DeMar Electronics sells a Twin Coil Latching Relay. It is advertised as dual coil 12 VDC with contacts being 1 amp @38 VDC. I would like to use this relay to control indicator lights on my control panel and ultimately some signals.
I am currently using the AC terminals of my power pack to throw the Atlas Snap Switches. Can I use this AC voltage to trip the relay, or will it fry?
Alternatively, can I use the fixed DC terminals on the power pack to throw the Snap Switches?
Thanks, Steve
It will probably buzz at best, overheat burnout at worst. Specs state DC… use DC.
A latch realy uses two coils, the operate coil and the reset coil. Both coils are rated for intermittent use only. The operate coil is energized to close the contacts and the contacts are held closed by a mechanical latch. The reset coil is energized to trip the latch and open the contacts. The DC coil may not operate on AC due to the high coil resistance. Some reed relays are made in latch form and the contact is held closed by a bias magnet. THe reset coil over powers the magnet and the contacts open.
A 50 cent full wave bridge will fix you right up, but most transformers have a 16VAC output which will give you about a 22 VDC output after rectification, way too much. So add a small resistor to the coils. Off the top of my head a 220 ohm should keep the voltage in tolerance for the coil. Just hook it up and measure the coil voltage and adjust the resistor value up or down to get say 10 to 12 volts at the coil when the power is applied. Also, some latching relays have a built in diode to shunt field collaspe. So the coil is polarity specific, hook them up backwards and the diode becomes a short circuit until it fries. This is easy to check with a vom. one way the resistance will be close to zero, the other way it will be say 400 ohms. The 400 ohm way is the correct way to hook her up. Fred
Thanks Fred,
But I’m pretty near totally ignorant about this type of electrical wiring. As I understand your post, the rectifier will change the AC to DC. Where would it be in relation to the switch on the control panel for the turnout and the turnout itself? Would the resistor be wired in one wire from the panel switch to the relay? If so would it make any difference which wire?
Thanks, Steve
I’ve got a bunch of those very same relays. They will “seem” to work on AC, but actually they don’t work properly and are unpredictable. Instead, use that magical turnout elixer, the Capactive Discharge circuit. It starts with the full-wave rectifier bridge that Fred recommended, but then adds a big capacitor and a couple of resistors. You use the output of the circuit to drive all of your turnouts, plus these relays. The circuit protects the twin-coil switch machines and the relays, too. If you Google for something like “model railroad capacitive discharge circuit” you will get several schematics you can work from. It’s about $5 worth of parts from Radio Shack. You can buy a CD circuit pre-built for about $30.
Although these are twin-coil relays, they are also polarity-sensitive, unlike turnout twin-coils. If you wire the coils with the reverse voltage, they will throw backwards.
I use mine to control LEDs for both signals and control panel indicators. All of these are simple turnout-position indicators (even the signals) so I just wire the relays in parallel with the switch machines. I typically use dual-color (red/green) 3-lead LEDs.
My last suggestion is to wire these up on the bench. Last night I was finishing up my control panel indicators, and I had to solder wires on to the very small relay pins while crawling around under my layout. You want to avoid that, believe me.
Yep, a CDU is the way to go for Turnout control, but he was wanting the latch relays to power signal lights I thought? Anyway, I have a basic CDU how to on the net at http://www.2guyz.info/Content/pa=showpage/pid=27.html
A latching relay for signals could be wired in after the momentary switch, one relay coil to each side, the resistor would go in series with the relay coil anywhere. The cdu will have plenty of power to trigger both. Fred
I would use the CDU to drive both the turnouts and the relay coils. The signal lights are a completely independent circuit, running through the relay contacts. This gets around the issue of having an AC supply but needing DC to drive the relay coils, even though the turnouts will work on AC.
On these relays, by the way, the common contact pins are NOT between the “red” and “green” pins like they would be on a DPDT toggle switch. Check the wiring diagram provided. I found out the hard way because I thought I knew everything.
I’d want to see one of these relays before purchasing at the price DeMar is asking. All Electronics (http://www.allelectronics.com) has a 3VDC DPDT Dual Coil Latching Relay for $1.35 each. The problem with these tiny relays is that the pins are very short, sharp, brittle, and difficult to solder to; and the pin spacing does not fit a DIP IC socket. From the sketch on DeMar’s web site, they are the same size as the ones sold by All Electonics but are 4 times the cost.
Well, OK…
The spacing on the pins fits into a PC board, which makes a neater installation.
I would agree that the allelectronics selection and price are probably better, though. This one has a 12v DC coil, as it says on the other side. The coil connections are on the left in this picture. The black wire goes to the center of the upper pole, with the top green and red wires on the switched side. I didn’t use the other pole on this relay. The guy on the coin is Roosevelt. No, I’m not quite old enough to have known him personally.
Thanks guys.
I think I’m begining to see the light. But, I have another question. The CDU that I plan to build discharges at 35 volt DC. The dual coil latching relays from allelectronics and DeMar are rated from 3 volts to 12 volts. Will they handle the 35 volt pulse of the CDU? If not, how do I reduce the voltage to the relay?
Steve