nolabama

I have sometimes heard a circuit as being described as a latch circuit. This is not using latch relays. Can someone describe a simple latch circuit? A question I posed the other day here about ramp speed of VFD let me know exactly how lacking my ladder logic skills really are. Lucky for me I have had some time with an engineer on how he draws his circuits. Anywho I am puzzled by what is ment by a latch circuit.

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stuiec

Not that I know, but sounds like another way to describe a holding contact.

aaaaand I would be wrong.

. electronics Also called: latch circuit a logic circuit that transfers the input states to the output states when signalled, the output thereafter remaining insensitive to changes in input status until signalled again — vb .

Mulder

Sounds like a holding contact on a motor starter, maybe?

Frank Mc

As the other guys have mentioned it sounds like a holding contact....I.E. When you press a start button...it will energise a relay/contactor and one of its contacts will short the start button causing the circuit to latch...Pressing a stop button (n/c) is normally used to unlatch the circuit...

HTH
Frank

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Tom45acp

Try this web page, about the 8th diagram down. I do not recommend using this diagram in real life as a broken wire in the stop button circuit will prevent you from stopping the motor. You would use a NC button with a NO (XIC) contact in the ladder diagram.

http://www.allaboutcircuits.com/vol_4/chpt_6/6.html

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Big John

Yeah, that's the most common type of latching circuit I can think of.

I'd describe it as a circuit that locks itself into a certain position, e.g.: When one of the NO contacts on a relay will feed power to the relay coil after it's initially energized.

-John

JRaef

Technically, a seal in circuit is a seal in circuit, a latch circuit is something different. But... A lot of people mistakenly call a seal in circuit a latch circuit, so you need to know the context.

A latch (latched) circuit would be one that retains its state regardless of power application. A seal in circuit will reset if power fails, that's why you use them on motor control circuits because for safety you DON'T want the motor to automatically restart after a power fail is restored (in most cases). You would use a latch circuit if you DO want the circuit to remain in its previous state when power is restored, for instance with a lighting circuit.

To do a true latch circuit, you need a control device that is mechanically bi-stable. A latching relay or latching contactor has a mechanical or permanent magnet that retains the state, so it needs to have 2 coils, a latch coil and an unlatch coil. There are also electronic circuits that will retain their state as well, but not forever. They usually rely on batteries or capacitors to maintain the logic (or restore it so fast that you can't tell). Technically though, a simple on-off selector switch is a latching circuit, it's just not one that you can energize electrically.

The other kind of logic circuit that people often misidentify as a latch circuit is a "flip-flop" circuit. That is one where you have one momentary control device like a push button, and you push it once to energize, then push the same device again to de-energize the circuit. This CAN be a latch circuit IF the control device maintains its state, but that is a separate issue because you can also have a flip-flop that resets on power fail too.

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Big John

Good reply. I didn't know there was a difference between mechanically held and electrically held.

-John

dronai

With PLC logic you can program either way. I hear it called Latch, or seal in. Holding contact is the same as seal in.

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