As shown in the figure below, the DC-IN is 24V, and the SW comes from the I/O port of the 51 single-chip microcomputer (with internal pull-up). The problems encountered now are: 1. When the SW outputs a high level of 5V, the transistor is not turned on, the multimeter The measurement found that the voltage of SW was pulled down to about 0V instead of the high level of 5V. Disconnect resistor R131 to measure SW. It is a normal high level of 5V. When R131 is connected, SW is pulled down;
2. If R131 is changed to 10K, the circuit can work normally again, SW outputs high level 5V, and the transistor is turned on;
Before, I thought it was the 51 I/O port drive capacity was not enough, but according to the second case, after the current limiting resistance is increased, it can work normally, so the problem of insufficient 51 pin drive capacity can be ruled out, so the above phenomenon can be eliminated. What will be the reason, everyone is welcome to discuss
Emi 发表于 September 16, 2020
Does the 51 pull-up resistor and R131 produce a voltage divider? If R131 is small, the voltage divided to SW is small
If R131 is larger, the divided voltage will be larger
You try to take the resistance value between 2K-10K, and then measure the voltage. If it is between 5V and 0V, then you are right.
Nana 发表于 September 16, 2020
It's not normal. It stands to reason that the switch tube circuit can work without the R131.
Take a look at the R131 in your hand, test it with a multimeter for any problems, and test it with the 9013.
Kenji 发表于 September 16, 2020
Nothing strange.
The I/O port with internal pull-up is an open collector (or open drain) structure.
At high level, VCC is sent out in series with a pull-up resistor, so the load capacity is limited.
When the value of R131 is low, the level of the I/O port is the result of the partial pressure, and it cannot be 5V.
Increasing the resistance of R131, the partial pressure result will be improved, the premise is to ensure that the following triode can achieve reliable conduction.
For the I/O port with internal pull-up, use its low level (internal tube conduction) output for control, and there is no problem with the load capacity. Of course, circuit composition and phase issues must be considered separately.
