View Full Version : Power Supply Options

11-09-2009, 11:46 PM
What kills a device: volts or amps?

I have built Brian's 595 board and 16 of Sean's SSR boards from a co-op order a few years ago and I'm finally ready to start using them. I've put every single LED on each of the boards. So my question comes down to the power source option for P1.

I have an RCA Universal AC/DC adapter model AH5. The sticker states:

OUTPUT: DC 3/4.5/6/7.5/9/12V
VA: 12VDCx500mA

When I place the voltage switch to 3 and test with my multimeter I get 5.32VDC.

I then switch my multimeter to 20A and it states that 1.989A.

So I'm a little concerned; the sticker states 500mA but my multimeter states almost 2A. The voltage is damn near but is it too much amperage for these boards?


11-10-2009, 12:14 AM
Well, i am no electrical engineer. But i am pretty sure that if you are just putting the amp meter in line with just the power supply (AKA no load) you are shorting the power supply which is not good.

See diagram

So your answer is.
Your AC to DC will only supply up to 500mA "RELIABLY"
If you short the terminals you will breifly get more than 500mA, until you smoke it.

So for a voltage of 5V the appropriate load to test current would be 10 ohm.
R = V/I (10 = 5/.5)
If you place a resistor of 10 ohms in line as the load then you should read 500 mA if its not smoked.

11-10-2009, 12:22 AM
What kills a device: volts or amps? pending on the device ...
But , majority of the time ... too high of volts will .
Amps --- as a general rule , think of it as your cars gas tank .... the more you have in it does not mean the more your engine will use faster , just means there's extra if needed . :cool:

Also , to be safe , use only regulated power supplies when available .

11-10-2009, 12:58 AM
You will need a regulated 5 volt supply.

Those universal wall-wart supplies are usually not regulated and will read a considerable amount higher than the rated output voltage, and will more than likely damage your 595 board.

The 500mA rating is the amount of current the power supply at the specified voltage (12V as you mention).
If you are reading almost 2A then you might be reading that with value your meter directly across the supply + and - leads, which would be dropping the voltage considerably and may cause damage to the supply.

The amperage is not as important as the voltage, and with the 595 board it should be 5V regulated with at least 1 amp of current output, especially if you have all LED's installed. If your supply is more than 1 amp output then you would have some overhead and this is OK, but if the voltage goes over 5.5V (up to 7V for some 595 variants) you will most likely damage the 74HC595 IC's. If it drops below 5V then you may have problems with erratic operation.

11-10-2009, 02:27 AM
Thank you very much for all the information.

I found an old Linksys transformer which outputs at 5.12VDC@2.5A. I put the Molex connector on the end and triple checked the polarity. I initially had an issue with multiple channels firing up but I remembered that I reset my BIOS and had to change the parallel port back to EPP.

It works! My first SSR distribution box works too!

Thanks again for all your assistance.


03-10-2010, 09:20 AM
Current overload is what kills a power supply. If your PS is rated at 500 ma (1/2 amp), then drawing 1 amp will eventually burn it up. Unless it is a regulated PS (recommended by RPM), then the output voltage will usually drop as the current is increased. Too much current will also burn up the regulator.

I have not used the 595 boards, so I am not familiar with current requirements. I would follow the recommendations of RPM.

SMV, to measure current, you have to put your multimeter IN-LINE with either the plus or minus lead from the power supply. Current cannot be measured by putting the ammeter across the power supply. Go to Ibouchers' website to see how to hook it up.

IBOUCHER, if the ammeter is in line with the power supply and not connected to a load, nothing happens. You have an open circuit and no current flows. If the bulb in your instructional were removed, no current would flow and the meter would read ZERO.

Another quick note: A load will consume only as much current as it is rated for. In other words, if your power supply is rated at 1 amp and the load (board) requires 1/2 amp, that is what it will draw. You now have a 1/2 amp "safety factor" or you can put another 1/2 amp (or less) board on the supply.

Note that the ammeter can be in either the PLUS or MINUS line of the power supply. The polarity of the ammeter has to be correct, however. If the polarity is reversed, the meter will try to read backwards. This is usually not a problem with the digital type multimeters, however.

03-10-2010, 10:18 AM
in regards to IdunBenhad (http://doityourselfchristmas.com/forums/member.php?u=2921) comment,
if your multimeter blows the fuses inside, current wont flow, an therefore no power.
i remembered that after 30min of scratching head.
fixed my problem. (i cant remember even when i blew them tho.)