flipping a relay with an open collector output

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davisrepair

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Hi, am trying to turn on a relay with an open collector output with 10k ohm pull up resistor and 5v DC. I get the 5v DC, but when it's wired into the relay the voltage drops to 2v DC. The relay only works at 3v-8vdc. Does anyone know why am getting the voltage drop or how to
get the relay on. Thanks.
 

hi,
If I follow your description, the 10k pull up resistor is to the transistor base.?

Consider the current in the resistor, 5V-0.7Vbe =4.3/10000 = 0.43mA base current.
Thats much too low to fully switch ON the transistor, try a 1K or even a 470R.

Lets know.
 
Hi,
Thanks for the reply. The open collector output and 10k ohm resistor are in a Datamax printer output and am trying to turn on a relay with that output. So am not sure where
the resistor is connected to. All I know is when the output is on it gives 5vdc until I wire
it to the coil on the relay then the voltage drops to 2vdc which will not turn on a relay.
Thanks.
 
Get a "LOGIC-LEVEL" NFet. The printer port pin is either at 5V or 0V, depending on what you write to the register bit. Apply the port-pin signal to the Gate of the NFet. Wire your external relay as shown. Get a Power supply to match the relay coil voltage.

There is no way to power your relay directly from the printer port.
 

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alphacat said:
Hey Mike.
Could you explain what R1 is for?
Click to expand...

Holds Gate at ground potential (Relay not pulled in) if the device gets disconnected from the Printer Port. Remember the admonition about leaving CMOS inputs "floating".
 
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