Connect with us

First project - stumped

Discussion in 'Electronic Basics' started by H. Dixon, Jul 23, 2008.

Scroll to continue with content
  1. H. Dixon

    H. Dixon Guest

    I'm using Slone's Tab book to learn the basics. It's pretty good so
    far although once in a while the book takes bigger leaps so I stumble
    a bit.

    What I've got so far is the the two 24V transformers with the
    primaries in parallel, the secondaries in series. I've tested the
    outputs from each secondary (about 28V each) and the secondary circuit
    common reads 50V. I've added a 250V 6A bridge rectifier and attached
    the free secondary leads from each transformer to the AC inputs on the
    bridge. The bridge shows 50V DC out.

    The book has me (temporarily) add two 10K resistors - one between the
    secondary circuit common and the bridge positive lead, and another
    from the secondary circuit common and the bridge negative lead.

    Testing DC between secondary circuit common and the + bridge lead
    shows +24V.
    Testing DC between secondary circuit common and the - bridge lead
    shows -24V.
    This agrees with the book. His next two tests don't work for me.

    Testing" AC" between secondary circuit common and the + bridge lead
    shows about 50V.
    Testing" AC" between secondary circuit common and the - bridge lead
    shows about 0V.
    His book indicates I should get about 12V for each. That I don't
    understand. Something just
    doesnt seem to read right here.

    I've rechecked what I've done with the schematic and everything looks
    good. All tests have agreed with the book except for these last two.

    Any advice much appreciated.

    H. Dixon
     
  2. If you are using one of the el-cheapo $3 Harbor Freight type meters, be
    advised that they read AC simply by using a single diode to half-wave
    rectify the signal. For an ordinary sine wave, it works pretty well, but
    any DC component will throw off the readings "big-time". It is best to get
    a true-RMS meter, but at least get one that uses a precision full-wave
    rectifier circuit. Any meter that has low AC volts and AC millivolts should
    work OK. But there are decent true RMS meters available on eBay and
    discount places for as low as $60 or so.
    http://cgi.ebay.com/Mastech-PC-RS23...photoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

    An acceptable meter (not true-RMS) can be had for about $15:
    http://cgi.ebay.com/Mastech-32-Rang...photoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

    Even for about $13 you can get a 4 digit meter that has autoranging,
    capacitance, frequency and duty cycle:
    http://cgi.ebay.com/Sinometer-Auto-...photoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

    I have ordered meters from Multimeter Depot before and have been quite
    satisfied.

    Paul
     
  3. H. Dixon

    H. Dixon Guest

    Thanks - I hadnt thought it might be the DMM. Its an old one I cant
    even remember where/when I got it.
    I see the radio shack <gasp> around the corner has an Extech True RMS
    DMM model 22-816. Brief googling seems to show Extech
    as acceptable in quality. It's 89$ but seems to have a lot of bells
    and whistles.

    Maybe I'll pick that up tomorrow (unless someone screams NO).

    Thanks

    H. Dixon
     
  4. neon

    neon

    1,325
    0
    Oct 21, 2006
    Nothing wrong with the cheap old meter you just don't understand that is all
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-