Electronics > Beginners

DIY 0-30V 0-3A ...Again

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Cliff Matthews:

--- Quote from: Jwillis on January 08, 2019, 02:06:14 am ---I give up!
This circuit is a complete piece of crap and isn't worth any further investigation.

--- End quote ---
I think the retailer over-rated this.. so expectations were too high (what did we expect for a $6 bag of parts?)
For me, the one thing that saved my op amps during testing was to put a Variac in front of my 88VA, 2 x 22v transformer.

*edit - Here's something for you to read from another builder (see what you think before you pull out that hammer..)
** - I'd like to add Floobydust's protections and shave-off 1.2v for U2 (as shown) to get 0-25v @ 2a
http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

Kleinstein:
With the given instructions the kit is really bad. With a reduced voltage (e.g. 18 V transformer and thus a maximum output of some 15 V), the circuit is not that bad and could work.

The typical board layout also has a poor connection for the shunt sensing. This can be fixed with a cut trace and bodge wire if one does not like hum in CC mode.

With the power transistor it's a good idea to do an initial test with a short to see if actually survives. There is a chance to get fake transistors from china. :horse:

The MC34071 looks good on paper, but might be difficult to in a DIP form factor. For the voltage regulator a 741 or TLE2021 (up to 40 V) should also work.

Tek Tech:
Hi Jwillis,

I wonder if you have given up on your project? I built several of these just because they are dirt cheap (less than $4 shipped), have a darn nice PCB, and they work really well within reasonable voltage and current range with a proper heatsink. I never had a problem with any of these. I did read about wrong parts being sent in the kit; that's how you got the 47 Ohm instead of 0.47. I never received the washers and nuts that should come with the 10k pots, which is so frustrating. BTW, these nuts are metric M7 x 0.75 (fine thread). I ordered cheap taps from China and will be making my own nuts out of washers.


--- Quote from: Jwillis on January 05, 2019, 11:25:43 pm ---You know you guys are right I neglected to notice that .The schematic even requires a  0.47 Ohm  not the 47 ohm that was supplied. Damn my poor vision. Now that explains whats going on.
A couple 0.25 in series should be alright to test.

--- End quote ---

Given the above information, your project worked just fine in the first place, as others have pointed out. The 30 mA current limit corresponded with the 47 Ohm resistance: 100x greater resistance gave you 1/100 of the max. limit - only 30 mA instead of 3000 mA.

Given your report of further problems, I'm wondering if you actually measured the two 0.25 Ohm resistors in series, which replaced the 47 Ohm? And, these should be each at least 2.5 Watt resistors - because when you short the output at the max. 3 Amp current they will dissipate 2.25 Watts each. Lower powered resistors will burn at full current.

As I said, I put together several of these kits and they worked perfectly, as expected, without any additional modifications.

I found unmodified schematic of the original kit that displays all parts and their values in a clear layout. Credit for the schematic goes to VoltLog and his youtube channel.

Kleinstein:
The main complaint about the kit is that the TL081 OP is only specified for 36 V supply, but with a 24 V transformer as suggested the voltage could go up to some 40 V, depending in the transformer (higher with a small transformer as the no load voltage is higher) and local mains voltage.  It would still have a hard time to deliver 30 V - even 25 V could be only with reduced current.
One could still be lucky and get a TL081 that can stand 40 V.

With a 20 V transformer things are much better. Maximum output voltage is than more like 20 V at low load and maybe 16 V at full current.

The filter cap is relatively small and the drop on the .47 Ohms shunt is rather high so it runs quite hot - so one could consider reducing the maximum current to some 2 A or consider the 2-3 A range only as a bonus with limited performance.

m3vuv:
Hi all,ive built one of these,works ok except when the cc led comes on it cuts the output to 0v,it doesent regulate the current,its all or nothing,any ideas where to look?

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