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-   -   newbie...relay problems (http://www.diychatroom.com/f18/newbie-relay-problems-20651/)

jfabes 05-05-2008 01:50 PM

newbie...relay problems
 
eventhough this is automotive related, i didn't post in that section because it has to do with general relay performance.

i recently added some 100w auxilary lights to my car with a new relay that is triggered from the parking light switch and they work fine.

what i'd like to do now is add another relay to have the lights come on as courtesy lighting when the doors are unlocked or opened...which would tie-in perfectly to use the interior light as a trigger.

when i tried it, the system works, but as the interior light "fades" on and "fades" off, it makes the relay buzz horribly as its coming up to power, and then buzz again as it shuts off. so, how can i stop the buzzing? i'm thinking that the "fade" signal must ramp-up in voltage and the relay is following the voltage curve. Can I get a relay that senses low signal inputs but can carry a 20 amp contact rating for the lights? Other suggestions?

I even built a mini circuit with a zener diode, resistor and transistor, ibut t it still buzzes.

Thanks in advance for any help you can offer.

BigJimmy 05-05-2008 02:21 PM

The simple thing would be a voltage sensing relay. They have adjustable on/off setpoints (usually expressed as a percent of rated coil voltage) and are designed to open/close at setpoint without chatter. Check out the Grainger catalog or simply google "voltage sensing relay."

jfabes 05-05-2008 04:20 PM

Quote:

Originally Posted by BigJimmy (Post 121122)
The simple thing would be a voltage sensing relay. They have adjustable on/off setpoints (usually expressed as a percent of rated coil voltage) and are designed to open/close at setpoint without chatter. Check out the Grainger catalog or simply google "voltage sensing relay."


Hmm..interesting. That makes sense. I tried looking, searched on Google, Yahoo and looked at Grainger and Digikey and unfortunatley I can't find anything rated at 12vdc with a 20 amp current capacity. They are either ac voltage, or much lower current. Did you have one in mind that maybe you've seen before?

micromind 05-05-2008 06:19 PM

If you can find a voltage sensing relay that's a lower current rating, use the contacts to energize the coil of a larger relay. The little one triggers the big one, then the big one turns the lights on.

Rob

BigJimmy 05-05-2008 08:13 PM

Quote:

Originally Posted by jfabes (Post 121153)
Hmm..interesting. That makes sense. I tried looking, searched on Google, Yahoo and looked at Grainger and Digikey and unfortunatley I can't find anything rated at 12vdc with a 20 amp current capacity. They are either ac voltage, or much lower current. Did you have one in mind that maybe you've seen before?

You know, after I entered that reply I wondered if that'd be a problem since my more recent application was a "loss of power" indication problem (120VAC). Let me look into this more tomorrow!

BigJimmy 05-06-2008 08:24 AM

Most relays have hyteresis and will pick up at a certain voltage and drop away at a much lower value. I'd try a zener diode with a zener voltage of say 10V. You can rely on the sharp knee in the reverse BD area to keep the voltage constant to the relay at or above the zener voltage. This should hopefully prevent any chatter. The trick is to get a zener voltage that is higher than the voltage at which the relay begins to chatter as the lights are dimming.

As someone else pointed out, if you need high current contacts, you can take your first relay and drive a second that is rated for the load you anticpate (in my business, we call this second relay, which simply follows the operation of the first, a "repeater"). Or, when your primary relay has several independent contacts, you can wire them in parallel to increase the effective current capacity as well.

Take care,
Jimmy

jfabes 05-06-2008 01:58 PM

Thanks guys for the help. I have tried two different zener diodes, a 9.1v 1 watt and a 8.2v 5 watt, thinking that would suffice, but neither worked. I put a meter on it last night and it basically "fades" on from 0 to 12v fairly linearly, its hard to tell the starting point, maybe around 4v...the relay starts buzzing immediately

Rather than looking for a low voltage sensing relay and potentially running into the same problem, what about a solid state relay? I found one that is dc-dc, and will accept 12v in and out and up to 20 amps. i'm not crazy about what i've read though regarding heat and inductive voltage spikes. But you got me thinking....what if i bought a very small solid state relay, rated for 3-5 amps and used it to trigger the standard automotive mechanical relay? Would that work?

