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chinese psu kit

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m3vuv:
hi all ive  one of these  https://www.ebay.co.uk/itm/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-Short-with-Protection/143085522518 its a 0-30v psu at max 3 amp,ive changed the  op amps for mc34071pg op amps,if i run 2 or 3 2n3055 pass transistors with ballast resistors in place of the original plus use a 30amp bridge in place of the original rectifier diodes along with beefing up the tracks and changing the current sense resistor from a .47 ohm 5w to a .22ohm 25 watt should it be pos to get say 6-10 amps from it also using bigger filter caps?,cheers m3vuv.

Kleinstein:
With a 24 V AC source the DC voltage be at around 28-33 V with load. At that voltage a 2N3055 is good for maybe 2-2.5 A. So 2 of the 2N3055 may allow up to 5 A with a really good heat sink. Even at that current level the simple circuit without tap switching is not really useful.

The circuit is supposed to be up to 30 V - however real world maximum is more like 25 V due to ripple and other losses, maybe 30 V with no load and a relatively small transformer and thus high no load voltage. However this puts the OPs at even greater danger.

The easy fix is to use a 18-20 V transformer and change 2 resistors (lower current and voltage setting) to make it a 15 V 2 A supply. It may take a larger filter cap at the negative supply


The filter cap for the kit is one the low side - though more capacitance adds to the ripple in CC mode - the layout is still the same crap for the last 20 years or so. It can be fixed with a bodge wire (green) and cutting the trance (blue)  - see attached picture.

m3vuv:
what is that going to achieve the track does the same as the wire,seems pointless

m3vuv:
im supprised by the lack of useful answers!

Kleinstein:
The change is to fix a layout problem:  The connections are the same, but the effect of trace resistance is different.
There is quite some ripple current to charge the filter cap. This can easily reach the 5 A range, possible 10 A at full load. How much current depends on the transformer and fuse used.

Though the traces are a little wider this gives a drop, especially in the short path from the diodes and the filter cap. The voltage at the shunt is relatively low, so that 1 mV ripply would leads to some 2 mA of current ripple. I have not measured it myself, but the problem came up in a German forum: the original board showed quite some ripple (just like the current) and the changes essentially removed it.

Adding an extra copper wire to thicken the current path would be a first fix.  It is not the worst point, but one that is relatively easy to fix - and for a kit that is sold for some 20 years now, it's a shame not to fix the layout, if it is so easy.

The worst point is having to much voltage at the OPs. Even if it may initially work, chances are the TL081 may not like 40 V very lond and fail. I don't know the failure mode, but failing with to high an output voltage is possible.

This circuit has been discussed over an over gain  :horse: :horse: :horse:

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