When using an open-drain output to drive an NPN transistor, can the base resistor be replaced by a pull-up?

Question

Assuming the value of R1 is the correct value for the base resistor for the given motor L1, is it okay to omit the additional base resistor, used in push-pull applications, for an open-collector output?

To my understanding, when the open-collector output goes low, the low impedance path will not affect the transistor negatively, as there is no/minimal potential difference between the open-collector output directly connected to the base and ground, therefore turning-off the transistor.

This is my proposed solution using an N-channel MOSFET, based on the helpful answer of @Spehro Pefhany:

I'd much appreciate feedback on this proposal. Will it work with a good margin both at 5V and 12V motor (L1) voltage and over a temperature range of 10°C to about 85°C?

These are the links to the datasheets:

• N-channel MOSFET: https://assets.nexperia.com/documents/data-sheet/2N7002BK.pdf
• Temperature sensor IC with open drain thermostat: https://www.ti.com/lit/ds/symlink/tmp101.pdf

Answer 1

Maybe, but you need to ensure that the Vol from the open-drain output guaranteed to be sufficiently low to ensure that the transistor is fully off under worst-case conditions with the value of R1 that you need to properly drive the base (typically something like the relay coil current divided by 10 or 20).

A guarantee of 300mV is usually sufficient.

Otherwise, you can use a pullup resistor and a voltage divider to the base, or maybe use a MOSFET, which could allow a higher value of R1.

For example, here are the typical characteristics of the open-drain outputs on the STM8S103F3 microcontroller datasheet (that I happen to have open at the moment):

None of the guaranteed numbers are directly useful, but combining the two it looks like 3mA would be safe with a 5V supply, so a 30-60mA relay could be driven as suggested.

Answer 2

is it okay to omit the (extra) base resistor for an open-collector output

YES

But don't assume R1 is optimal.

To get rated current in the coil, The datasheet will show its DCR use 10x that value for pullup scaled by the voltage ratio 4.3/12 thus if DCR = 80 Ohms, R1= 800 *4.3/12= 287 Ohms or nearest preference. 330 to 280.

Very high hFE transistors will use 20x factor.