[ARC5] Hamfest DVM modules (and now PSU modules from eBay).

Sun May 8 20:06:08 EDT 2016

G'day all,
On the topic of digital meters,  I bought a 3 wire digital meter (as is
the topic of discussion) and it worked.  I can't say it was the same
item discussed here by others.

Broadening the topic slightly to "small items bought from eBay" - I
bought a number of different power supply modules and the quality varies
from almost un-useable (and certainly never tested by the designer ) to
satisfactory.  To give some detail:  Many of the small power supply
modules quote the spec rating of the regulator (eg LM7812 is rated at
1000mA) but the heatsink and filter capacitor might be suited to 50 or
100mA.   I would use the word "junk".  Allowing 1000uF per Ampere, a
board with a 100uF filter capacitor might deliver 100mA (but maybe only
for a few thousand hours before the filter caps "die" from over heating
etc).  Yes - 100uF on a board advertised at 1000mA!

A second kit,  ( www.eBay.com/itm/231736128525  ) that I assembled was
rated at 1A (max).  The filter capacitor (2200uF/50V) is adequate.  The
pot controlling the output voltage came without the mounting nut (and
washer).  The holes for the switch and the output terminals don't line
up well (I had to bend and tweak the leads to fit them to the board).
The positive voltage appears on the black terminal, the negative on the
red.  Sure, I could (and did) disassemble (and re-assemble) the terminal
and now the (+)ve  appears on the red terminal.   However at AU$3.75 the
card is value for money, and I bought 4.   I'm not disappointed - but
the card could be even better with a little care/thought.

A third board ( www.ebay.com/itm/121712984604 ) came assembled.  The
heatsink is "beefier" than the one mentioned in the previous paragraph.
The filter cap (1000uF) is adequate, but not so generous as the 2200uF
mentioned in the previous paragraph.  Another 50 cents spent on a larger
cap would be worthwhile.  Anyway ...  It has mounting holes provided
(not seen on some cards).

So - some items are good, some nasty.  Caveat Emptor.  Look carefully at
the size of caps and heat-sinks used on cheap boards.

I intend to write a single page article and publish it in the local
Westlakes club magazine.

73 de Les Smith
vk2bcu at operamail.com

On Mon, May 9, 2016, at 06:39, Dave Merrill wrote:
> Check out item 191864367191 on eBay - the seller includes a wiring
> diagram in the description - scroll way down.
>
> On Sun, May 8, 2016 at 1:34 PM jeepp at comcast.net
> <jeepp at comcast.net> wrote:
>> That would be fine except that that there are only three leads on the
>> module.  In fact, one version he had, had only two leads, self
>> powering from the measured source.
>>
>> Jeep. K3HVG
>>
>> Sent from my Verizon 4G LTE Tablet
>>
>> ------ Original message------
>> *From: *____
>> *Date: *Sun, May 8, 2016 11:36
>> *To: *DSP3;
>> *Cc: *MRCA at mailman.qth.net;arc5 at mailman.qth.net;
>> *Subject:*Re: [ARC5] Hamfest DVM modules
>>

>> Jeep,I believe the power supply ground is not referenced to the meter
>> input ground. You have to power the meter with an isolated supply
>> like a 9 volt battery or from a power transformer secondary.Mike N2MS-----
>> Original Message -----From: DSP3 __To: MRCA at mailman.qth.net[1],
>> arc5 at mailman.qth.netSen[2]t: Sun, 08 May 2016 13:10:18 -0000
>> (UTC)Subject: [ARC5] Hamfest DVM modulesFor what its worth, I bought
>> two of those small, pop-in 3-digit, panel mount voltmeters at the
>> Hagerstown fest, yesterday.  No documentation, whatever, is provided.
>> The idea was to add them to a couple of LV power supplies I have that
>> are meterless.  Well, they don't work very well, at all, I believe.
>> They are powered by about 3-9v DC.  The ones on display appeared to
>> be just fine.  I wired both of them up to batteries and power
>> supplies and, with proper Vcc applied, got readings typically 20-30%
>> high along with a 2v-4v instability.  I thought that perhaps the
>> meters were responding to noise and/or ripple but the 9v batteries
>> connected exhibited the same error and instability. The meters do
>> appear stable with only Vcc applied and indicate 00.0v.   Anyone have
>> any experience with these little DVMs?  Is additional filtering
>> required?  Are they maybe AC meters with a high crest factor and a
>> bridge on the input?  I have no idea.......Jeep - K3HVG__
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