BigJimmy 05-06-2008 02:44 PM

Quote:

Originally Posted by jfabes (Post 121339)
Thanks guys for the help. I have tried two different zener diodes, a 9.1v 1 watt and a 8.2v 5 watt, thinking that would suffice, but neither worked. I put a meter on it last night and it basically "fades" on from 0 to 12v fairly linearly, its hard to tell the starting point, maybe around 4v...the relay starts buzzing immediately

Rather than looking for a low voltage sensing relay and potentially running into the same problem, what about a solid state relay? I found one that is dc-dc, and will accept 12v in and out and up to 20 amps. i'm not crazy about what i've read though regarding heat and inductive voltage spikes. But you got me thinking....what if i bought a very small solid state relay, rated for 3-5 amps and used it to trigger the standard automotive mechanical relay? Would that work?

Sure, the SS would be fine. If you're worried about noise, hang a capacitor across the DC input.

On the other hand, are you wiring the zener correctly? It sounds like you may be forward-biasing it. In order to work as a regulator, it needs to be rev. biased.

Take care,
Jimmy

jfabes 05-06-2008 03:20 PM

Quote:

Originally Posted by BigJimmy (Post 121344)
Sure, the SS would be fine. If you're worried about noise, hang a capacitor across the DC input.

On the other hand, are you wiring the zener correctly? It sounds like you may be forward-biasing it. In order to work as a regulator, it needs to be rev. biased.

Take care,
Jimmy

Actually, i'm not sure....i only know enough about EE to be dangerous :) let's start with the basics: the zener goes on the "fade" wire, but hich end should be connected to the wire and which end to the relay...the cathode end (with stripe) or anode end?

Regarding SS relays, they list "input voltage". Does that mean you have to fall within that range? If it says "3-10vdc in" I take it I can't use for 12vdc in, correct? Now, regarding a small pcb mount SS to turn on another relay, vs. a panel mount 20a capacity one, I don't understand the need for the diode to eliminate voltage spikes. Also, will the 20a SS really get that hot? Do i need the heat sink? Its only going to be on for 10-20 seconds at a time....when the interior light is on.

Thanks again for all your help.

elkangorito 05-06-2008 03:49 PM

I believe that the above approaches to your problem may be incorrect.

If you wish to trigger your relay via the internal light (for example), it is quite obvious that this light will have a constant supply (to its' control circuitry) to it somewhere. You need to tap into this supply instead of tapping into the "resultant" supply (aka the "light fading" circuit). An auto electrician should be able to tell you this. Since you obviously need someone like an auto electrician to help you find this "active" wire, you may as well get him/her to install the relay, which will be a normal relay.

Alternatively, you could try to interpret the amazing wiring diagrams that are produced for motor vehicles.

jfabes 05-06-2008 04:40 PM

Quote:

Originally Posted by elkangorito (Post 121349)
I believe that the above approaches to your problem may be incorrect.

If you wish to trigger your relay via the internal light (for example), it is quite obvious that this light will have a constant supply (to its' control circuitry) to it somewhere. You need to tap into this supply instead of tapping into the "resultant" supply (aka the "light fading" circuit). An auto electrician should be able to tell you this. Since you obviously need someone like an auto electrician to help you find this "active" wire, you may as well get him/her to install the relay, which will be a normal relay.

Alternatively, you could try to interpret the amazing wiring diagrams that are produced for motor vehicles.

thanks...and way ahead of you....the fade circuit is itself a timer relay with a +12v feed and 5a fuse. when the doors are opened or the unlocked, the timer relay is triggered for 15 sec or until the doors are locked or closed. the constant supply you speak of is just a normal +12v 5a fused line that's hot all the time. Using that as a trigger wouldn't make any sense. I want to use the resultant trigger because that's what's coming from the timer delay that gives me the fade I want. but maybe i'm misunderstanding you...

I agree, an "amazing wire diagram" would be the ultimate solution, and if this were a 1981 Chevy pick-up, i'm sure any auto parts store would carry one. should you find one for a 2004 BMW M3, let me know....

BigJimmy 05-06-2008 04:48 PM

Quote:

Originally Posted by jfabes (Post 121346)
Actually, i'm not sure....i only know enough about EE to be dangerous :) let's start with the basics: the zener goes on the "fade" wire, but hich end should be connected to the wire and which end to the relay...the cathode end (with stripe) or anode end?

I'll draw you a circuit tonight and post if I have time. The other post about trying to tap into the control as opposed to the fade output is actually a more straightforward/goofproof way to go (thanks Elkangorito; I was thinking WAAAY too hard on this one!). See if you can't dig up a schematic(s) for your vehicle on-line somewhere.

Jimmy


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