Delaying Relay

[kindred]

Hello everyone,

I have recently made a switching regulator that I am using to power my controller and several relays. I have noted that when I power the regulator my relays stutter before finally closing (you can hear them trying to close). The reason for this is because for safety reasons i have made my regulator ramp the output current up instead of giving the output voltage instantly. The safety reasons are basically inrush current on the input side.

I want to find a way to delay the time the relays turn on so that when they try to turn on the available output voltage is present. I have tried several methods I have thought of myself but sadly none of them have worked.

So far I have tried to drive a zener diode into reverse break down so that it conducts once my output hits 30V to turn on a transistor; in all honesty I'm not too experienced with zener diodes being used like this so I assume I have done something wrong. I have also tried a delay network with a capacitor but that only posses the same problem, as it ramps the voltage on the base of the transistor.

Heres a quick look at what I am attempting:




Technical Info:
Relay:EV200
Zener: 1n4750a
Transistor:TIP41c

I also tried a mosfet since the base current of the TIP41C is a little daunting...
Mosfet: Irf540n

Vout = 30V, limited to 3A max

This is for my final project for college so any help is greatly appreciated. Thank you for any info you can share,

Asa

EDIT: Had to clean up the links

[keebler303]

how about using a comparator and a voltage divider network to sense the 30V, then it will output logic high to drive a transistor to turn on the relays.

maybe a comparator driving a ULN2003 would be appropriate

Matt

[kindred]

Hello Matt,

Is there any particular reason you suggest using the darlington arrays? I have used the method you mentioned and it works with the mosfet i had before, irf510 (I mistakingly put irf540n).

The problem I have now is one that really surprises me. The two relays turn on separately but will not turn on properly when they are set to be turned on together. I have made it so that each really turns on about 1second apart, but they still try to close at the same time, this puzzles me. I am using an RC network on the input side of my comparators to create a delay that sets each of them apart. For what ever reason, they still try to close at the same time when connected in the circuit together. Does anyone have an idea about the typicall gate current of a mosfet is, I beleive they are supposed to be extremely low.

I have 12v on the gate with a 100ohm resistor in series. I selected a 100ohm because when I tried to turn on the relay and used a pot this is the value it turned on at.

Does anyone have an idea as to why I am having this problem?

Thank you, if you need anymore info let me know please,

Asa

EDIT:
My 12v regulator doesnt seem to be the problem in terms of current since my fan still runs constantly even when the relays wont power on. Most of the time i can hear the relays humming, so they are trying to close.

I have also noticed that if after the instance of the relays not turning on, i disconnect the power to the comparators and reconnect them one at a time both relays close and work properly. But if i completley remove power and try them again without connecting each comparator one at a time, they fail to turn on again.

[irving2008]

A MOSFET gate is not current driven but charge driven, however you need to be able to supply enough current to charge the gate capacitor fast enough to turn the MOSFET on into saturation. If you look at the spec for a IRF540 you will see the gate charge is 71nC at 10v Vgs. Supposing your comparator output switches from 0v to 10v and you want a turn on time of 1uS then your comparator needs to source (i = Q/t) 71 X10^-9/1 X 10^-6 = 71mA at the full 10v output. If it goes into current-limit while doing this then the MOSFET may not turn on, or will turn on slowly which may cause chatter in the relay.

I don't know what circuit you are using but an RC network on the input side of the comparators is unlikely to work. I'd use one comparator to signal that the supply volts were valid (and drive the first relay via a bipolar transistor) and then trigger the second comparator from that, see pic... rv1 adjusts the initial turn on volts for the first relay to pull in and rv2 the delay before the second relay pulls in (approx 1 - 2 sec if I got my calcs right)

[kindred]

Hello Irving,

I have spent the entire day working on it and thankfully I got it working . But thank you so much for your input Irving. My eyes are burning like crazy lol. It turns out the problem stemmed from the ramping up of my 30V from my switching regulator. The ramping of the 30V caused a ramping in the 12V regulator, the comparators (lm311) didnt seem to like this.

Personally I never knew .5 seconds would cause such a problem even though I delayed the relays themselve extensively before hand to offset that .5 secs. I had initially put the .5sec of ramping on the swtching reg because I damaged 2 chips that way and I didnt want it to happen again. But after reading the data sheet again for the regulator it said it had protection against this and soft starts werent required except if powered by a power supply. I suppose I had damaged them before because the heat sink for the chip at the time was possibly too small for the inrush current, I have a much larger heat sink now.

I also had to place a larger capacitor on the input side of the 12v regulator to make it work perfectly.

Unfortunatly I have yet another problem, woopie.

[IMG] [/IMG]

The problem now is that when I check continuity or the resistance, I can see a connection accross the red line shown above. Both relays are off (checked with continuity, and no power to the system when I measure the red line). If I remove the resistor from accorss the main relay I do no see any continuity or resistance accross the red line shown above.

Does anyone know the reason behind this?

Thank you,
Asa

UPDATE: So if i disconnect that connector between the two relays I still get a connection. I can also see battery voltage on the controller with the main connector to the secondary relay disconnected from the battery pack.