Diodes in Electronic Control Circuits

[StarCat]

Can someone explain the function of the diode here? Assume 24 VDC power supplied.

 

[GoldDigger]

It is a snubber circuit, needed when the load reactance has an inductive component. It allows current to continue to flow through the load as the magnetic field collapses when the liquid level sensor tries to turn the load off. Without some such provision (alternately an RC circuit or a device with a suitable breakdown voltage) the voltage across the contacts or semiconductor switch in the level sensor could be damaging to them.
The choice of a diode offers the longest delay (in milliseconds) before the load actually "turns off" but that is probably not an issue here.

 

[synchro]

It is a snubber circuit, needed when the load reactance has an inductive component. It allows current to continue to flow through the load as the magnetic field collapses when the liquid level sensor tries to turn the load off. Without some such provision (alternately an RC circuit or a device with a suitable breakdown voltage) the voltage across the contacts or semiconductor switch in the level sensor could be damaging to them.
The choice of a diode offers the longest delay (in milliseconds) before the load actually "turns off" but that is probably not an issue here.

Also known as a clamp diode, suppressor diode, etc. But my favorite term is "free wheeling diode". It just sounds like the diode is having a good time doing its job. That name is more commonly used when a switching supply is involved.
Just to add to what GoldDigger said, the rate at which the current thru an inductor will change is proportional to the amount of voltage across it ( dI/dt = V(t)/L ). So because a diode clamps the voltage to only about 0.7 volts, as GoldDigger mentioned it will take longer to discharge the inductor current than with a Zener diode or RC snubber that allows a higher voltage to be developed.
Also note that the polarity of a DC voltage across an inductor will determine the direction that the ramp in current vs. time takes from its present value in amperes.

 

[StarCat]

Ok, thank you for the resposes. I am slow with DC Electronics as have not been exposed to a lot of it at this level. In this case the Level Sensor is switching Negative line of DC from B to C. Why the arrow to the right on that line and then the current flow through the Diode is back to the left exactly when?
When the load is " De-Energized?"

 

[winnie]

The arrow to the right shows the direction of current flow when the switch is powering the load. When the switch opens, the current tries to continue flowing in that direction.

The picture is a little misleading however because you should imagine another arrow right beside the load, pointing in the same direction, and the situation is when the _load_ has lots of inductance. The wire between the load and the switch has negligible inductance, so in this circuit we ignore it. When the switch opens the current through the load tries to continue to the right, but can't continue on to the switch and instead follows the alternative path through the diode.

 

[PaulMmn]

The arrow to the right shows the direction of current flow when the switch is powering the load. When the switch opens, the current tries to continue flowing in that direction.

The picture is a little misleading however because you should imagine another arrow right beside the load, pointing in the same direction, and the situation is when the _load_ has lots of inductance. The wire between the load and the switch has negligible inductance, so in this circuit we ignore it. When the switch opens the current through the load tries to continue to the right, but can't continue on to the switch and instead follows the alternative path through the diode.

The diode should be rated for the current normally flowing through the load, and at least as much voltage.

I've seen them used a lot around relays-- helps guard the coils from the energy left in the coil when the circuit opens.

 

[GeorgeB]

Also frequently used with DC solenoid coils on hydraulic and pneumatic valves. Plug-on "DIN 43650" style are available with them from the connector manufacturers.

The "free wheeling" term is true ... the diodes slow the turn-off time of the solenoids.

 

[StarCat]

Ok the purpose is clear, but the action not quite because this device is shunted around the load. I get that we are in the Electronics world now.
Does this device admit some current when the load is powered? Also, is most of its job done when the load is de-energized? Reading other resources, this configuration is said to appear like a " short circuit " which it does. Thanks for all responses they are all helpful.

 

[PaulMmn]

Yes, the diode does most of its work when the load is de-energized. This all assumes that the load is a coil, solenoid, or something where the 'load reactance has an inductive component' as goldigger said.

While the load is energized, the diode is 'reverse biased,' meaning it blocks current trying to flow through it. This is where one of the diode's ratings, maximum reverse current (?), comes into play. It's the maximum current trying to push through the diode 'backwards.' Theoretically, the diode does no work while the load is energized.

When the load is de-energized, there's still energy in the magnetic field around the coil and its core. The magnetic field 'collapses,' and induces a voltage on the coil winding. This is like a generator-- a magnetic field moveing across a wire creates a voltage. That energy has to go somewhere! The voltage creates a current out of the coil flowing in the same direction the current was originally traveling. The diode, being the path of least resistance, loops that current back to the input side of the coil. The resistance of the coil eventually turns the current into heat, but no wiring is injured (hopefully).

