Author Topic: How to switch load using radio controlled module if it switches the ground?  (Read 1233 times)

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Offline ghostbanana34Topic starter

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I had old broken toy car. I wanted to use it's RC module to switch 12V fan. I measured original circuit, found the output, saw 3V there (fig. A). Then I assumed that RC module switches the 3V and designed my circuit accordingly (fig. B). It was always on. Only then I start analyzing it more deeply and to my surprise I found out that RC module output is permanently connected to 3V and only switches second output to the ground (fig. C).

I used NPN transistor in TO220 package from old PC power supply, as that was only power transistor I had. Is it possible to modify this circuit so that switching to the ground would switch it?
 

Offline capt bullshot

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Add a small PNP transistor like this
Safety devices hinder evolution
 

Offline Zero999

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Yes, it's very common to switch the 0V. It's often much easier, because N-channel MOSFETs have a lower on resistance than P-devices.

If the RF module can switch enough current for the 12V fan, then add another transistor, forming a cascode with the internal transistor inside the module.



If the module isn't rated to switch the current required by the fan, another option is to switch the motor's 12V with a PNP transistor.
« Last Edit: January 10, 2019, 12:51:59 pm by Zero999 »
 

Offline ghostbanana34Topic starter

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Thanks. The RC module isn't rated for such current, so I will have to add PNP.
 

Offline spec

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Hi ghostbanana34

Attached below is a schematic for another version:
 

Offline Zero999

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Hi ghostbanana34

Attached below is a schematic for another version:
I did consider this option and it might work, but it's risky because the RC module's output stage is not a simple switch, as shown in the schematics  posted previously. We don't know what the maximum voltage rating of the module's internal switching transistor and there will be a snubber diode across the transistor or an internal back-EMF diode to +V. I think the latter is most likely the case, as it's designed to switch a motor connected between +3V and the output. If this is the case Q3 will never turn off.

It might be possible to remove the internal back-EMF diode, but one needs to be sure the internal transistor is rated to switch over 12V.
 

Offline spec

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Hi ghostbanana34

Attached below is a schematic for another version:
I did consider this option and it might work, but it's risky because the RC module's output stage is not a simple switch, as shown in the schematics  posted previously. We don't know what the maximum voltage rating of the module's internal switching transistor and there will be a snubber diode across the transistor or an internal back-EMF diode to +V. I think the latter is most likely the case, as it's designed to switch a motor connected between +3V and the output. If this is the case Q3 will never turn off.

It might be possible to remove the internal back-EMF diode, but one needs to be sure the internal transistor is rated to switch over 12V.
Good points. I did not have this additional information.
 


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