Another PSU question

[seventenths]

While in the process of building a suitable enclosure for my first gainclone, I started picking up parts for number 2... this may get ugly
Anyway, I bought a 48VCT(24+24)/ 500VA tranny, carried it home and powered it up. I put a rectifier across one secondary and the center tap(potted type) and grabbed my meter expecting to see approx- 33-34 VDC . Instead I get a reading of 23V. I then did a quick search of this forum and saw in a post that I should load the output with a resistor (10K suggested). I did this... still 23V.
Am I missing something?


7/10

[BWRX]

Is your meter set to DC voltage? Since you have a center tapped transformer you want to measure the voltage between the center tap (ground) and each rail after the rectifier.

[seventenths]

Thank you for the reply...
Yes, my meter is set to DC.
Currently with a 10K resistor across the rectifier +/- connections I get a 23VDC reading (at +/- of rectifier). Set to AC and measuring either sec to center tap, I get 26.4 VAC.... 53.4 VAC from Sec to Sec.
I keep reading that your DC voltage should be 1.41 (?) times the rated VAC, but both of my PSU setups measure a DC voltage roughly equiv to the rated AC output of the XFMR.
My wavetek and my fluke concur...mostly


Thanks
7/10

EDIT* to add; I grabbed my el-cheapo analog meter and it too agrees... 23VDC

[jaybombalous]

so whats the voltage of just the secondaries on the tran.? Maybe u are getting 1.41 times as much voltage, you just haven't realized it yet. Maybe then again not and your problem lies else where.

[FastEddy]

Try your rectifier across the full secondary ... your primary may be 220 VAC input and the secondary output is relative to that ...

[sherelec]

seventenths,

You are getting the correct answer for your test setup. I am assuming your rectifier is a bridge so the waveform is a fullwave rectified sinewave.

The peak voltage is Vrms*1.414-2*Vd = 26.4V*1.414-1.2V = 36.1V
The Meter on DC will measure the average DC voltage.
Vavg = Vpk*2/Pi = 36.5*2/3.1416 = 23.0V

Close enough?

If you want to see what you were looking for you need to add a filter capacitor across the bridge output.

I hope this is helpful.
VSR

PS: my original post only assumed one diode. By using two at Vd=0.6V the answer comes out to almost exactly 23.0V

[hughmon]

Do you have some filter caps in place?? If not the DC range on the meter will read funny (as it tries to average the pulsating DC out of the diodes).

[FastEddy]

sherelec / hugemon: right, right, right ... you have to add some capacitance to the rectifiers to get the peak (stored) DC voltage reading ...

[seventenths]

I hope this is helpful.

Only a whole bunch That is exactly what I needed. Thank you all for taking the time to respond. Onward... oh whats that smell... could it be the sizzling 35V cap? Oh yeah some DA got in a rush and didn't reconfigure to the center tap after testing sec-sec


7/10

[BWRX]

Originally posted by hughmon
Do you have some filter caps in place?

I just assumed seventenths was using filter caps after rectifiers since this is a power supply thread It's always the details!