I'm using a littelfuse relay shown in the link to switch motor directions. I'm sending a pulse to the relay switch of 0.02s in order to switch between the L1 and L2 contacts. The pulse seems to be too short and the relay picks up on the pulse sometimes, not always, which is a major problem for me. If i increase the time of the pulse duration, the relay works perfect but the accuracy of my project reduces, so the shorter the time the relay picks up on the pulse, the better. What are some alternatives to this relay, or how can i improve my design to make sure the signal is picked up by my relay? Thicker wire, shorter wire, caps? I dont know. I'm only knowledgeable in the basics plus just a tiny bit extra and i can't figure this one out. Thanks in advance for your time and help!

edit: I've included a wiring diagram with part numbers available. The counter is counting up to a pre-set number XX. Once it gets to XX, counter sends pulse to Relay #2 and reverses motor through the speed control. The pulse duration needs to be AT MOST 0.02s in order for my accuracy to be sufficient. I was thinking if there were some kind of solid state alternating relays? The motor is a 90V DC from Dayton.

I hope that's a little more information. I didnt want to add too much so that you all didn't have to do all the work, but any and all suggestions are welcomed.

LittelFuse Relay


enter image description here

  • \$\begingroup\$ please edit your question and add some more details like type of motor, ratings and schematic if possible \$\endgroup\$ – Satish Singupuram Jan 16 '19 at 19:12
  • \$\begingroup\$ I would recommend you look on digikey, mouser, or your preferred vendor for some kind of actuation time rating. This will tell you how long it takes for the relay to actuate after you send the signal. I'm not familiar with alternating relays, but it looks like they're some type of single-coil latching relay? \$\endgroup\$ – Hearth Jan 16 '19 at 19:13
  • \$\begingroup\$ What is the motor voltage and DC resistance? How often is it activated. Surge current can be 20x the rated current due to back EMF making V up to twice normal /DCR = 2x Isurge 20ms is pretty fast for a motor with inertial loads \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 16 '19 at 19:14
  • \$\begingroup\$ " When the rotary switch is in the “alternate” position, alternating operation of Load A and Load B occurs upon the opening of the control switch S1. " So you don't need a short pulse, you just need to control the timing of the end of the pulse. \$\endgroup\$ – Phil G Jan 16 '19 at 19:18
  • \$\begingroup\$ @SunnyskyguyEE75 90V DC. Not sure about the DC resistance. Here's the site I got it from: link \$\endgroup\$ – Erick Jan 16 '19 at 21:17

The datasheet for the relays says that these are intended for operation from an AC supply. This is either 50 or 60 Hz.

You state that your pulse is only 20ms long and results in unreliable operation. Increasing the pulse width increases the reliability of the relay but degrades the accuracy of your process.

You may now see where I'm going with this.

Assume that the signal detect network for the control input to the relay uses a half-wave rectifier. The control signal timing is asynchronous relative to the AC mains that the relay operates from.

Depending upon the phase of the AC mains supply when the control signal is initiated, the relay may not see an entire half-wave of the control signal. It is entirely possible that the relay sees only two pulses: the end of one half-cycle and the beginning of the next half-cycle.

The datasheet does not mention a minimum control pulse width but I suspect that a minimum of two or three cycles of the AC mains is required for reliable operation.

In other words, the relay that you have chosen is most likely NOT suitable for your requirements.

  • \$\begingroup\$ I did not think about this at all !! That makes complete sense. What if I changed to a DC supply? Seems like that would be the next possible option by keeping a relatively similar wiring diagram. Thanks a lot for your response. Enlightening. :) \$\endgroup\$ – Erick Jan 16 '19 at 21:56

from https://www.grainger.com/product/DAYTON-DC-Gearmotor-90VDC-1LPW5 that torque and RPM computes to 0.02 HP output power or equiv to 16W or 180mA full load and 1.8Ap start and 3.6Ap change direction so a 15~20A relay is what I suggest. with an RC snubber for more contact life and EMI.

Required <=20ms

Operate Time 13ms
Release Time 10ms

enter image description here 12V or other options avail in datasheet https://www.digikey.com/product-detail/en/te-connectivity-potter-brumfield-relays/K10P-11D15-12/PB324-ND/254541


Your wiring diagram was useful. I can tell you what I would try but you may not be able or willing to do this.

I see two approaches that may allow you to keep using your existing Littlefuse relay. Both involve increasing the pulse width of the trigger signal without affecting the normally-closed contact on the counter module.

1) Use another fast-acting timer to stretch the pulse coming out of the counter module that feeds the Littlefuse relay. Problem is that this added relay may have the same limitation as the Littlefuse relay. But it is something that you can look at.

2) Use a diode & capacitor to stretch the pulse into the Littlefuse relay. This requires some experimentation on your part because we don't know the polarity of the trigger signal. Nor do we know the input impedance of the control signal pin. But this is the approach that I would look at.

Start by adding a diode (1N4005 or similar) in series with the control input right at the Littlefuse relay (pin 5). Test to see if the system still works as before. If it doesn't work, reverse the polarity of the diode and test again. If it still doesn't work, abandon this approach.

Now add a 1uF 400V capacitor between pins 3 & 5 of the Littlefuse relay. Try the system again. There is a really good chance that 1uF is way too large but it's a starting point.

Now just try different values of capacitor until you get satisfactory operation. I would iterate until I saw a small change between capacitor being present or not, then try a capacitor value perhaps 3 to 5 times larger than that minimum value.

Something like this is extremely easy for me to do - I'm an old-timer and I still have my old resistor and capacitor substitution boxes hanging around. These are very old-school and intended for working with tube-type equipment. The capacitors in the capacitor substitution box are rated at 600 Vdc.


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