Mike said:
If you want to find bad electrolytics, I don't think you can find
a better bargain. It gives the dissipation of the capacitor, so
you don't have to fool with lookup tables like you do with a
conventional ESR meter to see if the cap is good or bad. Besides
giving an easy way to check electrolytics, it also gives the
capacitance value to four digits.
Update : it actually displays 4 digits plus a decimal on large
electrolytics. But the two least significant digits dance around a
lot.
I switched to a 10uF 250VAC +/-5% film capacitor, and it read
10.045uF with the last digit fluctuating between 4 and 6. So I
conclude electrolytics are not very stable. But the readings are
quite consistent on a quality part.
I've been monitoring the battery self-discharge. It is terrible. It
loses a significant fraction of the charge in a week. Leaving it
plugged in has no effect. It does not trickle charge while it is
turned off.
One solution might be to simply leave it plugged in a turned on. It
says "Auto Power Off" under the display, so that might not work. But
it seems to stay turned on when it is measuring something, like a
large electrolytic.
I found it really is necessary to turn it off when moving from one
AC plug to another, otherwise it may give strange results. In fact,
it might be a good idea to post the operating notes found on page 5
of the manual:
Notes on Use
1. This meter is only for indoor use.
2. Turn off the TH2821A while switching the power supply between
battery and DC adapter or replacing the battery.
3. Although internal circuit protection is provided, DC voltage or
current may damage TH2821A. Before you measure a capacitor, be sure
the capacitor is fully discharged.
4. Charging may disturb measurement result sometimes.
5. Nickel Metal Hydride rechargeable battery can be used for the
power supply. TH2821A will not work normally when battery voltage is
less than 6V.
6. The 12V AC to DC adaptor is recommended to be used for TH2821A
power supply.
7. Perform Open and Short corrections for accurate measurement
especially when test fixture is changed.
8. The functions locked with password are not accessible by users.
My biggest problems have been from ignoring rules #2 and #4. I used
to freak out when it didn't do what I thought it should be doing,
but after following the notes, it works fine. And I will add
protection against charged caps, as stated in rule #3. I found that
human error can quickly wipe out a good instrument when you are
working with caps charged to high voltage.
Now I'm wondering if a plain 9V NiCd can be used in place of the
NiMH battery. They are supposed to be interchangeable, but the NiCd
may have a capacity of 120mAh, where the NiMh has 200mAh. So it may
last 3 hours on NiCd, and 5 hours on NiMh. The difference is not
significant for me.
But the NiCd has a much lower discharge rate, so you wouldn't be
running into the issue of leaving it on the shelf for a month, then
trying to use it when the battery is completely discharged. I'm
tempted to get a 9V NiCd and see how it works. If it solves the
self-discharge problem, that's the way to go.
So now I'm happy with the stability of the meter readings on a
quality capacitor, and I won't worry about fluctuation on
electrolytics.
But I'm very unhappy with the self-discharge of NiMh. That may be
solved by switching to a plain NiCd, so it is more likely to have
plenty of charge remaining when I want to use it for some quick
measurements. Afterwards, I can just leave it plugged in and turned
on to bring the battery back up.
I think that covers the issues up to now on this instrument. It is
still by far the best bargain for LCR measurements I have found
anywhere.
Mike