I built a linear supply for a Pass ACA amp. I don't understand the rectified voltage. It is much lower than I expected.
I attached the schematic.
My question is why is the voltage after the bridge rectifier 26.5v instead of 31v? My experience is that voltage would be [(22.2VAC x 1.4) - 0.6v].
BTW, the filtering seems to work pretty good.
Thanks,
Dan
I attached the schematic.
My question is why is the voltage after the bridge rectifier 26.5v instead of 31v? My experience is that voltage would be [(22.2VAC x 1.4) - 0.6v].
BTW, the filtering seems to work pretty good.
Thanks,
Dan
Wrong ground connection on the schematic above.
Disconnect the ground from the secondary winding.
Instead connect the ground to the rectifier, at the left corner of the square.
Also connect all 3 capacitor lower leads to ground, along with the load.
Post the corrected schematic here before you change the actual circuit.
If your circuit is already built correctly, and the schematic is just wrong,
lower than expected voltage can be due to many things.
Load too heavy for the transformer. Low AC line voltage. Too small first filter capacitor
What is the transformer secondary current rating? Should be a couple of amps or more.
What is the transformer VA rating? Should be 50VA or 75VA.
The series resistor before the last capacitor is too large.
It drops 4V at 1.4A, and so is probably about 3 ohms.
Instead use a smaller value, 1 ohm or less.
In fact you may be better off with an even smaller value, say 0.22 ohms or so.
That would also waste less power.
Disconnect the ground from the secondary winding.
Instead connect the ground to the rectifier, at the left corner of the square.
Also connect all 3 capacitor lower leads to ground, along with the load.
Post the corrected schematic here before you change the actual circuit.
If your circuit is already built correctly, and the schematic is just wrong,
lower than expected voltage can be due to many things.
Load too heavy for the transformer. Low AC line voltage. Too small first filter capacitor
What is the transformer secondary current rating? Should be a couple of amps or more.
What is the transformer VA rating? Should be 50VA or 75VA.
The series resistor before the last capacitor is too large.
It drops 4V at 1.4A, and so is probably about 3 ohms.
Instead use a smaller value, 1 ohm or less.
In fact you may be better off with an even smaller value, say 0.22 ohms or so.
That would also waste less power.
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Yep, corrected schematic attached.
Line voltage is 120VAC as expected.
10,000uF 35v. Could this be the issue?
20v @ 5A. It has a 100watt rating. Under load it is measuring 22.2VAC.
My question is why am I seeing 26.5v after the bridge rectifier?
Thanks,
Dan
I built two identical power supplies and they both do the same thing. I can measure the first cap, but do I have to remove it from the circuit to do that?
Thanks,
Dan
First, Is the bridge rectifier a big 35A block, or one of those DIP things? That wouldn't work.
If the rectifier is not a DIP, then disconnect one end of the input capacitor and measure it.
It should be ok, but you never know for sure without measuring it.
If you have a scope, look at the voltage ripple on the first cap. Is it at 120Hz or 60Hz?
If the ripple is at 60Hz, the rectifier (or a connection) is bad.
If the cap is ok, make up a dummy load with a suitable power resistor so you are certain of the load current.
Then measure the voltages again.
I agree that the DC voltage on the first cap should be around 30VDC.
Something is not right, and is the same in both supplies. Focus on the ripple on the first cap.
If the rectifier is not a DIP, then disconnect one end of the input capacitor and measure it.
It should be ok, but you never know for sure without measuring it.
If you have a scope, look at the voltage ripple on the first cap. Is it at 120Hz or 60Hz?
If the ripple is at 60Hz, the rectifier (or a connection) is bad.
If the cap is ok, make up a dummy load with a suitable power resistor so you are certain of the load current.
Then measure the voltages again.
I agree that the DC voltage on the first cap should be around 30VDC.
Something is not right, and is the same in both supplies. Focus on the ripple on the first cap.
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The coil is 1 Ohm. You loose 1.4V over the coil. You loose 4.09V over the resistor. That is 5.5V. 5.5 + 21 = 26.5V so that is as calculated and in reality. High impedance PSU too with strong tube amp PSU resemblance and that worked OK with relatively low current. This is a class A amplifier though with 1.4A current.
