The previous article about a nearly ideal LDO regulator gained quite some interest amongst readers. Meanwhile I managed to build two "barrel" multi-purpose power supplies based on this LDO circuit.
Casing was always the biggest doubt in my DIY projects. Here I resorted to nice and cheap stainless-steel cans from Ikea with covers cut from a 6mm thick MDF. Perfect solution... to be stuck behind the desk.
|120W barrel power supply|
Some say I am (almost) crazy: these days there are all varieties of inexpensive power bricks available on the market, which can provide virtually any imaginable output. Even worse: these two barrels that I am writing about did cost me quite some money (mainly for transformers) and time to build. They replaced some 7 switching power supplies - those I already had and simply put into a storage box in a basement chamber.
And I can tell you why: it's my battle against electromagnetic smog in our living space. Guess where is my micro-wave oven? Right, it's been donated to the cleaning team. My conscience still punishes me for that donation - I'd better have teared it apart for parts: what a nice high voltage 1kW transformer it had 🙂
The topic of electromagnetic trash in the air worth a full-blown thesis work. I'm thinking of returning to EMI issues on these pages one day...
|Quick casing solution: heat-sink with fan fits nicely|
|Components side: LDO + Mains FUSE|
|Soldering side (Teflon insulated + Kynar wires)|
There are endless variations possible around the LDO schematic layout I am posting here, all based on a particular application requirements. Let's review here just few simplest mods around the same schematic.
In all the schematic snippets below I kept the elements numbering from my first "MOSFET+tl431 LDO" publication.
Dual Transformer's secondary + Soft Start
Here's the simple mod - adding a soft-start circuitry: just one additional resistor R9.
|Most efficient raw power - dual secondary windings|
Sample selection of components:
- VD1, VD2 = Schottky rectifiers 8A 40V
- VD5-8 = small 0.5A 200V diode bridge
- C1 = 15kuF 25V
- C2, C3 = 47uF 25V
- C4 = 1000uF 35V
- R9 = 1kOhm
- C6 = 0.1uF ceramic
Note the increased value of C4. Together with R9 it makes sure that the pull-up voltage "V++" raises slowly at startup. Since the regulator's output could not go higher that the potential on the C4' positive pin minus the MOSFET's threshold - the output voltage will also raise slowly at the power-up.
Single Transformer's secondary + Soft Start
No Current Limiter
|TL431+MOSFET=great LDO regulator|
For parts values please see below.
Attention: proper fusing is the must! I would suggest installing a fuse rated twice the nominal load current right after the transformer's secondary. The fuse must be of a "slow" type (with letter "T" - time). I would highly discourage you relying on the fuse installed at the transformer's primary in case the transformer provides several secondaries for different loads. The reason is simple: while one secondary is about to smoke - the full load might still be below the rated max provided other loads went off.
LDO with the Current Limiter
Here is the equivalent of the originally published schematic. That one was simply re-drown in an easier-to-read manner, I hope.
|LDO regulator with Current Limiter|
Sample selection of components:
- R1, R6 = 2.2kOhm
- R2, R3 = 470Ohm
- R4 = 0.22Ohm 3W
- R5 = 12kOhm
- R7 = 2.2kOhm multi-turn
- C5 = 10nF ceramic
- VT1 = IRFZ40
- VT2 = 2N2222
- VD9 = 1N5244B (14V Zener)
Here is my tried and true TESTING LOAD. It helped me countless times in testing and tuning audio power amplifiers. Now it helped also giving hard times to the linear LDO voltage regulator rated at 12.6V 2A. The current limit was set to some 2.5A.
|8/4/2 Ohm 30/60 W test load|
Further developments of this idea
- External shutdown + soft start;
- Thermally controlled fan;
- Thermal shutdown / fuse;
- DIY kits for tube amps upgrade and beyond;
- Programmable supply...
Stay tuned - subscribe to my blog! 😉
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- Serge Patrushin.