Dynamic braking ideas for forward/reverse DC motor

Thread Starter

*Leeroy*

Joined Nov 29, 2020
6
Hi guys. So as the title suggests i'm looking for ideas. The application is a portable, battery operated power tool which requires repair due to burnt out contacts in the trigger mechanism. The tool has a variable speed in both forward and reverse which is determined by the trigger position, when the trigger is fully released the contacts should close and short the motor, creating the dynamic braking, it's these contacts that have burnt out.
The motor is DC 20v and as mentioned above is run through a speed control which i'm presuming is working as a variable resister rather than pulsing, the forward and reverse feature is part of the trigger assembly but seems to be a simple switch that reverses the polarity to the motor.
I'd like to recreate the dynamic braking with a compact electronic solution that will fit inside the tool if possible, so while the trigger is outputting to the motor in any speed in forward or reverse the motor contacts are open, as soon as the trigger outputs 0v the motors terminals are shorted and braking occurs. It was mentions that perhaps a triac might be a possible solution but this is far from something im familiar with, hence me creating this thread.
Any ideas welcome and appreciated!
 

Dodgydave

Joined Jun 22, 2012
10,027
Post pictures of the switcing mechanism the new can see it better, yes a Triac would do to short it out, or a Thyristor and bridge rectifier circuit.
 

Thread Starter

*Leeroy*

Joined Nov 29, 2020
6
Post pictures of the switcing mechanism the new can see it better, yes a Triac would do to short it out, or a Thyristor and bridge rectifier circuit.
Thanks for the reply Dave, i've included a few photos but they might not give too much away as the switch/speed control can't be disassembled. As you can see i have two wires out of the speed control to the motor and five wires going in, two of which are obviously heavier power wires.
 

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Thread Starter

*Leeroy*

Joined Nov 29, 2020
6
I happen to know the internals of the speed control have two copper contacts that short the motor out when the trigger is released, unfortunately it's typical for these contacts to burn out like they have in this case, its a very common failing which hasn't been revised even on new trigger assemblies.
 

Dodgydave

Joined Jun 22, 2012
10,027
I happen to know the internals of the speed control have two copper contacts that short the motor out when the trigger is released, unfortunately it's typical for these contacts to burn out like they have in this case, its a very common failing which hasn't been revised even on new trigger assemblies.
If you can isolate the contacts and bring them out separately, then you can use this circuit to trigger a Triac, the contacts will link the Gate to MT2 when the motor is not powered and acting as a generator will fire the Triac and short the motor out, it will turn off when the voltage across it goes to zero, use a BTA16-600b Triac. Use 1W resistors.

.IMG_20201129_135457_9.jpg
 
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Thread Starter

*Leeroy*

Joined Nov 29, 2020
6
If you can isolate the contacts and bring them out separately, then you can use this circuit to trigger a Triac, the contacts will link the Gate to MT2 when the motor is not powered and acting as a generator will fire the Triac and short the motor out, it will turn off when the voltage across it goes to zero, use a BTA16-600b Triac. Use 1W resistors.

.View attachment 223665
Thanks Dave, i really appreciate you taking time to draw that circuit up for me. Unfortunately the speed control can't be opened without destroying it, so repair of the internal switch to fire the triac isn't possible in this case. It might be possible to incorporate a small external switch the trigger would hit when released if we cant come up with a better solution, it definitely wouldn't be easy if it is possible but its certainly an option at this stage.
 

Thread Starter

*Leeroy*

Joined Nov 29, 2020
6
What tool is this out of? (Part number) Is it possible to obtain a replacement or donor part?
It's an impact wrench. Trigger assemblies are available but they're prone to this issue as the design hasn't been revised.
Dave's triac solution looks great but the requirement of a switch closing to make the circuit and fire the triac makes it very difficult to incorporate so i'm still looking for ideas
 
Could be worth checking out local classifieds for an entirely new tool if this has been a long-term issue. Facebook marketplace, Craigslist, etc. often have good deals on power tools. Might even be able to find a brushless impact for cheap if you're patient. Could end up being less of an investment than the time and labor of repairing this one.
 

Thread Starter

*Leeroy*

Joined Nov 29, 2020
6
Could be worth checking out local classifieds for an entirely new tool if this has been a long-term issue. Facebook marketplace, Craigslist, etc. often have good deals on power tools. Might even be able to find a brushless impact for cheap if you're patient. Could offset the labor/expense of repairing this one.
Thanks for the input but this is a relatively new, high end tool. In this case it's well worth some thought to repair and improve the design.
 
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