SPDT Latching Relay Unlatching Problem

Thread Starter

Penekis

Joined Jan 8, 2018
5
Hi all,

I recently purchased a SPDT latching relay (Takamisawa LZL-6H) and I'm having trouble delatching the relay. The relay has a nominal coil voltage requirement of 6Vdc and requires two opposite polarity voltage pulses to latch and delatch the relay. I know in order to create this reverse polarity signal, an H-bridge can be used (which is what I will be using in the final circuit). I first wanted to see how the relay works, before including it in a circuit with other components, in order to prevent having to debug an entire circuit if something goes wrong.

The problem is, I cant seem to get the relay to unlatch. What I am doing currently is simply connecting the coil pins of the relay to a DC power supply set at 6 V with no current limit. When I use one polarity pulse (manually connecting the positive and negative wires to the coil), the relay latches and the NO pin is connected to the COM pin. Then, when I use an opposite polarity pulse (again, manually switching the positive and negative wires), I can hear the relay make a 'clicking' sound (which I normally take as a sign that the relay jumper has switched between the NO and NC contacts), but the NO pin stays connected rather than having the NC pin reconnected to the COM pin.

I have tried this same procedure with and without a 54 ohm (5 x 270 ohm 1/4W resistors in parallel) load connected to an external 9V battery in series with the COM-NC as well as COM-NO connection to simulate the minimum switching load, but both situations result in the same problem as in the previous paragraph. I did notice that when connecting the coil to the 6V supply, the coil draws around 330mA, which doesn't quite match up with the 80 ohm nominal coil resistance. I bought two of these (in case I broke one), and both act the exactly the same way. So my thinking is that the problem is the method I am using, rather than the relay itself. The link to the website where I bought the relay: http://www.mantech.co.za/ProductInfo.aspx?Item=13M1705

I have searched online using various phrases and keywords, but most of the results either show how to make a latching relay using a normal relay, or just state that an H-bridge type setup is needed to produce the reverse polarity signals. So my question is, does anyone know what I may be doing wrong. Is there something obvious I might be missing in regards to delatching the relay or is this a common problem with latching relays? I have not included any circuit diagram, but if I need to upload one, that wouldn't be a problem since it is an extremely simple setup.

Any help/advice will be greatly appreciated!
 

MrAl

Joined Jun 17, 2014
7,140
Hi,

Some latching relays have a second coil that you energize to 'unlatch' the relay (really just latching in the other direction). If you have only one coil though then you must have the reverse polarity type so it sounds like yuou are doing it right.
The only other thing then is the voltage. If you have a 5v coil and you apply 5vdc one way and it latches one way, then when you reverse polarity it latches the other way.
If it does not do this then it is either defective or it is not really a latching relay.
Perhaps try another relay.

The two coil types are easier to drive because you dont need an H bridge.
 

Thread Starter

Penekis

Joined Jan 8, 2018
5
Hi,

Some latching relays have a second coil that you energize to 'unlatch' the relay (really just latching in the other direction). If you have only one coil though then you must have the reverse polarity type so it sounds like yuou are doing it right.
The only other thing then is the voltage. If you have a 5v coil and you apply 5vdc one way and it latches one way, then when you reverse polarity it latches the other way.
If it does not do this then it is either defective or it is not really a latching relay.
Perhaps try another relay.

The two coil types are easier to drive because you dont need an H bridge.
Hi MrAI,

Thank you for your quick response. As soon as I posted the thread, I realized I did not specifically mention it is a single coil relay, although I did describe it as such. I am now in the process of editing the post to make it a bit clearer.

As for the part being defective, I also thought this might be the case, but having two (brand new) parts both being defective lead me to think otherwise. I would prefer using a double coil latching relay for the simplicity, but I bought this specific one due to the low price - the double coil variations I have seen are more than double the price.

Lastly, after pulsing the coil and disconnecting the relay coil from any voltage whatsoever, the NO-COM connection still remains intact, leaving me to believe that it truly is a latching type relay. Am I assuming wrong with this regard?
 

MrAl

Joined Jun 17, 2014
7,140
Hi MrAI,

Thank you for your quick response. As soon as I posted the thread, I realized I did not specifically mention it is a single coil relay, although I did describe it as such. I am now in the process of editing the post to make it a bit clearer.

As for the part being defective, I also thought this might be the case, but having two (brand new) parts both being defective lead me to think otherwise. I would prefer using a double coil latching relay for the simplicity, but I bought this specific one due to the low price - the double coil variations I have seen are more than double the price.

Lastly, after pulsing the coil and disconnecting the relay coil from any voltage whatsoever, the NO-COM connection still remains intact, leaving me to believe that it truly is a latching type relay. Am I assuming wrong with this regard?
Hi,

How do you know for sure it is a latching relay if you can never get it to latch the OTHER way?
You are saying that you pulse it and remove the voltage and it "STAYS" in contact one way, but if you have no way to flip it the other way how do you know it is working at all? Can you manually flip it back?

The operation of these things is fairly simple so if it doesnt work, it doesnt work :)
If you have read the data sheet and followed that then i would assume the test is valid.
Maybe you got the wrong voltage unit it's actually higher or something.
 

Sensacell

Joined Jun 19, 2012
2,564
The data sheet makes it seem to be a totally standard non-latching relay

It may be a pulse latch on / pulse latch off type.
Think of a ball point pen, each click changes the state.
 

