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Old 4th Oct 2017, 2:51 pm   #1
WaveyDipole
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Default Cheap digital meter failure in car battery charger

Some time ago I ordered one of these to replace the meter that had failed in a battery charger.

http://www.ebay.co.uk/itm/DC-0-100V-...P/272873364922

Initially this worked just fine. However one day after turning the charger off and then back on in fairly quick succession, the meter failed and let out the magic smoke. The regulator (I assume that is what it was as I could not positively identify it due to said charring) was both badly charred and shorted. I removed this shorted part and supplied the neccessay power to the IC directly using a regulated PSU to test, but it was clear that all was not well. The display was partial and gradually grew more corrupt and eventually failed as well.

Since the meter can be supplied with up to 30v and this was nominally not exceeded, this being a standard 12v/6v car changer, I was trying to understand what might have caused the device to blow like that? Naturally it could have been cheap sub-standard parts, but I was wondering whether back EMF from the transformer might have contributed? If so, then will clamping the supply voltage with a 'fly wheel' diode suffice or would a MOV be better? It is almost not worthwhile bothering replacing this really, but if I did, I would like to prevent it from happening again.
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Old 4th Oct 2017, 3:21 pm   #2
kellys_eye
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Default Re: Cheap digital meter failure in car battery charger

What type of charger?

If it was one that used the battery as the 'smoothing' device (i.e. half wave rectified output) the digital meter might not have liked the 'pulsed' DC an unloaded charger puts out.
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Old 4th Oct 2017, 3:43 pm   #3
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Default Re: Cheap digital meter failure in car battery charger

A resistor in series with the power input? That would stop most of the surges.
 
Old 4th Oct 2017, 3:50 pm   #4
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Default Re: Cheap digital meter failure in car battery charger

Its a simple car battery charger, switcheable between 12v/6v secondary winding taps, a silicon rectifier bridge and then to output via an overload protection relay. There are no smoothing capacitors so I guess the battery would be used as a smoothing device. It is branded Davenset, Super 4.

Merlin, yes, I was thinking that along with the diode or Mov, but if a current limiting resistor would suffice, then even better.
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Old 4th Oct 2017, 5:42 pm   #5
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Default Re: Cheap digital meter failure in car battery charger

Back EMF is possible. Feeding 1/2 wave rectified to a linear regulator 'shouldn't' make it blow but it's a different matter if there is a buck or boost regulator !

dc
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Old 4th Oct 2017, 5:46 pm   #6
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Default Re: Cheap digital meter failure in car battery charger

I'd use a series resistor and a big 20V Zener - which won't conduct unless a spike comes along, nor will it affect the charger's operation (if your 12V charger consistently puts out 20V you've got other problems...)
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Old 4th Oct 2017, 7:04 pm   #7
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Default Re: Cheap digital meter failure in car battery charger

G6Tanuki, I had wondered about using a zenner, but was unsure about the about the size that might required to absorb the spike. Anything 'big' will probably cost more than is worthwhile spending and I suspect I will need something more beefy than than a standard 500mW part! Still that might be a way forward in conjunction with a current limiting resistor. The output, if I recall, is nearer to 14v, which would be about right for a 13.8v car battery, but shouldn't be anywhere near 20v!
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Old 4th Oct 2017, 8:09 pm   #8
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Default Re: Cheap digital meter failure in car battery charger

How about a 1N4007 And 10ohm in series from the main rectifier +ve to the meter supply +ve.
Main rectifier -ve to meter supply -ve.
1000u 35v capacitor across the meter supply should absorb any spikes.

The diode stops the cap discharging into the battery if it's a bit flat and the 10 ohm tames the current peaks.

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Old 9th Oct 2017, 3:48 pm   #9
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Default Re: Cheap digital meter failure in car battery charger

One thing I could add:

Never under any circumstances add a smoothing capacitor to the output of a lead acid battery charger. When the charger is switched off, they remain charged and connecting disconnecting the charger etc, it can result in charge and discharge currents & sparks which can ignite hydrogen produced by the battery charging, exploding batteries etc, not good.

