# Safety interlock

#### HighVoltage!

Joined Apr 28, 2014
177
Hello all. I have a DC energizing a project I am working. As for safety, I installed a micro switch on the door (interlock) which opens the circuit/power to the DC set. I know I can use relay logic (relays with PB) so that every time I want to restart set after closing door, I push a start button. However, I would like that the voltage only starts when the knob/input voltage is zero like some sophisticated sets I have seen that have "zero voltage start". I would like to include this without having to use another micro switch. Is there a simple basic circuit I can make that will only let you restart the set when voltage knob is at zero?

#### AnalogKid

Joined Aug 1, 2013
10,944
What "voltage knob"? Your question is not at all clear.
If the input power is DC, isn't that not zero by definition?
What is the input voltage?
What is the total system current being switched on and off?
If the system voltage is zero, what powers the control circuit and relay?

ak

#### HighVoltage!

Joined Apr 28, 2014
177
What "voltage knob"? Your question is not at all clear.
If the input power is DC, isn't that not zero by definition?
What is the input voltage?
What is the total system current being switched on and off?
If the system voltage is zero, what powers the control circuit and relay?

ak
-Voltage Knob of power supply
-Input voltage of voltage set is 120VAC. Output DC.
-Not more than 5A set can put out

#### HighVoltage!

Joined Apr 28, 2014
177
If the door was to be opened, I would like the set to NOT turn back ON when the door closes. I would like to have a zero voltage start and slowly raise voltage back up.

#### nsaspook

Joined Aug 27, 2009
12,796
I've seen initial physical position interlock switches in series with the power reset button used on variable controls with older equipment (rheostats and/or vari-transformers). Most were designed in the shaft bushing but a few were after the fact mods with roller-switches under a knob pointer or a indent on a round knob.

I've added position feedback to existing knobs but it's likely to be a hack it you want a electromechanical Safety Interlock interface instead of one designed into the control electronics for voltage setting.

Added roller-switch to indicate PRI battery position.

No idea what this is but almost all of my high voltage interlock systems do not zero program voltages on interlock failure, things all turn off because of lost enables but the programmed values are still sent to the high-voltage controllers. If a interlock door opens the interlock system turns off the master enable to all supplies. When the door is closed the master enable button is pressed and then each supply must be enabled for voltage output by the manual or auto-programming. Just closing the door and hitting the reset button WILL NOT enable high-voltage.

#### HighVoltage!

Joined Apr 28, 2014
177
Zero start meaning that after a door opens, the set cannot be turned on again unless you first have the knob set to zero. Once at zero, you may then turn on set and raise output voltage.

#### nsaspook

Joined Aug 27, 2009
12,796
Zero start meaning that after a door opens, the set cannot be turned on again unless you first have the knob set to zero. Once at zero, you may then turn on set and raise output voltage.
Sure, the question is how could the interlock check the knob is at zero and maybe, is this set to zero requirement a safety requirement worth the trouble or just a nice option to skip a startup step. As a designed in safety option it's pretty trivial with the correct hardware and software. To add it might be as easy as using a substitute or additional pot with a on/off switch at the start of rotation for the interlock if the power supply output voltage is controlled by a low level input voltage.

#### HighVoltage!

Joined Apr 28, 2014
177
Sure, the question is how could the interlock check the knob is at zero and maybe, is this set to zero requirement a safety requirement worth the trouble or just a nice option to skip a startup step. As a designed in safety option it's pretty trivial with the correct hardware and software. To add it might be as easy as using a substitute or additional pot with a on/off switch at the start of rotation for the interlock if the power supply output voltage is controlled by a low level input voltage.

Sure, the question is how could the interlock check the knob is at zero and maybe, is this set to zero requirement a safety requirement worth the trouble or just a nice option to skip a startup step. As a designed in safety option it's pretty trivial with the correct hardware and software. To add it might be as easy as using a substitute or additional pot with a on/off switch at the start of rotation for the interlock if the power supply output voltage is controlled by a low level input voltage.

The internal circuit is mostly a variable transformer with ac/dc rectifying bridge. What do you mean by "using a substitute"? What part number is that pot you provided?

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#### nsaspook

Joined Aug 27, 2009
12,796

The internal circuit is mostly a variable transformer with ac/dc rectifying bridge. What do you mean by "using a substitute"? What part number is that pot you provided?
By substitute I mean if your circuit used a pot without a off/on switch to vary voltage it could be replaced by one that does. It's just an example found by a google search for potentiometer. https://www.adafruit.com/product/3393
Not very useful if a variable transformer is used to adjust output voltage.

Usually a ramp down circuit is for the machine operational safety as the high voltage safety interlock system should provide the required operator safety.