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My External Power Source for the Canon EOS 10D


The circuit board

BP 511 housing with cable and power plug


Inserting the adaptor

The Schematic (click to enlarge)

The power consumption (click to enlarge)
Before beginning, I must of course give the following warning: this is a description for a self-made voltage converter for the Canon EOS 10D. I do not take over any warranty that it works, and mistakes can destroy your hardware and your camera.

The Concept

My Canon EOS 10D needs much power, and when steadily turned on, one battery doesn't last 12 hours. Taking a look at the price of the original external power source for the camera was horrible: another 128 €. I decided to do it myself.

The camera needs 8.1 Volts. A fully charged battery of my camera showed 8.4V. So I thought, using 8V as an external power source should be safe.

I opened the housing of a very cheap BP 511 battery imitation (9 €) by cutting around it with a sharp knife, and mounted a voltage converter inside it. It needs an input voltage between 12V and 38V (11V works also), and voltage dependent current. Using a standard 5mm-bushing, it can be connected to various mains adaptors, via a standard adaptor cable to a car's cigarette lighter, and of course to the power source of my telescope equipment. The housing is finally fixed with a small srew (placing the nut in a small pit of the housing), therefore I let space on the circuit board for a hole in the middle.

I bought a BP 511 battery charger with a 12V mains adaptor and a car adaptor cable, and both also works with my camera power source.

The Circuit

The circuit consists of two parts: a false-polarity protection, and a DC-DC switching step-down voltage converter.

To avoid power loss in a diode for doing the false-polarity protection, I used a PMOS transistor. The zener diode protects its gate by limiting the gate-source-voltage to 4.7 Volts. The used transistor type is an IRF9Z34N, it's cheap and provides 0.1 Ohms DS-resistance.

The step-down voltage converter is a complete module from TI, providing 8V @ 1.5A. It needs only an output capacitor. I did not find a place where to buy it, but you can order a sample at www.ti.com for free. A LED provides control over the function.

The part list:

  • PT78ST108H, TI step-down voltage converter
  • IRF9Z34N, PMOS transistor
  • 4.7 V zener diode
  • 220 μF capacitor
  • 0.1 μF capacitor
  • 10 kΩ resistor
  • 3 kΩ resistor
  • red 3mm LED
  • the housing of an old BP 511 type battery
  • cable, 5 mm bushing with 2.1 mm pin, and circuit board
  • a screw for mounting the housing afterwards
Altogether I paid approx. 13 €.

Important Notice

The DC-DC switching converter is always working, even if the camera is turned off. Including the LED, I measured a current of approx. 9.5 mA, relatively independent from the input voltage (9.38 mA @ 15V, 9.58 mA @ 24V). This is not much (less than a standard LED with 20mA alone). But if you run it with a battery (e.g. lead battery), you should consider, that you should disconnect it when leaving it alone for a longer time (e.g. when you go for vacations or etc.), or integrate a switch into the cable and turn it off. A current of 10 mA discharges a 10 Ah lead battery in 42 days...

Also consider, the power converter doesn't know about the power source (battery or whatever). So it just takes the power it needs. If you use an ordinary lead battery, please pay attention, that it gets not deeply discharged.

Power Consumption

I measured the power consumption of the 10D, supplied by the described DC-DC converter, for:
  • 2 different cases:
    1. The camera is idle and does nothing,
    2. the camera is in Bulb mode, and the shutter is pressed (interesting for the long exposures I need in astrophotography).
  • an input voltage range of approx. 11 to 24 Volts.
The resulting characteristics of the current I and the power P are plotted in the power consumption chart. Nicely, the curves show how the TI module decreases the current with increasing voltage, and so keeps the power nearly constant. Without the switching regulator, but with an ordinary linear one, the power would have been doubled.

What I additionally recognized, but did not quantitatively measure, was:

  • Pressing the focus (without releasing the shutter), without an autofocus lens, needs even more power than holding the shutter and mirror open. Maybe the display inside the viewfinder uses a light bulb ;-)))
  • The power consumption for lens focusing, sensor reading, CF card writing, menu or image display, is very high (up to > 1 A). But I had not the equipment to measure the integral, i.e. the used energy.
But for long exposure sessions, the characteristic should help estimating the required battery size.

© H. Lorch