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Old 23rd April 2004, 02:43 PM   #1
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Default psu problem

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
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Old 23rd April 2004, 03:14 PM   #2
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Hi

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


Federico
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Old 24th April 2004, 09:29 AM   #3
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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
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Old 24th April 2004, 11:50 AM   #4
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Hi

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



Red
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Old 24th April 2004, 12:09 PM   #5
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The loading resistors are 217Kohm and 215Kohm.

MB
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Old 24th April 2004, 12:24 PM   #6
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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
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Old 24th April 2004, 01:49 PM   #7
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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
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Old 24th April 2004, 01:54 PM   #8
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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
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Old 24th April 2004, 02:48 PM   #9
MBK is offline MBK  Singapore
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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 )
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Old 24th April 2004, 08:52 PM   #10
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> 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
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