Hiya! In need of some help, auto relay shutoff for DC less than 24V

mikemikecnc

Joined Feb 15, 2022
10
Hiya!

Ok so I have a 24VDC brake that is messing up and its because when the voltage to it goes to 24vdc or more, it deactivates (releases brake), but the drive going to it needs to shut off (go to 0Vdc) to apply the brake.

Normally 24vdc = brake release
Normally 0vdc = Brake applied

However.... I am getting 16vdc when I need it to goto 0vdc to apply the brake.

The voltage comes off a large servo amplifier board. Instead of buying a new board (tons of money), im tying to design a simple DC relay circuit to CLOSE when 24vdc or more is applied, yet OPEN the moment 20VDC or below is seen, with NO DELAY.

Can anyone help or point me to a design? I have tried many setups with simple relays, but the residual voltage is keeping the relay closed, until about 5+ seconds go by and then closes becauses of losses.

Thank you in advance! Yall are awesome!

Mike

Ya’akov

Joined Jan 27, 2019
5,642
How many amps does it have to handle?

mikemikecnc

Joined Feb 15, 2022
10
Hiya! I would say max 5 amps, gauge wire to it is 16 gauge

Ya’akov

Joined Jan 27, 2019
5,642
Something like this might work but I think I would use it to drive the coil of a larger relay because the relay on the board doesn't inspire confidence.

mikemikecnc

Joined Feb 15, 2022
10
HIya, I actually bought one, the problem is that it has as 5 second on and 5 second off delay.... unless I can modify it? I dont have a schematic thoo... maybe shrinking the cap size?

sghioto

Joined Dec 31, 2017
3,270

mikemikecnc

Joined Feb 15, 2022
10
Hiya, that may work, the challenge with that is the reference and power voltage is one in the same, however I could tap into the 24vdc voltage from the control for reference voltage, if it drop the it with trigger relay off sounds like?

MisterBill2

Joined Jan 23, 2018
11,874
Areyou able to show us the circuit of the amplifier board? We may be able to help spot the failure and you could replace a part and avoid the problem with no changes. a much better fix.

mikemikecnc

Joined Feb 15, 2022
10
Hiya MisterBill2, no can do, is a CNC Servo amplifier board from Haas... so probably not...

sghioto

Joined Dec 31, 2017
3,270
Hiya, that may work, the challenge with that is the reference and power voltage is one in the same, however I could tap into the 24vdc voltage from the control for reference voltage, if it drop the it with trigger relay off sounds like?
The relay module would be powered by a steady 24 supply and that would also be the reference. The 16 to 24 voltage would be the input.

mikemikecnc

Joined Feb 15, 2022
10
Awesome sghioto, this sounds like a great fix, just bought it and it will be here this weekend. TY! ill let yall know about how it goes

MisterBill2

Joined Jan 23, 2018
11,874
OK, no complex circuit required. You will need to add a relay and a resistor and a bit of wiring, though. The relay will need to operate on probably six volts DC, which should be fairly common, or it can be a 5 volt DC relay The one side of the coil will go to the 24 volt supply and the other side will go to the output that drops to 16 volts. So when the output is high there is no voltage across the relay coil, but when the output drops to 16 volts then you will have about 8 volts on the coil, which is why you need a series resistor between the +24 volts and the coil. So then the relay will operate when the brake should engage, and release when the brake should release. So the contacts must be able to handle the brake current.

I am not aware of the complexity of the servo amp board but it seems like the failed part is in the spike protection circuit area. Can you eve get to the board to examine it??

sghioto

Joined Dec 31, 2017
3,270
That actually should work. Dagnabit, I wish I had thought of that first.

Last edited:

sghioto

Joined Dec 31, 2017
3,270
I suspect the 24 volts when the brake is off is a reading through the brake solenoid or electro magnet to the 24 volt supply.
Disconnect the control wire from the brake circuit and connect it to the 5 volt relay and use the relay contacts as the control for the brake.

MisterBill2

Joined Jan 23, 2018
11,874
My preference is always to have one side of the load tied to common,and switch the source side. AND, as the feed to the loade would be pulling it up to 24 volts to release the brake, (Brake released ) There are spring applied brakes and spring released brakes. For an arrangement of the brake applied when the drive is off, that would be a spring applied brake.
But I am sure that the TS will be able to figure that part out.

AND STILL, it seems that it should be possible to repair that big board. I have repaired things via email before, so I know that it can be done. At least sometimes.

mikemikecnc

Joined Feb 15, 2022
10
Hiya MisterBill2, Thank you for this, the servo board is incased inside the machine and not fun to take apart....

However I do believe that your idea would work as well. I tried a simple 1 Ohm resistor to drop the voltage the problem with the relay coil wont shut off until it goes pretty low... so 24volts is decativated brake, and then it goes to like 16 volts, but this is still too much voltage and then drains and then goes to 0

mikemikecnc

Joined Feb 15, 2022
10
Really appreciate you sghioto and Misterbill2, I will actually pull the board tomorrow and trace wires to see where the brake circuit is at. Now that I am thinking about it, there are 2 wires to the brake, 0v and 24vdc. Ill need to chase them and see if the go to the servo board for the Z axis, or if they just goto the control board, which im thinking it does because E-stopping the system is what releases voltage, and applies the z axis brake to the motor

MisterBill2

Joined Jan 23, 2018
11,874
OK, and perhaps the problem is on the control board and not the servo amplifier board. Taking advantage of a smaller voltage change to engage a relay is a good cheating trick. And now I am thinking that the brake is spring applied and electrically released. Let me know if I have that reversed. But it should be that way to stop motion if the power goes out. It may be just one part has failed and it might run as much as $2 or even$5 for a replacement. But changing parts on a circuit board does demand good soldering skills.

mikemikecnc

Joined Feb 15, 2022
10
Hiya MisterBill2, totally agree with ya, Ill dive in this weekend and reach back to yall

mikemikecnc

Joined Feb 15, 2022
10
Hiya yall! Ok so i tried a lot of things to get it working. sadly the input (reference voltage) is very damaged, and all over the place.

So i simply added a seperate 24v power supply, rigged a simple relay to the Estop switch, and its fixed now.

Thank you all for your help!

I feel a reinvigeration for electronics now! Hahah

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