RC circuits relay delayed slightly, perhaps 0.05 seconds

Thread Starter

billrvolz

Joined Jul 20, 2021
21
I have two relays that I want to trigger from the same pulse from another controller. The pulse is a square wave 5V peak 0.10 second in duration (adjustable in 0.10 second increments).

However I want one relay to be delayed slightly, perhaps 0.05 seconds. I connected the controller supplying the pulse to one relay and then connected the same controller output through a 100 ohm resistor in series with a 1000 microfarad electrolytic capacitor (negative side toward the relay input) which should have produced a 0.1 second delay. However I didn't see any delay. I tried a 330 ohm and 510 ohm resistor and still no delay. I don't know the voltage level at which the delayed relay will trigger but it's somewhere under 2.08 volts. What's wrong with this circuit?

If I calculated this correctly, and I assume that the relay triggers at 1.0 volts, a 62 ohm resistor with the 1000 mf capacitor should attain 1.0 volts in 0.1 second (with 5 volt peak).

Thanks in advance.
 
Last edited by a moderator:

LowQCab

Joined Nov 6, 2012
1,078
Don't use mechanical Relays if You want accurate control over Switching-Timing.
Putting a Capacitor across one Coil will also slow Break-Time of that Coil,
a lot more than the Make-Time, but maybe You don't care about the Break-Time,
we don't know what You are trying to accomplish.
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Thread Starter

billrvolz

Joined Jul 20, 2021
21
Thanks,

I'm trying to operate a latching solenoid sprinkler valve. It takes 9-11 volts to open and reversed polarity to close close the valve.

I'm using this latching relay wired so that the contacts reverse the voltage on each pulse. That is fed to the common and NO contacts on this momentary relay to operate the solenoid.

In theory the relays should operate simultaneously, but in practice they don't. Sometimes the momentary relay operates first applying power to the solenoid, then the latching relay reverses the voltage causing a rather large transient from the solenoid that winds up shutting down the entire DC power buss. Right now the solenoid is powered by a 9V battery and that's absorbing the transients but it can't be good for the solenoid. I'd like to operate the solenoid from the 12V battery/solar power I already have available and not have to worry about replacing the 9V battery (it involves climbing up a 25 foot tall water tank in the middle of nowhere). To power the solenoid, I'm using a 16 ohm resistor in series with the solenoid to drop the voltage from the 12.5-14.5 V I have to about 10V for the solenoid. I don't have it now but I'm considering putting a diode in series with the 16 ohm resistor to prevent any negative transients from the solenoid. Because the solenoid needs two polarities, it does not have any diodes (I checked).
 

Ian0

Joined Aug 7, 2020
3,181
Try this.
F4694271-A97C-464B-BE8F-EB9A15DFD832.jpegIf it’s a digital signal, replace the switch that I have drawn with a driver IC such as a MCP1401,depending on the current requirement of the solenoid.
 

eetech00

Joined Jun 8, 2013
2,565
In theory the relays should operate simultaneously, but in practice they don't. Sometimes the momentary relay operates first applying power to the solenoid, then the latching relay reverses the voltage causing a rather large transient from the solenoid that winds up shutting down the entire DC power buss.
Is the momentary relay a significant distance from the latching relay?
Is there a diode across the coil of the momentary relay to suppress back EMF? If not, there should be.

Might be able to drive the solenoid directly using a small driver circuit.
Can you provide a part number for the solenoid so we can determine the operating characteristics?
 

Thread Starter

billrvolz

Joined Jul 20, 2021
21
Is the momentary relay a significant distance from the latching relay?
Is there a diode across the coil of the momentary relay to suppress back EMF? If not, there should be.

Might be able to drive the solenoid directly using a small driver circuit.
Can you provide a part number for the solenoid so we can determine the operating characteristics?
Thanks

The two relays are about an inch apart, mounted right next to each other.
My DMM says there are diodes on the coil of the momentary relay.

The solenoid is a Hunter DC latching solenoid part number 458200. I've found almost nothing on the specs for that solenoid. It's probably rebranded from someone else's solenoid.
 

Ian0

Joined Aug 7, 2020
3,181
Thanks

The two relays are about an inch apart, mounted right next to each other.
My DMM says there are diodes on the coil of the momentary relay.

The solenoid is a Hunter DC latching solenoid part number 458200. I've found almost nothing on the specs for that solenoid. It's probably rebranded from someone else's solenoid.
I believe that it is 9V rated. In the absence of any other specs, could you measure its DC resistance.
 

Thread Starter

billrvolz

Joined Jul 20, 2021
21
I believe that it is 9V rated. In the absence of any other specs, could you measure its DC resistance.
From the current and voltage it was determined to be around 47 ohm. It's up at a water tank 2 miles away so I can't measure it right now.
 

eetech00

Joined Jun 8, 2013
2,565
458200 DC Latching Solenoid Specifications:

458200
• Minimum opening/operating voltage: 6 VDC
• Maximum recommended voltage: 9 VDC
• Coil resistance: 4.8 ohms nominal
• Maximum operating pressure: 200 PSI
• Wire leads: 18' of 18 AWG black and red UL-approved wire

Still looking for pulse information.
 

Thread Starter

billrvolz

Joined Jul 20, 2021
21
How do you know if it is meant to turn the valve on or off?
A controller that delivers the pulse and keeps track of open/closing and makes sure it alternates. The controller only has 2 outputs - one is used to turn on/off ventilation fans, the other for the solenoid. I have a Raspberry available. Considering moving the fan control to the Raspberry as it's less important and then I can use 2 momentary relays and the two outputs in exclusive mode so I have definite open or closing. I just have to make sure that I don't pulse both at the same time (hence the exclusive mode) or else it'd be a short. One issue with the this design is if the valve is open and the controller reboots it loses the state of the valve.
 

Ian0

Joined Aug 7, 2020
3,181
If you have a simple control signal which is high for on and low for off, then it simplifies to this.21BB91D3-2BDD-4A9B-8E20-458ED8C6C4DD.jpeg
No relays required.
If it requires a longer pulse, then use a larger capacitor.
It won’t matter that the supply voltage is larger than the rated voltage of the solenoid.
 

Thread Starter

billrvolz

Joined Jul 20, 2021
21
If you have a simple control signal which is high for on and low for off, then it simplifies to this.View attachment 244084
No relays required.
If it requires a longer pulse, then use a larger capacitor.
It won’t matter that the supply voltage is larger than the rated voltage of the solenoid.
I wish it were that simple. The solenoid is a latching solenoid and takes a pulse to turn the valve on and another pulse of opposite polarity to turn it off. That way only a short pulse is needed and allows the sprinkler valve to use a battery operated timer to operate the valve. I have the timer, but I don't want to use that to get more control over when the valve opens and closes. With 2 x 9V batteries they will last up to 2 years so it's not a lot of amps. Thanks,
 
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