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Can Transistor outputs also sink current?

Created by: MuteMan at: 10/31/2022 8:52 AM (5 Replies)

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10/31/2022 8:52 AM	Rate (0)
MuteMan Member Joined: 3/28/2020 Last visit: 11/3/2022 Posts: 3 Rating: (0)	Hi, According to the manual each transistor output of the logo provides 24V and a mass. It is able to source 0.3A to the load, but there is no mention of it able to sink current as well... I have a setup of pushbuttons with indicator leds using a common anode connected to 24V. To activate the leds, I can pull each separate kathode to 0V. Can I connect the kathode of a led directly to the Q1 transistor output of the logo? When Q1 is activated, it has 24V and the led does not light up. When Q1 is deactivated, is has 0V, the led will light up but Q1 should be able to sink the current of +-0.01A Thanks

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10/31/2022 11:14 AM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 9/1/2025 Posts: 35909 Rating: (4496)	If the Qx output is deactivated, it does not have 0V in the sense of 0V for the power supply. The transistor is only just blocked. What you try to make is so not possible.

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10/31/2022 1:52 PM	Rate (0)
MuteMan Member Joined: 3/28/2020 Last visit: 11/3/2022 Posts: 3 Rating: (0)	Thank you for the info. So is it correct to assume that the outputs are open-emitter outputs and that they are not internally pulled to mass? I suppose I could add a resistor between Q and M to act as a pull down resistor, but that would mean a tradeoff between led brightness and power consumption ... Bit shame I'll have to add extra transistors to get the job done properly ...

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10/31/2022 1:59 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 9/1/2025 Posts: 35909 Rating: (4496)	As far as I know, they are no longer "pure" transistor outputs, but special ICs. They are short-circuit-proof and deliver a limited current of 0.3 A per output. In any case, the +24 VDC of the supply voltage is switched through to terminal Qx of the connections.

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11/1/2022 9:05 PM	Rate (0)
WWEE Bronze Member Joined: 12/13/2014 Last visit: 7/7/2025 Posts: 156 Rating: (14)	No, you can't. Search for Infineon ITS4140N and look at block diagram. It is used for LOGO with transistor output.

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11/3/2022 3:16 PM	Rate (0)
MuteMan Member Joined: 3/28/2020 Last visit: 11/3/2022 Posts: 3 Rating: (0)	Hmmm good to know! I took a look at the datasheet of the ITS4140N and basically it is a FET with a protected open drain output .. so actually, a well chosen resistor should work, but anyway, I think I'll go for a more elegant solution. Thanks for the info.

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