Thread Starter

Penekis

Joined Jan 8, 2018
5
Hi,

How do you know for sure it is a latching relay if you can never get it to latch the OTHER way?
You are saying that you pulse it and remove the voltage and it "STAYS" in contact one way, but if you have no way to flip it the other way how do you know it is working at all? Can you manually flip it back?

The operation of these things is fairly simple so if it doesnt work, it doesnt work :)
If you have read the data sheet and followed that then i would assume the test is valid.
Maybe you got the wrong voltage unit it's actually higher or something.
Hi MrAI,

After failing to have it unlatch by applying the opposite polarity signal, I decided to open them. I can manually unlatch the jumper contact from the NO connection point - it then moves back to the NC connection point. Thereafter I can pulse the coil again, and it closes to the NO connection point again, but still won't unlatch. My assumption is that if the NO connection stays closed, even after power is removed, surely it must be a latching type since NO cant be Sometimes Open? :)
 

MrAl

Joined Jun 17, 2014
7,140
Hi,

Yes i see "LATCH" and "UNLATCH" on the product sales page, but no such thing on the data sheet linked to.

What happens if you unlatch it manually then apply reverse polarity?

I've had both types of latching relays in the past and they are not this hard to operate, so something must be wrong somewhere.
 

Thread Starter

Penekis

Joined Jan 8, 2018
5
The data sheet makes it seem to be a totally standard non-latching relay

It may be a pulse latch on / pulse latch off type.
Think of a ball point pen, each click changes the state.
Hi Sensacell,

Yes, the datasheet doesn't mention anything about latching. A lot of times I find that information provided by that supplier is accurate, even though not stated specifically in the datasheet provided - so I took their word for it in good faith hoping they have access to some more accurate information than I have.

I also had the idea of latch on, latch off, but this does not work. When the jumper is at the NC position (after manually resetting), one pulse (doesn't matter the polarity) latches to the NO position, while further pulses of the same polarity does nothing - not even a 'click' sound. Once I reverse the pulse polarity, the relay 'clicks', but remains at the NO position.
 

Thread Starter

Penekis

Joined Jan 8, 2018
5
I did some experimenting, and it seems that at around 4.1 V, the relay latches and unlatches perfectly. I also found that the relay will latch to the NO pin at a minimum of 4.1 V, and unlatch from the NO pin at a minimum of 0.8 V (opposite polarity to the 4.1 V), with no load attached. At any voltage higher than 4.1 V, the relay unlatches, but then latches again to the NO pin very fast. As I decrease the operating voltage closer to 4.1V, I can actually see the movement, although still very fast.

Looking at the datasheet again, p.6 and p.7 actually agrees with this quite well. I was under the impression that the same voltage, albeit different polarity, pulses CAN instead of MUST be used to turn this single coil latching relay on and off. I think the best solution now might be to include resistors in the turn-off leg of the H-bridge, in order for the coil voltage to be low enough.

I am going to test and implement this. If I run into any problems, I will post it here for future reference.

Thank you very much for your help!
 

MrAl

Joined Jun 17, 2014
7,140
Hi,

It sounds like you could be seeing oscillation in the coil. If the coil oscillates even once, the current would reverse and that would cause the relay to latch one way and then as the coil is disconnected latch back the other way again.
Check to see what happens when you keep the coil current flowing in the desired direction instead of disconnecting right after it appears to latch.
 

Alec_t

Joined Sep 17, 2013
10,907
330mA is a surprisingly high coil current for such a small relay. Perhaps that causes permanent magnetisation of some part, sufficient to hold the latch position even when the unlatch pulse is applied.
I suspect the relay supplied does not match the one advertised.
 

philba

Joined Aug 17, 2017
960
If I had a choice of believing the data sheet or the vendor's write-up, I'd take the data sheet's version any day. If the vendor explicitly talks about why the data sheet is wrong I might believe.
 

BobTPH

Joined Jun 5, 2013
2,259
The datasheet in the sellers link is for a non-latching LZ series relay, while the relay being used is LZL series.

It might help to find the correct datasheet.

Bob
 

MrAl

Joined Jun 17, 2014
7,140
Hello,

So someone posts 100 years later on the exact same day and asks a question about a similar product Ok it was really 2 years later but it's strange that it was on the exact same day of year. Even stranger they joined that same day too.

The person that did this should really start their own thread it sounds like a different question anyway. A relay needs energy to switch and that energy has to come from somewhere.
 

MrAl

Joined Jun 17, 2014
7,140
This is a latch once relay. I didn't know these were still available. Sales were always low. (Tag: Sarcasm)
Hi,

Oh yes he he those kind are rare these days :)

Maybe the OP (or should i say the second OP) had a serious question but it should have been a new thread.
 

MisterBill2

Joined Jan 23, 2018
5,196
OK, so it latches and unlatches properly with a lower voltage. The latching mechanism is a permanent magnet, I presume. And I think that you stated that it appeared to function with a lower voltage. It was not clear if it both latched and unlatched with that lower voltage, but if that is the case then it may work to put a very thin non-magnetic spacer between the coil pole piece and the moving segment. That will reduce the magnetic force a small amount, which seems like it may be the solution to the problem.
 
Top