shitty chinese psu kit

[m3vuv]

take a look at the pic,its marked thermister but looks like a zener.

[MK14]

You were right about the title of this thread. It does seem to have quality control issues.

They should call it, power supply kit with built in, set of 20 complicated technical puzzles to solve. We have randomly changed component names, values and moved things around to create a wonderful electronics problem solving arena.

Anyway, I can see why people don't think too highly of these Chinese 'bargain' kits.

[MK14]

Anyway, it looks like a small signal diode (1N4148) or zener, not a thermistor. As someone else mentioned, they can be cased like that, but that is extremely unlikely, in this case.

Rarely diodes can be used as temperature sensors (because their voltage drop, changes somewhat consistently), so it could be temperature related. Another great puzzle to solve ?

[magic]

It looks like a thermistor. If it measures equal resistance regardless of polarity (near 10kΩ is probably the most common value) and decreases fairly fast with temperature, it's a thermistor.

[m3vuv]

well it measures about 6k both directions at room temp and drops to 100 ohms about if heated in a flame,its a lot bigger/thicker than a regular 1n4148!

[magic]

Not sure if I would want put one in flame, but those things can indeed take more heat than silicon.
Similar ones are found in some kitchen thermometers going up to 300°C, for example.

Schematic and board:
https://ae01.alicdn.com/kf/HTB1zsMyXPDuK1Rjy1zjq6zraFXao.jpg_Q90.jpg_.webp
https://ae01.alicdn.com/kf/HTB1atZ0XhrvK1RjSszeq6yObFXag.jpg_Q90.jpg_.webp

[MK14]

well it measures about 6k both directions at room temp and drops to 100 ohms about if heated in a flame,its a lot bigger/thicker than a regular 1n4148!

Thermistor then.

[MK14]

Labeled NTC (Negative Temperature coefficient), for the fan control, and VR5 adjusts the setting. Makes sense.
NTC = Resistance drops, somewhat dramatically as the temperature increases. So useful for fan control.

[m3vuv]

what a pile of shite!,well i hooked this up,firstly one of the caps exploded!,replaced all with higher voltage items,2nd try,it seemed to work,next thing i heard a crack and lost the display but still had output,the lm7812 smoked and got red hot,i just cut the leads too it and powered up the psu to find the input to the lm7812 has 50 odd volts on it,can anyone shed light on this?,im sure the lm 78 series regs spec a max input of 37v?,Tia.

[MK14]

what a pile of shite!,well i hooked this up,firstly one of the caps exploded!,replaced all with higher voltage items,2nd try,it seemed to work,next thing i heard a crack and lost the display but still had output,the lm7812 smoked and got red hot,i just cut the leads too it and powered up the psu to find the input to the lm7812 has 50 odd volts on it,can anyone shed light on this?,im sure the lm 78 series regs spec a max input of 37v?,Tia.

That's hilarious (sorry  ).

Would you ever trust it to be left on (unattended), and not attempt to set the place on fire ?

Or the output, when set to perhaps 5V, for it to suddenly go to 50V, and blow up the expensive/rare thing(s), connected to the output terminals.

On the other hand, I suppose you can eventually fix all the problems/issues. But even then, the overvoltaging and overloading, may have already wreaked havoc, on the remaining components.

[MK14]

the lm7812 has 50 odd volts on it,can anyone shed light on this?,im sure the lm 78 series regs spec a max input of 37v?,Tia.

If I've understood what you have said, it seems it should be coming from a rectified and smoothed, 15V AC tap, from the transformer. So, presumably, something has gone wrong with the transformer wiring, associated relays, or something around that area. Which may well explain why all those components, exploded, overheated, went pop and fizzled, etc.
Since it is a PCB, it shouldn't be able to be badly wired up as such (Correction: Assuming the external transformer wires, are connected correctly to the screw block connector). On the other hand, possibly poor instructions, and other issues with the kit, could explain it.

