UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

snowman_al · 14th Feb 2019, 7:15 pm

John Hi,
The statement in post no. 2 is correct.
If you try both set ups with no load you should see very similar results.
226 ac x 1.41 = 319 volts DC at the cap. (Assumes the caps are both the same in value and with no significant leakage of course.) If you do not, there is a problem somewhere.

Simplistically once you put a load on the end the 'resistance' of the transformer winding becomes significant. Just like putting a resistor in series with the load. The more resistance the lower the voltage at the load. So a bigger transformer has less 'resistance' therefore less voltage drop. Also it could have been designed to drop a few volts?

We do not know the relative sizes of the transformers to compare. But what Lucien says is also very important, 1/2 wave rectification only gives you approx. 30% of the AC current capability of the transformer.

The other factor is that 32uF is quite small, so again as Lucien says, the load may be such that the cap cannot charge to 'peak' voltage long enough and you end up measuring nearer the 'average' instead, which is about 90% of the RMS AC voltage. You could try adding another cap to the 32uF you have and see if that keeps the voltage up at all.

Phew, hope that helps a little, Alan

14th Feb 2019, 7:15 pm	#8
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	Re: Peak voltage John Hi, The statement in post no. 2 is correct. If you try both set ups with no load you should see very similar results. 226 ac x 1.41 = 319 volts DC at the cap. (Assumes the caps are both the same in value and with no significant leakage of course.) If you do not, there is a problem somewhere. Simplistically once you put a load on the end the 'resistance' of the transformer winding becomes significant. Just like putting a resistor in series with the load. The more resistance the lower the voltage at the load. So a bigger transformer has less 'resistance' therefore less voltage drop. Also it could have been designed to drop a few volts? We do not know the relative sizes of the transformers to compare. But what Lucien says is also very important, 1/2 wave rectification only gives you approx. 30% of the AC current capability of the transformer. The other factor is that 32uF is quite small, so again as Lucien says, the load may be such that the cap cannot charge to 'peak' voltage long enough and you end up measuring nearer the 'average' instead, which is about 90% of the RMS AC voltage. You could try adding another cap to the 32uF you have and see if that keeps the voltage up at all. Phew, hope that helps a little, Alan __________________ Never Leave Well Enough Alone...