chief
National Hazard
Posts: 630
Registered: 19-7-2007
Member Is Offline
Mood: No Mood
|
|
Strange rectifier behaviour
Can anyone help me on this ? :
I constructed myself a
==> full-wave, center-tapped rectifier,
==> out of 8 Schottky-diodes, 4*4 parallel
==> circuit as in the second picture of http://eece.ksu.edu/~starret/589/man/Efigs.html
The thing is: My clamp-amperemeter reads also an AC-current (40 A), besides the DC-current (80 A) ... ; I never made such a center-tapped circuit
before, but it looks right; first I thought the diodes might be bad, but isolating them and measuring them with the diode-test-voltmeter gave 0.2 V
forward-voltage, and OK reversely (no short) ; so the diodes seem to be right ...
Also: I measured the frequency: With the raw AC (as it should be) it is 50 Hz, and at DC it's 100 HZ, just as expected ...
The diodes are all such ones: http://www.datasheetcatalog.org/datasheet/irf/mbr2535ct.pdf
The max applied voltage in my circuit is 15 V (diodes rated for 45 V), each side of the center tap. The sum of the current is in 1/2 range of
theoretical capabilities ; the diodes don't get too warm (and as I said: After a while of operation they still measured good, all in parallel)
I completely don't know whats wrong, found nothing in the web: Where comes the AC-reading of half the DC-reading from ??? It shouldn't be there ...
The voltages (at the supposed-to-be DC termianals) read: 11.9 V DC, 4..5 V AC ... .
So: Somehow it is consistent with itself: Half the AC-voltage, half the AC-current, double the DC frequency, all at the high currents (80 A DC, 40 A
AC, no phantom measurements at these currents !), the diodes all measure good, don't get too warm ...
[Edited on 14-2-2009 by chief]
|
|
|
chief
National Hazard
Posts: 630
Registered: 19-7-2007
Member Is Offline
Mood: No Mood
|
|
Also I might add: The transformer is quite a powerful one: In case these diodes would be short, at the 30 V total, it would provide several hundereds
A ; definately even the primary main fuse would blow (known from experience), so either the diodes wouldn't survive with the OK test reding, or would
just fly around the environment in parts (protective glasses of course always on with this ...) ...
|
|
|
watson.fawkes
International Hazard
Posts: 2793
Registered: 16-8-2008
Member Is Offline
Mood: No Mood
|
|
| Quote: | Originally posted by chief
I completely don't know whats wrong, found nothing in the web: Where comes the AC-reading of half the DC-reading from ??? It shouldn't be there ...
|
You don't have a problem with the circuit; you have a problem understanding it. When you hook up a meter in
DC mode it will measure (more or less) (usually), a time average of the measured parameter. In AC mode, it will measure (more or less) (usually less
for a cheap meter), the RMS (root-mean-square) about the average. If you look at the output of the circuit (from the same page) you'll see the output
is not, in fact, direct current, but a function (approximately) abs(sin(t)). DC, as a function, is a constant function, f(t)=c. What you're observing
is that your energy is being partitioned in some ratio between DC and AC, about 3*sqrt(3). Without doing the calculus, I don't know if this is
correct, but it's an elementary, first-year kind of problem.
|
|
|
not_important
International Hazard
Posts: 3873
Registered: 21-7-2006
Member Is Offline
Mood: No Mood
|
|
| Quote: | Originally posted by chief
Can anyone help me on this ? :
I constructed myself a
==> full-wave, center-tapped rectifier,
==> out of 8 Schottky-diodes, 4*4 parallel
... |
Another issue is that paralleling diodes with using a low value resistor in series with each diode, unless the diodes are matched for I/V curve, is
likely to result in current hogging by the diode with the lowest drop. This can result in overcurrenting that diode, possibling destroying it.
|
|
|
chief
National Hazard
Posts: 630
Registered: 19-7-2007
Member Is Offline
Mood: No Mood
|
|
@watson: I thought of that, of course ; only I also have the impression, that with my former bridge-fullwave-rectifiers the meter did not display any
AC-content, neither of the voltage, nor of the current ... ... ; it's a not-so cheap voltcraft-clamp-Amperemter, measuring the magnetic field and
thereby finding the current without direct contact tot the wires ...
The meter is such a one ( Voltcraft VC 605) :
http://www.preissuchmaschine.de/trend/VOLT/Voltcraft-Stromza...
edit: sorry, those links don't work from this editor , ...
http://shop.cxtreme.de/scripts/wgate/zcxtreme/~flNlc3Npb249UDkwX0RFX1NJUzpDX0FHQVRFMTc6MDAwMC4wMTE1LmY3ZWQ4NDQ2Jn5odHRwX2NvbnRlbnRfY2hhcnNldD1pc28tODg
1OS0xJn5TdGF0ZT0yODAyOTg2MDU=?~template=PCAT_AREA_S_BROWSE&glb_user_js=Y&shop=CXE&zhmmh_lfo=&zhmmh_area_kz=&product_show_id=120593
&gvlon=&p_init_ipc=X&p_page_to_display=fromoutside&~cookies=&cookie_n[1]=cxe_hk_cookie&cookie_v[1]=CXT&cookie_d[1]=&co
okie_p[1]=%2f&cookie_e[1]=Thu%2c+19-Mar-2009+12%3a44%3a05+GMT&scrwidth=1280
http://www.mercateo.com/p/102-120167(2d)BP/STROMZANGE_VC_605.html
[Edited on 15-2-2009 by chief]
[Edited on 15-2-2009 by chief]
|
|
|
Eclectic
National Hazard
Posts: 899
Registered: 14-11-2004
Member Is Offline
Mood: Obsessive
|
|
You have no capacitor, so you are seeing the AC current waveform superimposed on the DC.
|
|
|
Texium
|
Thread Moved
19-11-2023 at 12:53 |
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
