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Low Budget Bench Supply: Digital Version

This one costs slightly more than the analog tattoo power supply:

Tattoo Digital Power Supply - front panel

Tattoo Digital Power Supply – front panel

The gold lion really spiffs up the Electronics Workbench!

Somewhat to my surprise, the circuit uses a switching power supply based on a Reactor Micro RM6302 controller that can produce about an amp at voltages up to about 14 V:

Tattoo Digital Power Supply - internal view

Tattoo Digital Power Supply – internal view

CAUTION: Everything on the input side of the transformer runs at line potential. I have my doubts about isolation, particularly under fault conditions.

The trimpot on the PCB seems to adjust the output voltage, although it’s not clear what’s going on.

The three wire AC line cord has a standard IEC entry block on the rear panel, albeit with the ground terminal not connected to anything inside the plastic case. It arrived with the hot wire soldered to a tiny fuse on the PCB and the neutral wire (red!) to the back-panel switch. There being no practical way to put the fuse before the switch, I rewired the hot side to the switch before the fuse and the unswitched neutral to the PCB; that’s as good as it’ll get.

I also flipped the AC switch to put the ON position at the top. Sheesh.

The two 1/4 inch jacks on the front panel are wired in series, so it didn’t matter which one got the tattoo needler or the foot switch:

Tattoo Digital Power Supply - jacks in series

Tattoo Digital Power Supply – jacks in series

I rewired the sockets in parallel to eliminate the need for a shorting plug, although I cannot imagine any need for two outputs.

The knob seemed unusually sloppy, which turned out to be due to a broken threaded sleeve around the pot shaft that prevented the crudely made nut from seating tightly:

Tattoo Digital Power Supply - broken pot threads

Tattoo Digital Power Supply – broken pot threads

Given that the builders stuck everything else to the front panel with hot-melt glue, I followed suit:

Tattoo Digital Power Supply - glued pot threads

Tattoo Digital Power Supply – glued pot threads

Which actually held it in place reasonably well, despite the hideous appearance. The knob covers the blob, so It Doesn’t Matter.

The output range extends from about 1.2 V to just over 14 V at about an amp, but the knob seems erratic and the digital meter has only a casual relationship to the actual output voltage.

I think if you regard this one as a parts kit, reverse-engineer the schematic (which surely descends directly from the RM6302 datasheet), and rebuild the electronics, it might work better.

Bottom line: The analog version seems to be better as a low-budget power supply, not least because it has a metal case and an actual power transformer for galvanic isolation.

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