psu problem

[metebalci]

Hi,

I have built the circuit below. V1 is our main line (220Vrms). T1 is a step-up power transformer which has a primary 0-220V, secondary 0-300V. C1 and C2 is rated at 450V.

Since 300Vrms = 420Vpk-pk, I am expecting to measure 420V between (+) terminal of C1 and ground. However, DMM displays 393Vdc. Why? Is it because of ripple (but ripple must be low) ?

Thanks in advance...

MB

 

[fscarpa58]

Hi

Maybe your transformer has an equivalent primary resistance of
2300 Ohm.
Try measure DC resistance of primary and secondary.


Federico

 

[metebalci]

No it does not.

Primary DC Resistance = 11.4 ohm
Secondary DC Resistance = 16.2 ohm

I have measured secondary rms voltage, that is around 300V, which is expected. However, voltage between (+) terminal of C1 and ground is 393Vdc which is expexted to be 420V.

Any other idea?

MB

 

[redwine1118]

Hi

Double check the loading resistor 220K ohm see if they are the correct value.



Red

 

[metebalci]

The loading resistors are 217Kohm and 215Kohm.

MB

 

[redwine1118]

Hi,

1) Change the loading resistor to 300K above, see if there is any increase in the output voltage. For sure , it will. But measure the output voltage by DMM. Monitor the difference.

2) Does the 2 pcs. of reservoir capacitor a new one?

3) If not a new one, if you have some extra capacitors, try to change it and re-measure the output voltage.

RED

 

[metebalci]

1) I dont have much resistor in stock, I will try it next week.
2) Yes the caps are new. They are Kendeil 05.

Can caps quality cause such a problem ?

MB

 

[redwine1118]

Hi,

The assumption I made is to find out the real problem of you psu

If the electrolytic is an old one, the chemical inside maybe dry out and affect the filtering of the AC ripple, so you cannot get the 420V output.

As the cct. is so simple, input is ok, output is abnormal, so the problem should be either the resistor or the reservoir cap.

Rgds,
Red

 

[MBK]

Is your secondary truly at 300Vrms? Many DMM's don't display RMS voltages accurately. The ones that do are sold as "true RMS DMM's" and more expensive .

Besides, with your small caps you likely have substantial ripple left. S ay, a 10% remaining ripple would mean that the actual DC voltage is lower than the potential one you could get with larger caps, the DMM of course ignoring the remaining AC peaks. 420-393=27V, that's less than 10%.

Both ways I wouldn't worry as long as V+ and V(-) are the same in absolute value. Reality is probably just fine.

(Occam's razor: the simplest explanation is usually the correct one )

 

[metebalci]

> Is your secondary truly at 300Vrms? Many DMM's don't display
> RMS voltages accurately. The ones that do are sold as "true
> RMS DMM's" and more expensive

I must certainly get a True RMS DMM. However, if DMM is measuring the main line voltage, I think it also measures transformer secondary correctly, because there is no significant DC component and it also measures same value without load.

> Besides, with your small caps you likely have substantial ripple
> left. S ay, a 10% remaining ripple would mean that the actual
> DC voltage is lower than the potential one you could get with
> larger caps, the DMM of course ignoring the remaining AC
> peaks. 420-393=27V, that's less than 10%.

If I calculated correctly, there must be a little ripple. However, I will check it twice or more.

Actually I have seen this problem when the following regulation stage does refuse to regulate because of less input voltage than expected. I must find a way to overcome this.

Thanks for advices.

PS: What does True RMS DMM measure for a Vdc + Vac potential, -I have seen different definitions for this- ?

MB

 

[EC8010]

PS: What does True RMS DMM measure for a Vdc + Vac potential, I have seen different definitions for this?

The only definition of RMS is that it is the DC amplitude (current or voltage) that would give the same heating effect as the AC waveform. It is calculated by taking the Root of the Mean of the Squares. At RF, it is measured using a thermocouple to measure heating effect.

In most cases, you don't need a true-RMS DVM. The only time you need one is if you are concerned about heating effect and your waveform is distorted. We don't like distortion in audio, so a cheap and cheerful mean-sensing calibrated RMS of sine wave meter will do perfectly well. That is, any old £15 junk.

My true-RMS meter (I wanted a 0.025% accuracy meter, and true-RMS was included) can display the AC and DC components independently, which is occasionally useful.

 

[metebalci]

Thanks so much...

MB