Need to short DC motor terminals when not in use

[supak111]

Hey everyone, I'm making a gadget out of a 12v DC geared motor. When the motor is not being used I need it to stay in as close to the same location where it stopped as possible. I don't want to add any complicated mechanical brakes or anything like that, this is a cheap little gadget. I tested the gear motor when the 2 terminals are shorted and its super hard to turn so I think this would be the easiest way to keep it from moving.

Does anyone know of any simple circuits that I can add between my Geared DC motor and the 12v batter that would just short the terminals anytime there is no power applied from the battery?

 

[(*steve*)]

The super simplest way would be to wire up a relay powered by the external power source which shorts the motor wen the relay is not energised (and disconnects the motor from the power) and reconnects the motor when power is supplied.

 

[supak111]

Oh yea thats a great idea Steve. I actually have some 15mm relays, they don't take up much room and they draw super low current.

 

[Minder]

If you are using manual on/off switching, then a simple DPDT toggle switch will perform both.
M.

 

[Gryd3]

Hey everyone, I'm making a gadget out of a 12v DC geared motor. When the motor is not being used I need it to stay in as close to the same location where it stopped as possible. I don't want to add any complicated mechanical brakes or anything like that, this is a cheap little gadget. I tested the gear motor when the 2 terminals are shorted and its super hard to turn so I think this would be the easiest way to keep it from moving.

Does anyone know of any simple circuits that I can add between my Geared DC motor and the 12v batter that would just short the terminals anytime there is no power applied from the battery?

How is the motor controlled?
Do you require speed control... or is it simply on/off?

Take a peek at this:

It's a common cheap circuit in electric windows and locks on automobiles.
When at rest, both wires of the motor are shorted together on the + or GND terminal depending on how it's wired.
In the picture, there is an 'unlock', and 'lock' wire. Think of this like a forward reverse.
As soon as one of the inputs gets power, the corresponding relay toggles one of the motor wires to the other contact (Power / Ground) and the motor moves forward or back.
If you accidentally tell the motor to go forward and backward at the same time, you end up simply shorting the motor wires together again, but on the opposite pole then it's resting position. No harm done, and the current won't shoot through like it would when using a cheap H-Bridge design.

 

[Colin Mitchell]

Don't forget, the motor will creep when shorted.

 

[Minder]

Don't forget, the motor will creep when shorted.

Only when back fed by load or inertia.
M.

 

[supak111]

I was hoping to do it with only one relay, maybe something like this:

 

[Gryd3]

I was hoping to do it with only one relay, maybe something like this:

I have no reason why not. I recommended the two relay method on the assumption that the motor would be used in a bi-directional fashion, and the circuit could be use to control as well as short the motor leads together.

 

[supak111]

I won't be using the motor in both directions, I just need it to be shorted when motor is without power.

Space is kinda of a big deal so using 2 relays could be a problem. Like I said I have some mini 15mm relays "HK4100F", but even putting two of them might be hard in the space I have.

 

[Gryd3]

I won't be using the motor in both directions, I just need it to be shorted when motor is without power.

Space is kinda of a big deal so using 2 relays could be a problem. Like I said I have some mini 15mm relays "HK4100F", but even putting two of them might be hard in the space I have.

Only thing you may need to worry about is ensuring that the relay actually switches right away. For a moment, it will be a short circuit to power as well.
You may want to revise the pinout a little.

Ground on Pin 85.
Power to the Momentary Button, and the other side of the momentary button to Pin 86 and 87.
Pin 30 will go to the + of the motor.
Pin 87A will go to the - of the motor.
Ground will also go to pin 87A.

At rest, pin 30 and 87A are shorted together. Which will short the motor leads together and tie them to ground.
When power is applied from the button (Ensure the button is rated to handle the motor current) the relay toggle from holding the + motor wire to ground, and swap to power. This arrangement should ensure you do not accidentally short he power supply but will REQUIRE a Double Throw relay.

 

[supak111]

Oh nice, that arrangement definitely works better. I'm running this gadget of a 12v car battery so shorting it out even for a tiny bit with my other arragement probably wouldn't be a good idea

 

[Minder]

So what is picking the relays up?
M.

 

[(*steve*)]

This isn't the tidiest diagram I've ever done, but it should give you the idea.

 

[Minder]

So if you are manually switching why the relays?
See post #4.
M.

 

[Gryd3]

Something is most certainly wrong with me...
I should have just vouched for the SPDT switch, or Drawn up Steve's design.
I need a holiday

 

[supak111]

Thanks for everyones help. Soooo after trying the relay successfully which was great I found out that even the mini relay won't fit inside my gadget by a few millimeters.

What I ended up doing is using a momentary DPDT switch to control the gadget in both directions and I just soldered 2 diodes, 1 to the top 2 pins, and one to the bottom 2 pins in opposite directions.

Works good. Just have be careful when connecting the battery to the switch to connect it the right way. Connecting it the wrong way would short out the battery.

 

[Gryd3]

Thanks for everyones help. Soooo after trying the relay successfully which was great I found out that even the mini relay won't fit inside my gadget by a few millimeters.

What I ended up doing is using a momentary DPDT switch to control the gadget in both directions and I just soldered 2 diodes, 1 to the top 2 pins, and one to the bottom 2 pins in opposite directions.

Works good. Just have be careful when connecting the battery to the switch to connect it the right way. Connecting it the wrong way would short out the battery.

This would create an voltage drop, but if polarity is a concern, you can always put a diode in-line with one of the battery terminals to prevent current flow in the wrong direction

 

[supak111]

Why would this create a V drop? Wouldn't the diodes only be used when the motor is NOT running?

I was thinking about adding a diode in line but that now would create a 0.07 voltage drop.

 

[Gryd3]

Why would this create a V drop? Wouldn't the diodes only be used when the motor is NOT running?

I was thinking about adding a diode in line but that now would create a 0.07 voltage drop.

Correction. My post above mentioned the voltage drop, but it was intended to be only for the diode that is connected in series.