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### Topic: Current Limiting Power Supply (Read 1 time)previous topic - next topic

#15

#### basirim

#16
##### Dec 07, 2015, 09:21 pm
Be aware that even PP3-sized 9V batteries can be a risk to (human) life if you place enough
in series, so above 40V or so take the same precautions you would with mains electricity.  It is
a relatively simple way to get a high voltage supply at low current, and you can experiment with
the voltage you actually need too.

Poor little insects!
This is certainly a mistake I could (would) have made haha so thanks for the heads up!

#### basirim

#17
##### Dec 07, 2015, 09:26 pm
Adding up all the facts, I think you need a switched "high-side" constant current source of 100uA.
And 5 or so 9volt batteries in series to supply this CC source.
Leo..

Leo, would you mind talking me through the circuit diagram so I can understand it a little better? I've looked up all of the parts used in it but it would be useful if I understood the theory behind the design a little better. If you have some time, you can explain it here or in a PM.

#### Wawa

#18
##### Dec 07, 2015, 10:58 pmLast Edit: Dec 07, 2015, 11:28 pm by Wawa
This is a classic constant current source.
When the Arduino pin is high, the BC547 turns "on".
Current from the battery stack now flows through the two diodes and R2 to ground.
Creating a constant voltage across the two diodes (~1.4volt).
The BC557 uses this constant voltage to drop a constant voltage across R1.
This is one diode drop less, because of the BE junction.
So there is a constant voltage across R1 of ~0.7volt.
Constant voltage and constant resistance of 6k8 is constant current.

The two diodes can be replaced by a common red LED.
Change R1 to ~10k for 100uA.
Leo..

#### MarkT

#19
##### Dec 08, 2015, 01:36 pm
Better choose high-voltage transistors BTW - the BC547 isn't a great choice.  Go for a rating double
the max expected working voltage for a reliable design.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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