DIY power supply...no output

[tronadave]

Hello all;

I assembled 2 kits; power supplies with 24vac input & variable dc output. Tested OK after assembly, but now when powered with no load the over current diode lights and vdc out -0.66. The 10k pots for volts set at min & amps at max.. have no effect when rotated. The 24vdc fan works OK.

Input is from a Honeywell 120vac to 24vac 40va transformer

47ohm resistor goes to 30degC (in 2 min)

Nothing else heats up over 26deg C

I may have powered them up at one time with the 2 control pots out of the circuit, but I’m not sure.

The 3 op-amps are TL081CP, all other components are listed on the inst. & a wiring dia



I am an old retired geezer with just enough knowledge to get into trouble.

I thank you all in advance………….tronadave

 

[Tha fios agaibh]

Specs say 24vac max.
I'm thinking your "24v" Honeywell transformer might have exceed that. ?

 

[davenn]

Input is from a Honeywell 120vac to 24vac 40va transformer

Specs say 24vac max.
I'm thinking your "24v" Honeywell transformer might have exceed that. ?

agreed, definitely could be a problem .... you should have aimed for a 18 - 20VAC output on the transformer

47ohm resistor goes to 30degC (in 2 min)

not sure if that was a typo ? ... there is no 47 Ohm resistor .... it is 0.47 Ohm
if you are referring to R7 on the negative rail output ?

 

[tronadave]

Yes, sorry, it is 0.47ohm...... The 24vac transformer puts out 27v open circuit. If this is too much, can I put my variac on the input of the transformer & drop it down a few volts?

 

[duke37]

Yes, sorry, it is 0.47ohm...... The 24vac transformer puts out 27v open circuit. If this is too much, can I put my variac on the input of the transformer & drop it down a few volts?

Yes, that is what a Variac is for.

 

[Tha fios agaibh]

Sound like something is shorted. I'd suspect a diode somewhere.

 

[Audioguru]

The kit is a Chinese copy of an old Greek kit that is a project at www.electronics-lab.com. The project has many overloaded parts and cannot produce 30VDC at 3A. The overloaded parts fail soon then the project is dead. Even if the transformer does not produce more than 24VAC with no load then the max allowed supply voltage for the old opamps is exceeded.

I helped fix it by using better higher voltage opamps, a higher voltage and current transformer, two output transistors to share the heat, calibration trimpots and higher power ratings on many of the resistors.

This Chinese kit is now sold at a clearance price because they have an improved one.