Topward 4303 5V/3A ch supplies 7,5A

[DigitalD]

Hi All, I have this Topward power supply that works well but the 5V/3A supply channel does not limit the current to 3Amps as per the specifications. It gives a lot more current out than 3A, 7.5Amps!

I understand that the load determines what the current draw will be, but I'm interested in getting to the bottom of why this circuit is giving more current than it should. I have a shunt resistor which I used to measure the current with a Fluke 87V.

Attached is an image of this circuit, the LM340T-5 max current it should give according the the datasheet is 1Amp, so I assume the extra 2Amps will have to come via R303. I measured this resistor and got 0.7 Ohms and not 0.2 Ohms, which is what doesn't make sense to me, then the output current shouldn't increase, it should decrease, true? Unless Q302 if faulty, will most likely not route any additional current through the voltage regulator.

Could the more clever guys please help with this? What could be the problem?

[coromonadalix]

if its not 0.2 ohms    yes the current limitter is bad

BUT

q302 is supposed to conduct when  around 0,6 volts is between emittor collector

But i dont see how this circuit is pushing 3 amps  since the lm340 is 1.5 amps max,  normally you must add an power transistor to boost the current,  the schematic seems incomplete or has some errors ??


Check the lm340 datasheet, you'll see what i meant by  boost transistor and current limitting

[lowimpedance]

if its not 0.2 ohms    yes the current limitter is bad

BUT

q302 is supposed to conduct when  around 0,6 volts is between emittor collector

But i dont see how this circuit is pushing 3 amps  since the lm340 is 1.5 amps max,  normally you must add an power transistor to boost the current,  the schematic seems incomplete or has some errors ??


Check the lm340 datasheet, you'll see what i meant by  boost transistor and current limitting

The cct is not complete and there is multiple versions as such !, and I have attached 3 that I have.

@OP what is the unloaded ouptut voltage ? , and then what voltage is accross the load that produces 7.5A ?.

Use the attached ccts to help trace what you actually have in the supply itself.

[floobydust]

There is no current limiting- just a red LED to show overload. The Topward 5V 3A regulator schematic has several errors.
I did these pics and schematic from a TPS4000 but have to dig for it.
It's your basic 7805 with a booster discrete Darlington, the big pass transistor MJ2955 is on the large heatsink.

edit: found my schematic. The 2SA699 tries to shunt base-drive from the big pass transistor if the output current goes too high. I don't think Topward ever got the circuit to work properly, they were changing it all the time.
OP, check again R303 is not 0.2ohms, maybe your multimeter's test leads are adding 0.5 ohms to the reading?
Check that R303 did not go open-circuit and the 2SA699 (TIP31) has shorted E-B. Does the red LED light up at 3A up?

[DigitalD]

@coromonadalix, yes I agree, after reading the posts and seeing the circuits I think my circuit diagram I have is incomplete and incorrect. I should've taken the board out and trace sketch my own circuit diagram. Apologies. I also did not understand how it could give 3Amps...

@lowimpedance, thank you for highlighting that there are multiple versions, I believe I found mine amongst the ones you shared. The unloaded voltage is 5.042V and I used a 150A50mV shunt resistor to determine the max current given, so basically no load voltage. Got a rating of 2.5mV on my 87V, which equates to 7,5A of current in this case.

@floobydust, that image is the supply I have, I haven't traced the circuit myself, I should've! Stupid of me to forget about test lead resistance, they only give me 0,1 Ohms extra resistance. I will check R303 and 2SA699 later this evening and get back to you. Yes the LED does light up.

[DigitalD]

Hi All

Apologies for the delay in my reply, I managed to solve the issue at hand but not exactly to the said specs of the Topward power supply.

Firstly, the power transistor was not isolated from ground, someone who previously worked on the unit must have neglected to put in a Mica insulator.

Secondly, I replaced Q302 with a TIP32C, and didn't touch the 0,2 Ohm resistor. The max current being drawn is almost 4Amps. So the tolerances of the transistor and the resistor most probably is the cause for the difference in current. If it was exactly 0,6V (transistor switching on)/0,2 Ohms one would have exactly 3Amps. But these are not precision components and there is no adjustable point to adjust it accordingly. If the transistor takes 0,7V to switch then that is a 500mA difference for 0,1V change in transistor value to switch on.

Am I thinking correctly here? Do I need to characterise the components to get it to supply exactly 3Amps? With reference to the circuit diagram taht Floobydust posted.

[floobydust]

I measured current-limiting happens around 5.5A. The Overload LED comes on dim at 0.75A and bright at 1A up.