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25mA to 100mA 12v trigger

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Sameian

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25mA to 100mA

hi,
i need to raise the current output of a 12v 25mA trigger to 12v 100mA to drive a relay. i found a few sites showing a circuit using a transistor to accomplish this but im having a hard time understanding the schematics. i also read that i may be able to use a low current relay thatll be driven by the trigger i have along with a seperate 12v 100mA power supply to drive the higher current relay, i just cant seem to find such a relay. my trigger is a denon home theater receiver and the relay is part of a da-lite motorized screen. any help is greatly appreciated.

Sam
 
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You need to know the actual connections in the Denon, otherwise it's going to be much more complicated than it needs to be! - essentially all you probably need is a transistor, a diode, and a resistor.
 
hey Nigel,

thanks for the quick reply, i hope im understanding you correctly, the denon has a 1/8" miniplug 12v+ and ground. the video projector interface (the relay) has 2 wires coming from it, red and black. the red goes to the 12v+ and the black to ground. i hope its ok that im posting this link.... http://www.scooter.cx/~scott/screen.html this person had almost the same problem im having only his was with his projector not pushing enough current to drive the relay. he ended up changing the relay to a dpdt relay. thats one of things im confused with on this schematic, im not changing the relay... i dont understand where the 2 wires coming from the relay are and also is he using a seperate 12v power supply along with his projectors trigger in this circuit? how would i go about using the transistor, diode and resistor suggestion you made?

Sam
 
His diagram does just what you need, connect the relay coil to the collector of the transistor, and the other side to 12V - don't forget the diode across the relay!.

His choice of resistor was strange, basically because he didn't know what he was doing, a 6.8K would be fine (in fact better than his choice).
 
hey Nigel,

great, could you please just explain to me; do i have to use a seperate 12v power supply along with the 12v 25 mA trigger? my relay draws 88mA and my trigger outputs 25mA, can i still use the 6.8k resistor?

thanks for all your help Nigel,
Sam
 
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Sameian said:
hey Nigel,

great, could you please just explain to me; do i have to use a seperate 12v power supply along with the 12v 25 mA trigger? my relay draws 88mA and my trigger outputs 25mA, can i still use the 6.8k resistor?

The relay need a seperate 12V supply, you can't make power from nothing!.
 
so basically, the transistor is acting like a low current relay? thats where i was having trouble understanding, i thought the transistor circuit was amplifying the currrent coming out of the trigger.
 
I don't like the guy using a transistor gain of 100. the fairchild datasheet shows a min of 40 in certain circumstances. I would size the base resistor for that value - 3K will work just fine and will allow the relay coil to see up to 133 mA, Since it only draws 88 mA, it will be fine. you don't need to actually draw 25mA from the Denon since the transistor will amplify the current to at least the 88 mA needed. This circuit will draw around 3 mA from the Denon.

It's not clear to me where your 12V power is coming from. Does your Denon supply a seperate 12V power? My Denon 3803 does not have a 12V power output, just the trigger (20 mA) out. Does your DaLite screen have a 12V power output? I suspect you will need a 12V power supply to drive the relay. You might be able to tap into the DaLite power for the 12V (that's what I would do but I'm comfortable with this stuff). If you don't feel ok doing that then I would get a 12V regulated wall wart. Jameco has one that should work for you - https://jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=220898
be sure to get a matching jack.
 
yes but it's about the current, not the voltage. You need "extra" current to boost it to 88 mA. The power supply provides the extra current.
 
thank you very much for your help. i also have a denon 3803 and i just got done building this circuit, i ended up puttin in a 3.9k ohm resistor (only thing i could find), from what ive read, im guesstimating it'll push around 100+ mA. gonna go try it out on my denon, see if itll trigger a relay i have here (hope its the same type of relay thats on the screen)
 
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couldnt i just buy a relay that could get triggered from a 25mA source? and just put in the 12v 200mA supply on the relays output to drive the screens relay. the more i look at this the more it looks like the transistor is just acting like a relay. i dont see how its amplifying anything... it seems to me as if the 25mA triggers, switches, drives, turns on (whatever its termed) the transistor allowing the 12v power supply to pass through to the screens relay. basically acting like a relay itself. i hate to be a bother but i really want to understand this.
 
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The transistor acts as an amplifer and switch, you could use a 25mA relay, but that would be more expensive than a transistor, and still require an external 12V supply just the same.
 
Well, I suppose that could work if you could find the relay that would do that but you would still need to find a power supply, case and so on. I don't see how that saves you much. In fact, your relay will cost quite a bit more than a transistor and resistor.

if you insist on a relay, look at the omron G5A relay line. there are several 12V coils that draw less than 25 mA. I would stay below 20 mA.
 
I appreciate the info!

I was very glad to find this discussion as I had the exact same issue of using a Denon AVR to trigger a Dalite electric screen.

I built the circuit and am using a wall wart. However, my wall wart has a variable voltage selector and when I choose 12V, I actually get 17.9V on my digital multimeter. In fact, I get between 17.7V and 17.9V on every single voltage setting. The adapter has a switch to choose between 1.5V and 12V, and each setting outputs nearly 18V. Does this sound right, or do I have a defective adapter?

thanks in advance.
Jeremy
 
It's probably an unregulated supply (transformer, rectifier and, maybe, a filter cap). It is rated for a voltage and current. what that means is that it will deliver thqqt voltage at that current. When you measure it by just putting a DMM on the output with no other load, the voltage will be a fair amount higher. what you have to look at is the voltage under the specificed load.

It sounds like the switch doesn't do anything but I wouldn't worry too much about it if you are using a voltage regulator (like a 7812). I'f no VR, I'd look at the relay datasheet - if it can handle 15-18V, I'd not worry about it.

By the way, I've had problems with the one switchable wall wart I have. It seems flakey but may have been killed by my wall-wart eating PIC programmer. but that's a different tale...

Phil
 
yeah, eats 'em for breakfast.... seriously, I've got a PIC programmer (EPIC+ from MELabs) that has killed 3 WWs over the course of about 2 years. it takes 16V in and uses a 317 and 7805 to get 13VDC and 5VDC, respectively. I use a lot of WWs and have never had another go bad. current draw is reasonable (10s of mAs, even when programming) and I've used 500 mA and up rated WWs. Programmer still works just fine otherwise.
 
Power Supply Questions

I have had the same problem with a trigger from my Sony Projector to a Draper screen. I built a circuit like the one linked to in an earlier post but I am still having trouble. Is my power supply (12VDC 1000mA) too high in mA for the transistor listed in the link ? I have tested the circuit and it seems that the trigger output is working properly with the transistor but when I apply the 12VDC power supply it trips the relay whether the trigger is on or not. I thought I was close to figuring it out but now I'm at a loss. Any help is sincerely appreciated.
 
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