[m3vuv]

Well according to the paperwork,i need a taps of relative to 0v of 15v then 25v then 35v,i was just using 13v instead of 15v and a 36v tap,am i right in thinking to get the rectified smoothed voltage from the transformer secondary raw ac, i need to times it by 1.414?

[m3vuv]

if thats so it comes out at 49v using a 35v tap,that seems over the top for 78 series regulators!,am i correct?

[MK14]

am i right in thinking to get the rectified smoothed voltage from the transformer secondary raw ac, i need to times it by 1.414?

Yes, it is. There is a slight increase (perhaps 5% or 10% or so, depending on transformer specifications, usually bigger transformer = smaller percentage, called transformer voltage regulation) if the transformer is very lightly loaded on that secondary. You will lose around 1V or 2V, when a diode (or bridge rectifier), has lots of current. There are other more minor effects, if you really read up on transformers.

So basically, yes, multiply by root 2 (1.414 ..).

[MK14]

if thats so it comes out at 49v using a 35v tap,that seems over the top for 78 series regulators!,am i correct?

Well a quick look at the schematic:
https://ae01.alicdn.com/kf/HTB1atZ0XhrvK1RjSszeq6yObFXag.jpg_Q90.jpg_.webp

Seems to show, it should be the 15V tap of L2, Not the 35V one, at that point. But, I'm still not anywhere near 100% sure how you have wired it up.

The voltages (in AC mode on a multimeter), should read the approximate voltages on the transformer tap PCB screw connector block. If not, that could explain why the 15V AC line to the 12V regulator, is actually the 35V AC one.

[MK14]

if thats so it comes out at 49v using a 35v tap,that seems over the top for 78 series regulators!,am i correct?

Sorry for the triple post, but no one else seems to be responding at the moment.

So, you seem to be using 2 transformer taps, when the circuit is expecting 3 transformer taps ?

If so, how come the 35V AC tap, is getting into the 15 V AC tap connector ?
Hence creating around 50V, instead of around 18V to 20V.

[m3vuv]

Tbh in not sure,i replaced the 7812,also the tip42,( i used a tip122 instead as it was all i had),also using a 17v tap and a 23v tap,it now seems to be working fine only goes 0-28v but i can live with that,cheers all,may post a pic when its finished.

[m3vuv]

may fitanother meter and a 1l resistor in it with some sprung loaded jacks for testing zener voltages,also a usb port running off of a 7805 for phone charging etc,seems a handy tool,just using it to charge my prc-320 battery,good to see the current limit is working ok,currently set to 28v 400ma charge,battery has come up 1v so far,4ah battery pack nicad so 1/10c 400ma seems ok ?.

[m3vuv]

a pic of it charging my clansman battery,its just loosly put together,quite happy how its turning out,the case is an old pc sub woofer

[m3vuv]

another pic,this showing the current thru my fluke,the psu readout is a tad off,need to adjust it.

[magic]

Shitty build of a shit PSU kit

Sorry, I don't think there is any way to end up with 50V on the 7812 without screwing up transformer wiring somehow

[Gyro]

another pic,this showing the current thru my fluke,the psu readout is a tad off,need to adjust it.

That has to be in the running for shitty eevblog photo of the year. 

[m3vuv]

yes truly crappy pics,i was using my phone,will take some proper ones with my dslr when its finished,sort of been beefing it up,made all the tracks that carry power a lot bigger ,fitted a 22.000uf phillips 63v smoothing cap+ a 50 amp bridge rectifier,and 5x2sd1047 pass transistors with 0.22 ohm 5 watt emitter resistors,these are bolted with allen bolts to a pc copper heatsink with heat pipes and fan,hope to get about 30+ amps from it,the transformer alone weight 6kg,fingers crossed it works out,if it does it should be a handy piece of kit.,i hope to have it finished in 2 or 3 days all beeing well.

[m3vuv]

forgot to say i may use 12v car glow plug 50a relays if needed.but i will see how the stock ones fair.

[xavier60]

Even with 5x2sd1047, you will need to avoid short circuiting the output at 30A.
What is the expected voltage drop across the output BJTs?
https://www.farnell.com/datasheets/2124326.pdf