ElCheapo PSU

[autir]

a heavy copper trace on its pcb

Can't I build it on stripboard?  :-\

Its heatsink tab should measure very close to 0V but its heatsink makes poor electrical contact through the thermal grease.

Why very close to and not exactly 0 V? I have not used thermal paste yet.

You shouldn't use an ammeter in series with the load.

I have seen PSUs which had ammeters and voltmeters. How do they do it?
I thought I should install an ammeter in order to monitor the power consumption of my circuits. It is supposed to be connected between the 200uF's "+" and the 330nF's "live" connections, is this the correct location?

The ammeter has a voltage drop which reduces the voltage across the load (and therefore reduces the current through the load).

No offence, but isn't the ammeter's resistance supposed to be too small to produce more than the slightest voltage drop?

 

[audioguru2]

Can't I build it on stripboard?

Use very short spacing between the IC's 0V pin and the 0V of the main filter cap and the load's 0V connection. Solder a heavy wire on the stripboard trace.
Use very short spacing between the IC's output pin and the connection for the load. Solder a heavy wire on the stripboard trace.

Why very close to and not exactly 0 V? I have not used thermal paste yet.

The IC has a small resistance in the metal of the tab and between its ground pin and the tab.

I have seen PSUs which had ammeters and voltmeters. How do they do it?

The current sensing resistor could be inside the regulator's negative feedback loop where its resistance doesn't matter.
Or you can buy cheap LCD meters with 200mV full-scale. You could add an opamp circuit to get as low a voltage drop as you want.

I thought I should install an ammeter in order to monitor the power consumption of my circuits. It is supposed to be connected between the 200uF's "+" and the 330nF's "live" connections, is this the correct location?

It is better between the rectifier and the 200uF (200uF is too small for much output current), then the input voltage of the IC will be more steady when the load changes.

No offence, but isn't the ammeter's resistance supposed to be too small to produce more than the slightest voltage drop?

My cheap DVM is made to measure car battery and appliance currents where a voltage drop doesn't matter. It has 1.25 ohms which drops 0.5V at 400mA.
My good DVM has 0.03 ohms and drops only 12mV at 400mA. Its connecting leads drop more voltage. ;D

 

[autir]

It worked!
I attached the power resistor directly to the breadboard and measured the voltage across its ends. It was 4.87V, dropping as the IC began to heat up and became stabilised at 4.78V.

In the lab I've attended we were used to work with external and/or PSU ammeters + voltmeters, plus a net of cables with banana clips. So much for old habits 

200uF is too small for much output current

Typo. It is 2200uF 

The IC has a small resistance in the metal of the tab and between its ground pin and the tab.

Can I attach many ICs in the same heatsink without problems such as short-circuit? (with all tabs being ground, of cource.)

inside the regulator's negative feedback loop

What/where is that? 

It is better between the rectifier and the 200uF (200uF is too small for much output current), then the input voltage of the IC will be more steady when the load changes.

Won't the analog ammeter's coil be damaged by the unregulated DC passing through?

 

[Ldanielrosa1]

p.s.: Could you explain this:

Quote
(the rectifier current might be 20A pulses)
?

The rectifier is only conducting when the input voltage is higher than the filter capacitor voltage.  This is probably a very low duty cycle so the instantaneous current is much higher than the average current.

 

[audioguru2]

It worked!

Great!

200uF is too small for much output current

Typo. It is 2200uF

 

[autir]

There isn't a problem attaching ICs or transistors without insulators to a heatsink when their cases or tabs have the same voltage. Each one should be wired separately because they make a lousy electrical connection to the heatsink.

What do you mean "wired seperately"?
If there is a resistor an the tab of the TO-220s and the heatsink is soldered to the neutral of the board, won't we have unwanted current flowing through the heatsink?

So you propose that the best place for an ammeter in this circuit is the one in the picture?

View attachment 37550

 

[audioguru2]

What do you mean "wired separately"?

Don't just use a single wire from the heatsink to the common point of the pcb and assuming that their cases and tabs are properly connected together on the heatsink. Solder each one's common terminal to the pcb or use a separate wire from each one to the pcb.
 

If there is a resistor an the tab of the TO-220s and the heatsink is soldered to the neutral of the board, won't we have unwanted current flowing through the heatsink?

You might have a small "ground loop" kind of current in the heatsink but your wires should be thick enough to reduce their voltage drops.

So you propose that the best place for an ammeter in this circuit is the one in the picture?

Correct. ;D

 

[Staigen1]

Hi there

Audioguru, placing the ammeter between the rectifier and the smothing cap just dont feels right, the resitstance of the ammeter reduce the charging current a lot, its only short pulses with a lot of current, compared to the total current going to the regulators. The ammeter should be placed between the 2200 cap and the caps connected to the inputs of the regulators. And use an ammeter with a low voltage dropout, 50 or 100 mV!

//Staigen

 

[audioguru2]

Hi Staigen,
You have a good point! ;D
I agree with you.

 

[Staigen1]

Hi there Audioguru

In an earlier reply you also say that the caps across the rectifier diodes is removed in newer equipment. They are suddenly not, only in cheap suplies! An examle, i bought a pair of speakers to my computer, cheap ones, 160(2*80) whats, they where powered by a 6 VA, 12 Volt wallvart(hehe), and have problem with hum when the volume was turned up, not much, but it was there. I then replaced the wallvart with an another one, 12 VA, 12 Volt, and the problem was gone. I then pryed the old wallvart apart, and found that the caps was not there, it was place for them, and the mounting holes was there, so i mounted 4 caps and the wallvart worked as it should, the hum was gone. Now i use the 12 VA one, i belive the other was a little bit to small for the 4 ohms speakers. Hehe, a 6 VA wallvart for a 160 Watts Amp! Ridiculous! The speakers inside is 5 Watts ones, and i belive that the amp put out less than that, with a decent sound!

//Staigen

 

[audioguru2]

Hi Staigen,
He, he. They should never put Whats ratings on audio equipment. ;D ;D

If the rectifier diodes have a quick reverse recovery rating, then I don't think they would cause mains buzz. The slow ones are still conducting when they should be shut-off. ???

Maybe it is the opposite. The rectifier diodes switch too quickly. Adding capacitors across them smooths their switching speed. ;D

 

[Staigen1]

Hi again Audioguru

I dont think 1N400X diodes are either fast or have a quick recovery rating, but they are good at causing mains buzz and hum and all sort of noice ;D ;D

//Staigen