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Old 10-28-2008, 01:10 AM   #1 (permalink)
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DC/DC Converter Indicator Light

I have a DC/DC converter in my Citicar to power the 12v items (headlights, turn signals, etc.) In addition, it keeps a small 12v gel battery charged (used as a backup/surge suppressor).

There is no indication that the DC/DC converter is working, so I made up a circuit that used a bi-color LED (tri-color yellow if both LEDs are on at the same time, not overvoltage) to indicate the 12v status.

Cheaper and uses less dashboard room compared to another meter just for the 12v system. I have not installed it yet as I am running it for a few days to see if it is robust enough to warrant a permanent installation.

I put it in this forum as it seems most of the DIY forum folks don't really use a DC/DC converter...

In any event, here is the description. If you copy/paste the text below and change the font to Times New Roman, the table should line up.

DC/DC Function Indicator Light

This simple circuit will indicate if your DC/DC converter is functioning normally.

When this circuit is used with a DC/DC converter assisting a 12v accessory battery, here are the indications -

Battery and DC/DC on 13.4v - Green
Battery on, DC/DC off, 12.8v Slightly yellow green
As aux battery is discharged, LED will become full yellow at 11.5v, fading to red. At approximately 10.5v, it is full red.

An additional indication is overvoltage. Above 14.5v, the LED will show bright green red. I wouldn't run this circuit above 14v for long as it will probably burn out.

0 1 2 3 4 5 6 7

A X B1 X X D+ R3 X

B C1 R6 E1 E2 B2 C2 D-

C LG R5 R2 LR R4 R2 R1

- X R5 L- X R4 X X

+ X X R6 X X R3 R1

Board population. Rows (1 to 7) are tied ABC (vertically).
+ and - rows are tied horizontally. X = (no component)

Parts List
R1 - 10 ohm
R2 - 220 ohm
R3 - 470 ohm
R4 - 4.8k ohm
R5 - 12k ohm
R6 - 15k ohm
D1 - 5.1v zener diode (band end D- on chart)
Q1,Q2 - BC327 PNP transistors
(listed as C1,B1,E1 and C2,B2,E2 on chart for collector, base, emitter)

L - Bi-color LED. LG is longest lead, center lead is L-, LR is other lead. (yes, L is normally an inductor...)
You will also need a small perf board from the local electronics store. See 'board2' picture for the style needed.

Components are inserted from the painted side so the component leads stick out the copper side where they are soldered.

You will probably want to solder extension wires for the bi-color LED so the circuit board isn't hanging from the LED. You don't need the entire board, and can save a few grams (less weight = more efficient ) by trimming to the size you need.

Here are a few pictures of the circuit board and a sketch of the circuit.

Attached Thumbnails
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Old 10-28-2008, 09:40 AM   #2 (permalink)
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Looks overcomplicated for a simple task. I'm sure I can design a simpler circuit that does the same thing.

To save some weight, have you considered using an automotive A/V capacitor (many farads) instead of a battery?
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Old 10-28-2008, 09:47 AM   #3 (permalink)
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I was thinking that a large diode hooked to a relay that keeps a normally cosed relay on/open as long as your power suply is working, when it stops working the relay closes and turns on the warning light on.

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