Gate/Base Current Source

While the overall purpose of this model is quite different, I'm throwing the remix flag on this one because I used...

LR44 / AG13 Battery Holder

... originally posted by jeplans on March 05, 2017.

https://www.thingiverse.com/thing:2155413

I changed his model a little, then buried it into mine.

This is a mounted, general purpose, stand-alone supply for tiny gate or base currents. Of course the ground of this circuit should be tied to the ground of the circuit it's supplying or the dreaded "...unpredictable results..." might occur.

I hate breadboards. Can't live without them; can't live with them.

A recent project involving some small thyristors required gate currents down around 100 uA (that's uA, millionths of an Amp). The components for that on a breadboard are pretty wonky - breathe on it, or even near it, and something opens or changes.

I've become a believer in general purpose "black boxes" with properly constructed soldered components for various functions that can be done off the breadboard. These can be connected to the circuit on the breadboard with a few wires and are stable. It reduces the clutter on the breadboard and eliminates the unreliable breadboard connections for the components in the black box.

So, this is a 3D printed base to which my gate current supply board is fastened. I've done this kind of base several times for similar things (see way below for a list) and it works out well, keeping the board upright on the bench and protecting its back side.

In addition to the 3D model, I've included all the details for building the board. I can't imagine anyone being interested in this 3D model for its own sake.

The base incorporates an integral battery holder for 3 X LR44 button cells into the model (someone else's design - see remix details above). There's a 3.5 mm hole though the block directly below the center cell, providing a way to get it out when you need to replace the batteries. Once the center one is out the others are loose.

If for some reason you want to push more current than the LR44s can supply another 4.5 V source can be connected in parallel using one each of the red and black pin headers adjacent to the LR44 connections.

The circuit board is fastened to the base with M2.5 machine screws, engaging brass threaded inserts in the bottom of the block. The holes are shouldered and the inserts can be pulled in with a screw. They should be flush with the bottom of the base when shouldered.

There's a pocket in the 3D printed base below the circuit board to clear the solder connections.

The circuit board is made on strip board and includes male pin headers for GND (black, 5 pins in two places, one used for the LR44 connection, all common); a + V connection bus (red, 3 pins but one used for the LR44 connection, all common) for temporary connection of parallel batteries or for reading battery voltage; output, on two pins (yellow, common, probably only needed one). There's also a pair of pins with a jumper cap (green). Removing the jumper cap provides connection points for an ammeter to read the gate current directly.

I used a 200K multi-turn pot and a 910 ohm fixed "Magic Smoke prevention resistor" in series, just in case the pot inadvertently gets turned down to zero. Theoretically, with the pot at zero, that should limit the output current to around 5 mA, which may be more than the LR44s could ever pump out anyway. You can substitute other resistor values when building the board as appropriate for your project(s).

I put in a slide switch and a pushbutton in parallel; either can be used. I included the pushbutton because the thyristors will latch in the on state, so the gate only needs to see current briefly to turn the device on. The switch is handy when setting the pot.

Materials:

• 3D printed base
• M2.5 X 4 mm brass threaded inserts (qty. 4)
• M2.5 X 10 mm Allen head machine screws (qty. 4)
• LR44/AG13 button battery (qty. 3)
• 200K multi-turn trim pot (qty. 1)
• 910 ohm 1/8 or 1/4 W resistor (white, brown, brown - qty. 1)
• Tactile pushbutton (qty. 1)
• PCB slide switch (qty 1)
• PCB jumper cap (qty 1)
• Assorted male pin headers
• Miscellaneous wire

PCB Details:

See the images included with this post. The sketch is pretty cluttered. I did that image in layers and had the ability to turn them on and off as needed. I had to include all the layers in a single image here. It should be possible to make sense of it.

The board is made on strip board (Vero board). The layout images show the copper side of the board. Ignore the fact that it's the copper side and pretend it's the component side. Lay everything out just as shown, as if the image was of the component side, except...

The only detail that requires extra care are the two strip cuts (blue lines). Those have to be done on the opposite side from the components, of course, so are going to be mirror imaged from the components. Be extra careful that you get it right. I find a die grinder with a tiny ball-end carbide burr is a good way to cut a strip.

There are only two jumpers (solid yellow lines). I used 22 gauge solid wire for those; they could easily be smaller wire.

The battery wires are 28 gauge solid wire. There are two holes at each end of the battery holder. The bare wire end passes into the holder from the outside, then back out through the other hole. The free end wraps around the standing part of the wire outside the holder. The part of the wire loop inside the holder provides the battery contact. No solder is necessary. Once the battery cells are in place, nothing is going anywhere.

The other ends of the battery wires wire wrap and solder to the strip board pin headers (red and black) on the + and - Bus strips.

Pay attention to the wire colors when soldering to the board and when installing the batteries. There's nothing to prevent the batteries going in backwards; the wire colors are the only indication.

If the pot is oriented as shown in the photos, the adjustment screw will be CW for increasing current.

PCB Image Legend:

• SW = PCB Switch
• POT = 200 K multi-turn trimmer pot
• RES = 910 ohm, 1/8 or 1/4 W resistor
• A = Capped pin header for ammeter connection
• PB = PCB pushbutton
• OUT = Output pin header, yellow
• GND = Negative Bus pin headers, black, two places
• + = Positive Bus pin header, red

Misc.

Hole Pattern: 40.64 X 27.94 mm (1.7 X 1.2 in.)

Pocket size: 51 X 38 mm

Printing Details:

Material: PLA Nozzle: 0.4 mm Layer Height: 0.15 mm Infill: 16% Shells: 0.8 mm Supports: None

Other Breadboard "Black Boxes" I've Created

Flexible Bench Load - 20-100 Ohms https://www.thingiverse.com/thing:7004423

USB Bench Load https://www.thingiverse.com/thing:6975603

Voltage Divider Enclosure https://www.thingiverse.com/thing:6027183

Expect more...