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0-30 Vdc Stabilized Power Supply

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Hi Rufinus,
What value and where in the circuit is the fuse that blew?

Your current regulator is not working properly. It is supposed to sense the voltage across R7 caused by load current and reduce the output voltage so that the load current is the same as the setting of the current pot. U3 is the current reguator and its output reduces the voltage of the output stage through D9.
With the current pot at half rotation the output current should be regulated and limited to only about 1.7A.

Your voltage from pin 6 of U2 does not go high enough up to about 34V so the project's output cannot go up to 30V. Is your transformer only 24VAC?

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Hi audioguru. Thank you very much for your reply.
Fuse is inserted straight after secondary winding before rectifier in order to protect the winding. It had a value of 2.5 amps and before everything was working excellent.
I do have a 24v transformer and for building this psu i used your 24v mod parts list.
So, do you suggest that D9 or U3 is dead?

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Hi Rufinus,
The current regulator has a max current output of about 3.4V if the value of the pot is a little high. You should increase the value of R18 to reduce the max output current to match the value of your fuse.

Test the current regulator by turning down the current pot to about half, turning down the voltage pot completely, adding a load to the project then measuring the output voltage. As the voltage pot's setting is increased then a voltage will be reached where the current regulator will stop the output voltage from rising and the LED will light.

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With the load of 1 ohm voltage stays at 0, it can not be regulated. Current pot is in the middle. LED is off.
With the load of 680 ohm voltage can be increased to 17.6v and then stops, but LED is still off. Current pot is in the middle.
LED goes on only when current pot is completely down.
Both loads are cold. It looks as there is no current basically  ???

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Here are the voltages measured at U3 with the current pot half way, load on (680ohm):
pins:
1 - -5.6
2 - 0v
3 - 1.48v (can be regulated with the current pot 0-3v)
4 - -5.6v
5  - -5.6v
6 - 31v
7 - 31,8v

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A 680 ohm load causes only 37mA if the output voltage is 25V. The current pot would need to be turned up to less than only 1% of its rotation for the circuit to begin regulating the cuurent.

Use a much lower value for a test load resistor to make enough current for you to see if the current regulator works.

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With the load of 1 ohm voltage stays at 0, it can not be regulated. Current pot is in the middle. LED is off. 
Voltage pot does not change anything. Current pot also does not change anything.
There is no current regulation. Ammeter in the series with the load (1ohm) shows 0.03amps independent of current or voltage pot adjustment.
Load resistor stays cold.

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Hi Rufinus,
The LED is off so U3 is not reducing the output voltage, something else is.
Maybe Q2 is destroyed by the very high current you had when the fuse blew.
1 ohm is much too low for testing. If the output voltage is 25V then it would try to draw 25A!
Try 12 ohms to 20 ohms. If the current regulator doesn't work then the max current will be about only 2A.

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Tried with 20 ohm. I did set voltage pot to 25v and current pot to half-way. When i connect the load, voltage drops to 0.6v and current is about 0.03-0.04 amps.
Current and voltage regulation pots do nothing then.

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Hi Rufinus,
With the 20 ohms load connected, the voltage pot set to 25V and the current pot to half-way:
1) Measure the output voltage (pin 6) of U3. It should be high which is about +31.8V in your circuit.
2) Measure the input voltage (pin 3) of U2. It should be about +8.1V.
3) Measure the output voltage of U2. It should be about +26.4V or higher.
4) Measure the base voltages of the output transistors. They should be about +25.5V or higher.

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D10 was shorted.
Many thanks goes to audioguru, the man, who cares.
Respect.

Now i wont connect ammeter between emitter of Q4 and output. I will better use voltmeter to measure across R7.
I did build one based on 7107 and it works fine (as a panel meter for this project). Now i have one more, also based on 7107, configured for 200mV full range and with a voltage divider at the input to have full scale of 20.00v. I guess, i can use it as ammeter if i measure voltage drop across R7?

Another issue is grounding. Here in Germany we have a grounding pins on the plugs. Should i connect the common of the psu to the ground, or just leave it floating and ground only the enclosure?

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Hi Rufinus,
Good, you fixed it. ;D
Now you can measure its performance. The voltage regulation is much better if you do not connect a current meter to its output. Measuring the voltage across R7 is a very good way to measure the current used by a load.

All my test equipment are not grounded. They all have "approved" power transformers.

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I building this circuit on a multi-purpose board. Kinda confused about RV1. The middle pin of RV1 is connected to R1. Other 2 pins to which pins of U2??? I think one is connected to pin 6 (o/p of U2) but the other????

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I building this circuit on a multi-purpose board. Kinda confused about RV1. The middle pin of RV1 is connected to R1. Other 2 pins to which pins of U2??? I think one is connected to pin 6 (o/p of U2) but the other????


End pins of RV1 are connected to pins 1 and 5.

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Subodhthok,
You are correct. The schematic does not show the pin numbers and the text in the project is not clear in using the terms "Offset Null" to describe this connection.
To understand more about what RV1 is doing, you can perform an Internet  search for "op-amp and Offset Null". You will find some pretty descriptive pages.
Hope that helps.

MP

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