On Wed, 5 Nov 2008, Matthias Urlichs wrote:
> If we only have a couple of DS2405 lying around, this gets a bit more
> complicated and we need to add a few more parts:
+5V +5V
e| |
Tpnp---R2----*---R3
c| b |
R1 |
| DS2405
Relay |
| |
GND GND
> A simple general purpose PNP transistor like the BC557 switches 100 mA,
> its Uce is around 0.2V. So R1 is roughly the same value as the sole
> resistor in the previous schematic.
Even easier is a N-channel mosfet connected to the DS2405, with a, say 10k
resistor R1 to 5V
V+
|
load
| +5V
s|| |
||-g--------*---R1
d|| |
| |
| DS2405
| |
| |
GND GND
In this case your voltage can be arbitrarily high, and you can easily
control 10A with a suitable mosfet (BUK456 family is a common one). You
don't get isolation, but the cost and simplicity is unbeatable. The limit
to power consumption is in fact the turn on time of the MOSFET - a common
value for the mosfet capacitance is 200nF, with a turn on time of say
200nF * 10kohm = 2ms. This means that for 2ms, whilst the mosfet is
charging up you will be dissipating about half your load power in the
mosfet. So if the load is 10A * 24V (240W) you'll be dissipating 120W *
2ms = 0.5J. This should be fine with a reasonable heatsink, at the speeds
you can drive 1-wire.
(If you take this route, your power supply needs to be able to float with
the 1-wire ground, off the shelf double insulated switchmodes work just
fine)
njh
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