SS805 base current setting upper limit problem

tongshaoqiang

SS805 base current setting upper limit problem [Copy link]

 

As shown in the figure above, transistor 8050 is used to control 2 relays (4100F-5V, HF46F-5V). The input of the coils of both relays is 5v, and the coil resistance is 125 ohms. Considering the space of the circuit board, saving parts as much as possible, and considering that the maximum output current of the microcontroller IO port is about 10ma, the transistor base resistor is omitted. Later, because the relay was mistakenly used with 24v, the base current was measured to be 30ma. I was a little worried that there would be problems with the transistor. I checked the manual of 8050 and didn't see the upper limit of the base current. With a base current of 30ma, the transistor can work in a saturated state far away, and there is no problem with the test. I am worried that there will be problems after long-term operation, so I come to ask everyone for advice.

1. Is there a maximum upper limit for the base current of ss8050 or other transistors?

2. After testing the IO port output, the base potential is about 0.8v. I think the output impedance of the IO port is too large, resulting in too small base voltage. I couldn't find the output impedance in the microcontroller manual. Please confirm.

If you don't know how to insert accessories, you can check the manual of 8050. The magnification is 120--400

chunyang

The maximum base current limit of the transistor should be found in the manual, but I'm afraid it will require a very detailed version. However, it doesn't matter in general, because when the collector current reaches the permitted limit, the corresponding base current still has a considerable margin from its limit. Therefore, when designing, just follow the base drive current value corresponding to the target current, and leave a proper margin. You don't have to consider the base limit current, but the base limit current resistor is necessary.

chunyang

Regarding the issue of IO directly connected to the base, this is obviously not allowed. Without a base current limiting resistor, IO will compete with the transistor for a level, which can easily lead to base current overload or IO output overload, which is very detrimental to reliability.

The output impedance of the MCU is generally not stated in the manual, because it is related to the manufacturing process and even temperature, and the digital circuit must ensure the logic level. Without the correct logic level, the digital circuit loses its meaning. Therefore, the manual will give the maximum sink current and source current of the output under the effective logic threshold. The limit that IO can withstand is obviously greater, and the MCU will also leave a margin when designing.

chunyang

In general, when designing transistor circuits according to specifications, there is no need to know the limiting base current parameters of the transistor. Similarly, when designing MCU application circuits according to specifications, there is no need to know the output impedance parameters of IO. Both of them have a considerable margin from the limit under the constraints of standard design, so they do not need to be stated in general manuals. Of course, if the circuit may be affected by some factors and cause overload, protection is required, but there is no need to know these two parameters. When making protection circuits, it is enough to ensure reliability by not leaving the threshold too outrageous.

maychang

tongshaoqiang

chunyang posted on 2021-3-4 17:18 The maximum base current limit of the transistor should be found in the manual, but I'm afraid it will require a very detailed version. But in general it doesn't matter, because in the collection...

Thanks to teacher Chunyang for his guidance.

According to the maximum parameters: the maximum collector current is 1500ma, the minimum β is 120, the base current is calculated to be 12.5ma, and it is assumed that the margin is 20ma. Now the tested base current has reached 30ma, so there are some concerns about reliability.

tongshaoqiang

chunyang posted on 2021-3-4 17:18 The maximum base current limit of the transistor should be found in the manual, but I'm afraid it will require a very detailed version. But in general it doesn't matter, because in the collection...

Thanks to teacher Chunyang for his guidance.

According to the maximum parameters: the maximum collector current is 1500ma, the minimum β is 120, the base current is calculated to be 12.5ma, and it is assumed that the margin is 20ma. Now the tested base current has reached 30ma, so there are some concerns about reliability.

maychang

Note what Chunyang said on the second floor: "The base current limiting resistor is necessary."

It is recommended that you add a current limiting resistor to the base of the transistor.

chunyang

tongshaoqiang posted on 2021-3-4 18:10 Thanks to teacher chunyang for his guidance. According to the maximum parameters: the collector is 1500ma at most, β is 120 at least, and the base current is calculated to be 12.5ma ...

There is no need to worry about the base current limit, but we should worry about IO. According to the collector limit current of 1500mA, the DC amplification factor is calculated as 100, corresponding to the base current of 15mA, 30mA is only twice as large, which should be much smaller than the limit. Maychang on the 5th floor gave the base current limit in the detailed manual. DC is direct current, which has always existed. The limit is 300mA, and the short-term burst value is even 1A. It can be seen that what I said before is true.

tongshaoqiang

maychang posted on 2021-3-4 18:15 Note what Chunyang said on the second floor: "Base current limiting resistor is necessary". It is recommended that you add a current limiting resistor to the base of the transistor.

receive.

Thanks to teacher maychang for his guidance.

tongshaoqiang

maychang 发表于 2021-3-4 18:09

Thank you, Mr. Maychang. The parameters in the manual I have are not as detailed as the ones you provided. Thank you.

tongshaoqiang

chunyang posted on 2021-3-4 17:27 Regarding the issue of IO directly connected to the base, this is obviously not allowed. Without the base current limiting resistor, IO will compete with the transistor for a level, which may easily lead to excessive base current...

Regardless of whether a resistor (2K) is connected in series between the IO port and the base, the base voltage is always around 0.8v, which is the BE junction voltage drop. I want to confirm whether it is because the internal resistance of the IO port is too large.

A 100k resistor was added between the base and ground, and the base voltage could not be increased when io output a high level.

tongshaoqiang

chunyang posted on 2021-3-4 18:19 There is no need to worry about the base current limit, but it is time to worry about IO. Based on the collector limit current of 1500mA, the DC amplification factor is calculated as 100, ...

Senior Chunyang, what you said makes a lot of sense! The microcontroller manufacturer said that the output drive current of IO is only 10ma, but I measured 30ma, which may be the IO output overload you said. So it is necessary to add a current limiting resistor to the base.

chunyang

tongshaoqiang posted on 2021-3-4 18:35 Regardless of whether a resistor (2K) is connected in series between the IO port and the base, the base voltage is always around 0.8v, which is the BE junction voltage drop. I want to confirm whether the internal resistance of the IO port is too large...

The emitter junction voltage drop can be considered to be basically a constant, and even if the IO internal resistance is very small, this reality cannot be changed. If the IO internal resistance is small enough, the result is that the base current is too large until the transistor burns out.

tongshaoqiang

chunyang posted on 2021-3-4 18:40 The emitter junction voltage drop can be considered to be basically a constant, and even if the IO internal resistance is very small, it cannot change this reality. If the IO internal resistance is small enough, the result is that the base voltage drop is...

Thanks again!

maychang

tongshaoqiang posted on 2021-3-4 18:21 Thank you, Mr. Maychang. The parameters in the manual I have are not as detailed as the ones you provided. Thank you.

The company that published this manual is PHILIPS, and the model is PSS8050. The data published by various companies are slightly different.

annysky2012

When designing this kind of circuit, it is important to consider the driving capability of the microcontroller IO