a mechanically loose connection or a broken conductor in that connector rather than anything electrically wrong.

A Zodiac is a defunct PDA product that was aimed at gaming and multimedia applications.
 
Gah. I think I found the problem: a poorly designed DMM. Even though it was set to DCV the red probe was plugged into the unfused 10ADC hole. This is the only meter I've encountered that has the 10Amp line completely isolated from anything not amperage measurement, and I'm guessing that it runs through the amperage circuitry no matter where the dial is. :eek:

I feel stupid and betrayed at the same time... :(
 
tom61 said:
Gah. I think I found the problem: a poorly designed DMM. Even though it was set to DCV the red probe was plugged into the unfused 10ADC hole. This is the only meter I've encountered that has the 10Amp line completely isolated from anything not amperage measurement, and I'm guessing that it runs through the amperage circuitry no matter where the dial is. :eek:

I feel stupid and betrayed at the same time... :(
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i've seen tons of multimeters with a 10A line..

it's meant for when you have to measure high currents...

by unfused you mean broken fuse??
 
Whoa, yeah, that'll do it. Most multimeters, actually, have a separate hole for current vs. voltage. But not including a fuse is, as you say, just bad design
 
RancidWAnnaRIot said:
i've seen tons of multimeters with a 10A line..

it's meant for when you have to measure high currents...

by unfused you mean broken fuse??
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It says right on the label of the hole:
10ADC
10Amax
unfused
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I had moved it over to that plug when I was measuring high current (well more than the 200mA that the normal hole allows). If I recall right, with the meters I have used before, that having the probe connected to 10Amps plug, and the dial set to DC voltage, it measured DC voltage...

mikeblas said:
What brand/model DMM were you using?
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Err... I think 'DMT7' is the brand. :( When I said cheap, I meant cheap. So cheap that I stole the probes off a $7 DMM, because that one didn't have audible conductivity indicator, and the probes on this one weren't very good.

I'm feeling even dumber right now. :( I wonder why I never got a better meter, once I had a bit more money.
 
Yeah, trying to measure voltage with the plugs in the current sockets is a good way to blow a fuse/ruin a multimeter. It's likely that the a/c adapter is still good, but your DMM may be toast.
 
Well the DMM basically acts like a short when measuring current because that function is meant to be used in series, so you had the effect of shorting out the terminals you were measuring. That won't do any circuit much good.

It's not really fair to put all the blame on the meter, as was mentioned that functionality is very common even on expensive high-end brands.
 
Better meters do help, though. The Flukes, for example, will beep if the probes are plugged into the current jacks while the meter is set to read something other than current.
 
agent420 said:
It's not really fair to put all the blame on the meter, as was mentioned that functionality is very common even on expensive high-end brands.
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IMO, a poor design is a poor design, irregardless of how high up the model chain it goes. There is only one setting on this entire meter that uses that plug, so why was it on when that setting wasn't selected? This meter gets off with the fact that it's so cheap, that the extra wiring and/or switch were likley eliminated for cost reasons.

Anyways, it turns out the Zodaic wasn't severely damaged. From what I can tell, it was actually kicked into a debug mode. Which wasn't functioning, as it was likely disabled to prevent rom rewirte to add another measure of theft-deterence. I plugged it in recently, and it fired up just fine. I had recently read about a built-in debug mode common to nearly all PalmOS PDAs, which can be accessed by holding down and reset at the same time at power up. Doing this caused the same symptoms as after the short.
 
tom61 said:
IMO, a poor design is a poor design, irregardless of how high up the model chain it goes. There is only one setting on this entire meter that uses that plug, so why was it on when that setting wasn't selected? This meter gets off with the fact that it's so cheap, that the extra wiring and/or switch were likley eliminated for cost reasons.
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um... no

its your mistake. sorry...
 
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