Triac PC Linuxcnc

[Doddy]

Your first schematic confuses me a little - because you show one transistor on the BoB and one on the machine. If the one on the BoB is intended to represent the output stage of the BoB (fairly common thing to do) - the problem is that it's not likely to represent the actual output stage of the cheap Chinese BoB (they use push-pull outputs rather than open collector) - the net result of this is I wouldnt recommend that diagram (I apologies - the earlier answer answered against the schematic, but I don't think the schematic represents quite what you're likely to achieve with the BoB).

The second schematic ("this is another diagram I found on the net") makes no assumption or assertion about the implementation or design of the output stage of the BoB, but instead just shows that (5V TTL...) driving a transistor. Two signals - Step/DIr, each with their own base resistor and transistor. That circuit implements the open-collector drive as shown in the BoB on the earlier diagram. This second schematic is "better".

The behaviour is, as the input (5V TTL) rises "high" (5V) then the transistor conducts, and the voltage on the collector ("12V from motherboard") will be shunted to ground. So, 5V in = 0V out.

When the input (5V TTL) drops "low" (0V) then the transistor stops conducting, and the collector->emitter is high resistance, and the 12V from the ("12V from motherboard") remains unaffected.

So 5V in = 0V out; 0V in = 12V out.

Note, and this is important if you don't want to blow the transistors up. The "12V from motherboard" I expect is either the Step or Dir input to the existing stepper driver - it's not connected directly to 12V, but rather through an internal resistor on the stepper driver to 12V. It's that resistor that presents the voltage drop when the transistor is conducting. So don't connect the circuit to a 12V supply, but do connect it to a Step/Dir input (... that might read 12V on a meter).

Just to demonstrate (a picture is worth a thousand words), I've sketched out your schematic in a circuit simulator (LTSpice)...



The circles - the one on the left in the "BoB" is a 5V voltage source, that starts at 0V and spikes to 5V after 1 second, then returns to 0V....

The one on the right represents the machine's 12V supply. The resistor is the aforementioned internal resistor.

Simulating this for 5 seconds gives the following waveform...



The lower, green trace is the 5v signal from the BoB. The upper blue trace is the resulting voltage measured on the input to the Stepper driver.