Relay control help ?

[Dave B]

Pardon the rough circuit sketch but I would like to trigger the 12 vdc relay on whenever the LED lites. Previously I was triggering a solid state relay across the LED which worked fine as the trigger voltage could range from 3-30 vdc. That's a 12 volt battery with a DC/DC converter dropping the LM3914 circuit voltage down to 6 vdc. The relay activation voltage is 12 vdc and the coil measures 33 ohms. I have several 2N3055 transistors and a variety of resistors figuring this would be a pretty straight forward switching circuit to bias the base and switch the 12v from collector to emitter with the relay coil in series. Must be I need to go back to Electronics 101, I hate to admit I cannot make it work. Anyone like to show off their circuit expertise to help me out ? I really would appreciate it. Thanks,  Dave B.

[Flux]

Dave there is something odd about the LM3914, it drives leds without limiting resistors. I suspect the thing is strobed.

I will try and draw something later, it takes a while to put pictures on here. Maybe that someone else will post a solution before that, but I should have time this evening. You can do it with one pnp and one npn transistor. 3055 is over kill but would work for the npn although leakage may be high.

Flux

[Dave B]

Thanks Flux,

  There must be something strange like you said with the way the LM3914 works. It seems to want to shut down or limit the LED voltage or current as soon as I try to pull the bias from that leg, I cannot get the transistor to see the base voltage so it won't switch the 12 volts. I'm no  expert in circuit design for sure but this one I figured I could handle without too much trouble, not so. I sure appreciate the help from you or anyone else who may have a solution, it seems so simple. I'll be checking back, thank you.  Dave B.

[commanda]

Use a pnp transistor first. The base-emitter junction goes in series with the led. Emitter to positive. Base to the pos end of the led. Collector goes through a resistor to the base of the 3055. 3055 emitter to ground. 3055 collector to relay. other end of relay to positive.

The circuit is in my dump load controller if you want to go and look at it.

Amanda

[Flux]

Thanks Amanda I hoped you would come to the rescue.

I hadn't thought of current driving the pnp, I would have used a resistor and voltage driven it, but the current drive is clever.

Don't forget a freewheeling diode across the relay with cathode to 12v pos. That should work fine.

If Dave could sketch what he thinks we mean then we can check.

I think the freewheel diode will hold the relay even if the led is strobed but you imply that you have done this so it should be fine.

I don't think you will get a round tuit at that price.

Flux

[commanda]

With the LM3914 the led is not strobed; it is driven by a current source. The current is set by the value of the resistors in the reference circuit (from memory). Read the datasheet for the exact details.

Amanda

[Dave B]

Thanks for the reply. Are you talking 2 transistors here ? I looked for your controller circuit you mention and could not locate a drawing describing your solution. I am a rookie when it comes to circuit design so I am getting a little lost with sketching this out. I'm not lazy, just don't get it. A drawing sure would help, thank you.  Dave B.

[Dave B]

Maybe this is the circuit you describe ? If so let me know and what would be a good PNP and NPN to use ? Thank you,  Dave B.

[Flux]

Yes that's right. You need a diode across the relay coil with cathode ( plus) to 12v and anode to NPN collector.

You should have a resistor from NPN base to ground but it will work without it.

PNP BFX88   NPN BFX85 TIP41 or even 2N 3055.  You may have trouble with the Continental numbers, there will be 2N equivalents but I don't know what they are.

The resistor can be 1k.

Amanda is right about the driver chip it is constant current and dissipates the heat within the package so no problem there.

Flux

[commanda]

BC557 and TIP31 would be first candidates out of my junk box.

Amanda

[Dave B]

Hi Flux & Amanda,

  Is this looking closer to what you describe ? I wasn't quite sure about the resistor you describe from base to ground. I have a TIP3055 NPN and a TIP42 PNP various resistors and I think some signal diodes ? Does it sound like I have the right stuff to make this work ? Maybe not ideal but at least enough to make the circuit functional and tweak it later if I need to ? I appreciate your patience, electronics lab was a long, long time ago. Dave B.

[commanda]

The diode across the relay is pointing the wrong way.

Otherwise yes.

Amanda

[Dave B]

Thank you Amanda,

  I'll turn the diode around. Any idea on the values for the resistors to start with ? The relay coil measures 33 ohms. Our company gave these out to their employees several years ago, sorry it's not for sale.  Dave B.

[commanda]

pnp collector to npn base, 1K. Must supply enough base current to make npn transistor saturate (fully on).

npn base to ground. not particularly critical. I typically use 2K2. It just has to bleed off any leakage current when the circuit is in the off state.

Amanda

[Flux]

That's fine Dave, just one small point, I noticed that you said you have some small signal diodes. The 1N4148 type signal diodes will be pushed to the limit across the relay. Use a 1A rectifier such as 1N400? any of the group will do.

Flux

[Dave B]

Thank you Flux and Amanda,

  I'm sure I have some higher rated rectifier diodes as you mention, I just have to find them. I can't wait to give this a try and I'll keep you posted as to my progress. Never enough time to do all you want, I hope to have time to work with this later today. Thank you again for your help, Dave B.

[Dave B]

Just an update on the control circuit. I've bread boarded it and it seems to be trigering the relay fine at any selected led. I have not connected the heating elements yet but plan to let this run for a while first. Thank you Flux and Amanda for your help on this, it looks like you found the answer.  Dave B.