25mA to 100mA 12v trigger

[Nigel Goodwin]

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.

I would suggest the problem is the pathetic quality of your wall warts, these 'made in China' are so poorly made it's not true!.

 

[philba]

I would suggest the problem is the pathetic quality of your wall warts, these 'made in China' are so poorly made it's not true!.

I don't disagree that the quality isn't high but I've never lost another one and have at least 15 or more devices that use WWs - many in constant use. This says it's more than just bad quality WWs.

 

[philba]

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.

if you mean this circuit http://www.scooter.cx/~scott/screen.html, it should work if properly wired. Make sure you have the transistor wired correctly. If you have DMM, measure the voltage between the receiver side of the base resistor and ground. it should be around 0 when the trigger is off, and 12V when it's on. the only way that circuit, properly wired, can fire the relay is if there is voltage applied to the base of the transistor. If wired properly, the voltage is correct and you still get incorrect behavior, get a new transistor. if you don't have a dmm, get a cheap one.

In a power supply, the voltage is important but the current only needs to be above a certain level. The circuit will take only what it needs so having 1000 mA available when only 100 mA is needed is ok though it wouldn't hurt to have smaller (250 mA or so in the above case). Note that an unregulated DC WW will deliver higher voltage when the current draw is less than the spec'd limit. so that 12V ww will probably provide 14-15V at 100 mA but that won't hurt the circuit. I'd use a requlated WW just to be sure, though.

 

[audioguru]

The transistor doesn't have a resistor from base to emitter to turn it off. I would use 10k.
If shielded cable isn't used from the projector to the transistor (like in the article) then radio waves or mains hum pickup will turn on the transistor.

 

[philba]

pull down resistor is worth a try but the outputs I've seen are totem pole so it should pull low when off. You can verify by disconnecting from the projector and shorting the input to ground. relay should be off. then connect the input to +12 and the relay should be on. you probably should have done that test before hooking it up in the first place.

edit: come to think of it, if the PJ is off, you may have that situation anyway, the resistor should be there.

 

[DerekL]

Problem Solved! User Error!

I guess it would have been better if I had not wired the transistor backwards! The system is working perfectly now.

Thanks for all the help!

 

[audioguru]

I guess it would have been better if I had not wired the transistor backwards!

A backwads transistor avalanches like a zener diode at about 8V. If the current is medium for the transistor then it gets fried.

 

[Tarsil]

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

Just replace the transistor and the relay with a MOS-FET driven trought a 4.7k resistor. It costs only 1$ and it's so simple!

 

[ronnol]

I know this is an older post and not sure if I should do new but was hoping someone might be able to help me figure out a 50ma to 100ma. I have a NuVo Grand Concerto system and an older amp, without any trigger, powering one of the zones. Thus I tried to add a Russound ACT-1 (voltage triggered AC outlet) to power on/off the amp with the 12v trigger of that zone from the NuVo. Problem is the NuVo outputs 12v@50ma and the Russound requires 12v@100ma to trigger. Thus it is not triggering the ACT-1. Taking a 12v 250ma power supply to the Russound triggers it fine and the 12v output of the NuVo triggers a new amp, with a trigger, fine. I added a relay, IDEC RH1B-U DC12V, and you could see the coil trying to flip but the NuVo's 50ma was not enough to trip the relay. I tried the diagram previously posted but didnt have success. Any suggestions or changes I need to make, this just keeps going over my head? Thanks for the help.