Wall Wart woes

[windozeuser]

Well, I found an old wall wart rated at 9VDC @ 300mA Output. I cut the end connector off, and replaced it with a 9V battery connector. I'm trying to use it as a replacement for the 9V battery on my BASIC Stamp Board. I plugged the snap connector into the battery prongs of the BASIC stamp board. Then nothing happened! I Measured the output of the wall wart while connected to the BASIC Stamp board, and was reading 15VDC! My BASIC Stamp board still worked after that endeavor just with the 9V battery. I infer that the transformer needs to be loaded down with a resistor?

Need a six volt drop at 300mA

R = (15 -9) / 0.3A = 20 ohm Resistor

Am I correct? Please help

Thanks

P.S - Thank God my BASIC Stamp Board still worked (Damn thing was $80) :roll:

 

[Nigel Goodwin]

Well, I found an old wall wart rated at 9VDC @ 300mA Output. I cut the end connector off, and replaced it with a 9V battery connector. I'm trying to use it as a replacement for the 9V battery on my BASIC Stamp Board. I plugged the snap connector into the battery prongs of the BASIC stamp board. Then nothing happened! I Measured the output of the wall wart while connected to the BASIC Stamp board, and was reading 15VDC! My BASIC Stamp board still worked after that endeavor just with the 9V battery. I infer that the transformer needs to be loaded down with a resistor?

Need a six volt drop at 300mA

R = (15 -9) / 0.3A = 20 ohm Resistor

Am I correct? Please help

No!, you're not right!.

You have a crappy unregulated wallwart, when loaded at 300mA it will output roughly 9V, but with VERY high ripple. What is the power supply on the STAMP?, does it have a 5V regulator?, if so it may be fine as is? - but these unregulated wallwarts are really bad news, for a little more money you can buy far superior regulated ones.

A resistor won't help you much, it will need changing for every slight change in load - which is basically what a regulator does automatically.

 

[upand_at_them]

And it should be pointed out that most wall warts are like this. The 9V, or whatever, is the minimum they put out, because the regulation is taken care of in the device being powered.

Mike

 

[Dr.EM]

You might be best building a regulator board for it yourself, then using an AC or DC wallwart adapter of 12v or more at a suitable current. Its not difficult to do with 3 pin regulator ICs and a few other components. Let me know if thats something you'd want to do.