Contact David Goodsell at dsgoodsell@verizon.net.
■ FIGURE 4. Interior view
shows battery box, 0.100 ohm
current sense resistor,
and the PCB.
high voltages. So, I settled
on 20V and 40V. Plus, I
added a label — “40V MAX”
— near the input connectors
to specify the upper limit.
Construction
Okay. Now that the
design was frozen, it was
time to package it. In the
Digi-Key catalog, I selected
a wrinkle-finish black Serpac
SR251 enclosure because it
had an internal battery box
and plenty of room for all
the switches and their
labels. Then, I drilled the
holes, applied the white
rubdown letters, and
sprayed it with Krylon Matte
Finish #1311. DISASTER! The finish looked
terrible — all mottled and uneven. I was sick.
I checked the spray can. It definitely was
compatible with plastics and had worked on
other projects in the past. What was going on?
Fortunately, I had another Serpac SR251
box so I started all over again, new holes, new
lettering. What a pain! Then, I took an old
spare Serpac box and sprayed it with three
different plastic-compatible Krylon sprays:
Matte Finish #1311, Low Odor Clear Finish
(Matte), and Low Odor Clear Finish (Gloss).
The Low Odor Clear Finish (Matte) looked the
best, so that’s what I used on the box shown
in Figure 2. However, please do your own
tests before using any of these sprays on your
next project.
As for the PCB, I considered using surface-mount components, but there was plenty of
room between the switches for through-hole
parts, as illustrated in Figure 4. I included a
test point (TP1) so the low battery condition
could be tested.
SPECIAL NOTE: It is important that the
wires soldered to the current sense resistor,
R14, are connected at a spacing of 1.310”, as
per the spec sheet. This assures the accuracy
of its value, 0.100 ohms +-1%.
As much as I like the Martel V-Series
PARTS LIST
ITEM
C1,C2,C4
C3
C5
1% RESIS
OTHER RES
R14
Q1
Q2
U1
U2
SW1
SW2
SW3
SW4
J1,J5
J2
J3
J4
J6
J7
P1( 4),P2
P3
P4
P5
LCD1
LED1
PCB
BOX
LETTERS
DESCRIPTION DIGI-KEY
(unless noted otherwise)
0.1 µF, 50V, Bypass P4525-ND
2. 2 µF, 20V, Tant 478-1869-ND
10 µF, 20V, Tant 478-1840-ND
Metal Film, 1%, 1/4W value+XBK-ND
Carbon Film, 5%, 1/4W value+QBK-ND
0.100 ohms, 1%, 3W 13FR100E-ND
2N7000, N-chan MOSFET 2N7000FS-ND
ZVP2106A, P-chan MOSFETZVP2106A-ND
LMC555 Timer LMC555CN-ND
LTC1440 Comparator LTC1440CN8#PBF-ND
DPDT, Toggle EG2400-ND
3PDT, Toggle EG2425-ND
SPST Pushbutton, N.O. Mouser, 10PA322
DPDT, Center-Off, Toggle EG2414-ND
2.1 x 5. 5 mm Panel Jack SC1049-ND
2. 5 x 5. 5 mm Panel Jack SC1048-ND
1.3 x 3. 4 mm Panel Jack Mouser, 163-4015-EX
3. 5 mm Panel Jack Mouser, 161-1640-EX
8 pos LCD Socket, 0.1" S4108-ND
9V Battery Connector Mouser, 121-1026T-GR
2.1 x 5. 5 mm Plug ( 5) SC1052-ND
2. 5 x 5. 5 mm Plug SC1051-ND
1.3 x 3. 5 mm Plug CP3-1003-ND
3. 5 mm Plug CP3-1005-ND
LCD Voltmeter, 0.5"digits 227-1060-ND
LED, Red 67-1066-ND
PCB w/Mask & Silkscreen ExpressPCB
Serpac, w/Batt Box, Black SR251B-ND
White Ltrs&Nums, 4 sizes FLS Discount #681012
February 2011 45
