Hi,
I have assembled the board, run all tests successfully, also taken Analogue
input that is satisfactory.
Now the very last step failed for my use of the board, I want to control a relay with
a digital output. Everything looks fine when just regarding the LED for the DIG-out (chan1).
Have connected “clamp” to +10V, and “grnd” to ground
however the output provided on digital port (1) (which has its LED going on & off)
only show not up to 1V as output, which is too little to draw my relay.
(highly dissappointed)
The digital “outputs” on the K8055 are not actually outputs in the classic sense, in that they do not produce or deliver any power. They are what is known as open collector outputs, meaning that the digital out terminals on the device are connected to the collectors of eight NPN transistors internal to IC4. The emitters of these transistors are connected to ground, and their bases are connected to eight outputs on the PIC micro-controller.
Open collector output:
When you command a digital output ON the PIC supplies a forward bias voltage to the selected output transistor’s base, turning that transistor ON and providing a pathway from the collector, through the emitter, to ground.
To operate a relay it needs to be wired as shown below in an image taken from the K8055 manual, highlighted to show the needed connedctions:
When the digital output is turned ON current will flow from the external supply, connected to “clamp” and one of the relay coil’s terminals, through the relay coil and the K8055’s open collector output to ground–energising the relay.
Hi ! and many thanks for explanation from christmas Florida !
Though I thought I had understood more or less what you tell about open collector,
I guess I didnt take it in fully. The near one 1V I measured on the output port would
then most probably be the forward voltage of the transistor (to ground), and that
my construct for connecting the relay was at err. Will check, many thanks. /g
One more thing when being at it. The solidstate relay says it has an impedance of 300ohms,
which would yield some 30mA when working 10V, what current can you output transistors
sustain ? (doesnt say in spec)
[quote=“gech”]One more thing when being at it. The solidstate relay says it has an impedance of 300ohms,
which would yield some 30mA when working 10V, what current can you output transistors
sustain ? (doesnt say in spec)
BR/georg[/quote]
The maximum values for the digital outputs are 50V and 100ma