All the ratings have to line up-- the diode has to be able to handle the current from the coil, and the coil has to be able to stand the (temporary) short-circuit caused by the diode.

 

[Besoeker3]

Many of our control circuits were 24Vdc. The relays all had a diode fitted across the coil. A 1N4004 or similar.

Vishay 50V 1A, Diode, 2-Pin DO-204AL 1N4001-E3/54 | RS Components

uk.rs-online.com

Cheap snubber.

 

[LarryFine]

Ok the purpose is clear, but the action not quite because this device is shunted around the load. I get that we are in the Electronics world now.
Does this device admit some current when the load is powered? Also, is most of its job done when the load is de-energized? Reading other resources, this configuration is said to appear like a " short circuit " which it does. Thanks for all responses they are all helpful.

The diode's polarity in the circuit is such that it does not conduct under normal use. It only conducts when the power across the load and, if inductive, creates a voltage from the magnetic field collapse. It's really to protect the surrounding circuitry.

 

[StarCat]

Yes, the diode does most of its work when the load is de-energized. This all assumes that the load is a coil, solenoid, or something where the 'load reactance has an inductive component' as goldigger said.

While the load is energized, the diode is 'reverse biased,' meaning it blocks current trying to flow through it. This is where one of the diode's ratings, maximum reverse current (?), comes into play. It's the maximum current trying to push through the diode 'backwards.' Theoretically, the diode does no work while the load is energized.

When the load is de-energized, there's still energy in the magnetic field around the coil and its core. The magnetic field 'collapses,' and induces a voltage on the coil winding. This is like a generator-- a magnetic field moveing across a wire creates a voltage. That energy has to go somewhere! The voltage creates a current out of the coil flowing in the same direction the current was originally traveling. The diode, being the path of least resistance, loops that current back to the input side of the coil. The resistance of the coil eventually turns the current into heat, but no wiring is injured (hopefully).

All the ratings have to line up-- the diode has to be able to handle the current from the coil, and the coil has to be able to stand the (temporary) short-circuit caused by the diode.

Paul. Thank you. This makes things more clear.

 

[StarCat]

The diode's polarity in the circuit is such that it does not conduct under normal use. It only conducts when the power across the load and, if inductive, creates a voltage from the magnetic field collapse. It's really to protect the surrounding circuitry.

This comes off similar to bleed resistor on start capacitors for CSR hermetic compressors, and the big resistors I have seen on WYE-DELTA starters on Centravacs in the 1000 ton range. I have only had to start dealing with DC Electronics circuits at this level within the last few years. Until then it was mostly AC relay logic with Solid State controls ahead of some of that. Its a different world and some things about it come off as strange.
As an example, an MR16 track light with the lamp at 24VDC has a PSU in the fixture base to convert from 120 AC. If the DC output circuit does not see a load, it will not give any voltage and that missing power cannot be read with a test meter. These kinds of things come off as counter-intuitive.
Thanks for all responses.

 

[Besoeker3]

As an example, an MR16 track light with the lamp at 24VDC has a PSU in the fixture base to convert from 120 AC. If the DC output circuit does not see a load, it will not give any voltage and that missing power cannot be read with a test meter. These kinds of things come off as counter-intuitive.
Thanks for all responses.

To me, that makes no sense.
If you rectify AC, you get a DC output voltage whether or not you measure it.

three phase rectifier diagram - Google Search

:

 

[StarCat]

To me, that makes no sense.
If you rectify AC, you get a DC output voltage whether or not you measure it.

three phase rectifier diagram - Google Search

:

It also made zero sense to me. The Tech support guy told me that this is how those track lights behaved. I had a Fluke 87 on a ladder, with some wire stubbed out of the MR-16 connector and alligator clipped on. This guy said if there is no load it will not output power. Quote. Go Figure.

 

[LarryFine]

It's not unusual to see a switching power supply that operates like that; it's easy to program in, like having to cycle power after a short.

 

[drcampbell]

I have seen battery chargers that refuse to turn on unless the leads see a reasonable voltage of the correct polarity; I can't see any reason that a track-light power supply couldn't refuse to turn on if there's an open circuit.

 

[LarryFine]

I have seen battery chargers that refuse to turn on unless the leads see a reasonable voltage of the correct polarity;

Yep, and they won't charge deeply-discharged batteries unless you add an additional load for a while to "trick" it.

 

[tallgirl]

I have seen battery chargers that refuse to turn on unless the leads see a reasonable voltage of the correct polarity; I can't see any reason that a track-light power supply couldn't refuse to turn on if there's an open circuit.

Battery chargers are different from circuits which are designed to sense a load. With a battery charger, it will typically sense the voltage at the terminals -- which would be there if the battery had any charge -- then decide to turn on, or not, the charging circuit.