First project - stumped

[H. Dixon]

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

 

[H. Dixon]

On Jul 23, 6:21 pm, "Paul E. Schoen" <pst...@smart.net> wrote:

"H. Dixon" <huwdi...@hotmail.com> wrote in message

news:c355097c-162f-4a04-b0b1-7c38202cae07@26g2000hsk.googlegroups.com...



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

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-RS232-Interface-True-RMS-Digital-Multi...

An acceptable meter (not true-RMS) can be had for about $15:http://cgi.ebay.com/Mastech-32-Range-Digital-Multimeter-Diode-Continu...

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-8-Function-Digital-Multimeter-Volt...

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

Paul

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