When the charger's transformer, which has a leakage inductance, is under a moderate load and it gets disconnected, the collapsing field can result in high voltage spikes (in automotive work the analogy is the load dump), that is probably what wiped out the meter. Since meters draw little power the idea of a series resistor and a zener, or a diode & resistor and capacitor to filter the meter supply would probably save the next fragile electronic meter.

Actually, if you want a meter to monitor the output of a lead acid battery charger, a volt meter is not helpful and has limited utility. The best type is an analog amp meter in series with the output, because they automatically average the pulsed current, be it half wave or full wave to give a sensible reading. There are some nice auto meters and motorcycle ammeters that do this, you really don't need anything with fragile IC based electronics in it.

People are finding this out now with Arduino projects, putting them in cars where the electronic environment is a lot more hostile (unlike a domestic appliance application) with power supply spikes and the IC's are dropping off like flies or just resetting and you end up having to surround them with multiple TVS devices analogous to having to hide them in a cotton wool box inside a case made of 6 inch thick lead.
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Old 9th Oct 2017, 5:04 pm   #10
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Default Re: Cheap digital meter failure in car battery charger

Quote:
Originally Posted by Argus25 View Post
One thing I could add:

People are finding this out now with Arduino projects, putting them in cars where the electronic environment is a lot more hostile (unlike a domestic appliance application) with power supply spikes and the IC's are dropping off like flies or just resetting and you end up having to surround them with multiple TVS devices analogous to having to hide them in a cotton wool box inside a case made of 6 inch thick lead.
I found out that the same is true for Raspberry Pi projects as well. I like the idea of 6 inches of lead though.
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Old 10th Oct 2017, 10:50 am   #11
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Default Re: Cheap digital meter failure in car battery charger

I often wondered why there was such effort put into Automotive semiconductor devices, but it is now evident that it is a very hostile environment to electronic devices, spikes all over the place. In the "old days" it was magnetic voltage regulators and dynamos, and spark plugs etc. then alternators and electronic ignition. It's another world.
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Old 8th Nov 2017, 4:06 pm   #12
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Default Re: Cheap digital meter failure in car battery charger

I have now received and replaced the meter and added a current limiting resistor and diode. Although the voltage reading is reasonably accurate, the current reading seems a long way off! I am getting readings at just a count of two high at a few milliamps, but for an amp or more it gets rather silly. I tried connecting a 12v/21W auto bulb across the output and got about 1.74A on the DMM but about 2.54A on the cheap meter. I also tried with a 12v/35W tugsten bulb which from memory read something like 2.87A on the DMM, but around 4.5A on the cheap meter! I have checked against two different DMMs which gave consistent readings, which readings seem mathematically close enough to what might be expected.

There is an miniature pot adjuster on the back of this gizmo which had to be set at one of its extremities just to get the reading correct at the milliamp level. I guess its Ok for an indication but pretty inaccurate otherwise. Then again, what might one expect for a couple of quid. At least it hasn't blown up yet!
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Old 8th Nov 2017, 4:14 pm   #13
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Default Re: Cheap digital meter failure in car battery charger

I suspect the 'odd' current readings you're getting are due to the meter expecting to 'see' DC or sinusoidal-AC, but the waveform of the rectified output of a battery-charger is very much not like either of these!

Old battery-chargers always used to have moving-iron ammeters: these behave semi-sensibly when measuring full-wave-rectified AC. Same goes for car ammeters measuring the 'rough' and ripple-frequency-variable output from dynamos and alternators.
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Old 8th Nov 2017, 4:58 pm   #14
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Default Re: Cheap digital meter failure in car battery charger

I've been shopping around for a new 60AH battery. Looking at the Yuasa data sheets, I noticed that they are recommending a bench charging current of no more than 4 amps, less than the C/10 figure that's been used historically. I guess this is all to do with the "maintenance-free" technologies. Tends to imply that having a reasonably accurate ammeter is a "nice to have" feature, though I suspect many chargers don't have.

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