Maybe the transformers and/or the main filter caps 10,000 µF are not high quality? Or are the Schottky diodes becoming too hot giving higher than normal Uf? They are relatively light rated for the purpose.
All far from elegant designing to be honest. An LDO regulator design will give better results. In all cases with unregulated or regulated LPS you will lose energy to useless heat much more than with an SMPS. That comes with class A, therefor the SMPS was chosen.
Maybe the transformers and/or the main filter caps 10,000 µF are not high quality? Or are the Schottky diodes becoming too hot giving higher than normal Uf? They are relatively light rated for the purpose.
All far from elegant designing to be honest. An LDO regulator design will give better results. In all cases with unregulated or regulated LPS you will lose energy to useless heat much more than with an SMPS. That comes with class A, therefor the SMPS was chosen.
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Yes, strange. Same voltages on both power supplies.
If a part was broken, then they wouldn't be the same voltages. Really odd.
Ok, with the power off, try adding another similar large cap in parallel with the first cap.
What happens to the DC voltage on the first capacitor? By the way, where did you get those caps?
The load current appears to be correct, since the voltages on the 3 ohm resistor are consistent.
Well thanks for the tough love!
I've never used an LDO. I actually had to google it. My PSU design starts and ends with Morgan Jones' "Valve Amplifiers."
I'm really just wondering why my voltage after the rectifier bridge is only 26.5v when I expect it would be about 30v.
I do think the caps are good quality:
I didn't want to drop so much voltage. I just wanted to try CLCRC filtering on Nelson Pass' ACA class A amp. A Meanwell wall wart is recommended for that project.
But I made some mistakes along the way.
First being, I thought each channel draws 1A, but it draws more like 1.5A. So I designed my inelegant PS expecting 2, not 3 amps.
Then the transformer I bought is spec'd at 20VAC CT, but it is measuring 22VAC with the load. So I have more voltage than I expected.
Also I bought a choke rated at 2A, but the circuit draws 3A. (I can go on!)
I think my biggest error was getting a choke rated at 35mH, 2A and 0.79ohms (but measures 1 ohm), when I should have gotten lower mH and lower DCR.
This is what I wanted to get 24v per channel, but based on 1A per channel:
oh well.
Dan
Attachments
Normally I am not too eager on advocating the use of SMPS but in this specific case (power amplifier, class A) I think the SMPS with the necessary filter is really is a better choice when having 2 channels drawing 1.5A per channel continuously.
Some considerations:
1. With 3A for 2 channels the diodes definitely are chosen too light!!! They also have to charge the filter cap. I think you will need one of those chunky chassis mounted 35A rated bridges. Also for correct cooling.
2. The chosen filter caps are good quality. However, they are not of the snap in type that are the right type for this purpose with regards to allowable ripple.
Some considerations:
1. With 3A for 2 channels the diodes definitely are chosen too light!!! They also have to charge the filter cap. I think you will need one of those chunky chassis mounted 35A rated bridges. Also for correct cooling.
2. The chosen filter caps are good quality. However, they are not of the snap in type that are the right type for this purpose with regards to allowable ripple.
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The rectifier diodes all measure 120-123F after being on for about 5 minutes.
I am making all measurements connected to the ACA amplifier.
I derived the load using the measured resistor values.
I've used the Meanwell LRS-100-24 SMPS on these ACA amps before with great results - I just bolt it in the chassis. (I was just curious about a linear supply.)
I don't have any suitable caps around to add to the first cap.
I figured Nelson Pass used the SMPS sold on the DIY sight to eliminate the need to have to deal with the mains. I thought it was a compromise so no legal liability.
I think this experiment is officially over.
Thanks everyone for your comments and suggestions.
Dan
I am making all measurements connected to the ACA amplifier.
I derived the load using the measured resistor values.
I've used the Meanwell LRS-100-24 SMPS on these ACA amps before with great results - I just bolt it in the chassis. (I was just curious about a linear supply.)
I don't have any suitable caps around to add to the first cap.
I figured Nelson Pass used the SMPS sold on the DIY sight to eliminate the need to have to deal with the mains. I thought it was a compromise so no legal liability.
I think this experiment is officially over.
Thanks everyone for your comments and suggestions.
Dan