# 12vdc transformer measures 17.39 with meter. Bad?

Discussion in 'The Projects Forum' started by JeffreyNHouston, Feb 18, 2009.

1. ### JeffreyNHouston Thread Starter Member

Feb 16, 2009
26
0
Hello Friends,

I am running a DC motor connected to a 555 timer kit, which is rated for between 5 to 15v. I took the plug-in transformer from my dead Hewlett Packard flatbed image scanner (Could over voltage be the reason for peripherals early death?), which is tagged 120VAC 60Hz 7W input / Output 12VDC 300mA and measured 17.39.

I finished building my timer kit and was about to power it, so I decided to use my VOM to test. I expected the transformer to measure 12v, maybe 13-14v like a car battery that is supposed to be 12v. I did not hook it up to the 555 mono kit, But I did run the motor to make sure it worked.

Would 17.39 fry my 555 kit? I over voltage probably the reason my scanner died?

Can I use this transformer with a resistor of some sort (RED flag, I am a newbie) to get back to 12V?

Thanks,

Jeffrey

2. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
Try measuring the transformer voltage with a load connected to it, something between 10 and 100 mA. John

3. ### JeffreyNHouston Thread Starter Member

Feb 16, 2009
26
0
John,

Would the 555 monostable timer circuit be between 10 and 100mA? The instructions say it can handle up to 100mA loads, but doesn't say itself what load. Would the circuit overload before it drove a load?

4. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
For testing the transformer, I would just use a resistor. At 12V (target), 120 ohm = 100 mA at 1.2W, so a 1/2 W resistor will work, but get hot pretty fast. 1200 ohm = 10 mA at 0.12W, so any resistor would work. I would suggest the greater the load the better for testing purposes. Use Ohm's law to figure out what you need.

As for the 555, you need to look at its quiescent current. For the NE555, it's about 3 to 10 mA without load. Of course, with a load it is higher. But I would still recommend testing your power supply with a simple resistor as load. Your question was whether the power supply is defective, and the resistor presents fewer variables to consider.

John

5. ### italo New Member

Nov 20, 2005
205
1
LM555 can source and sink 200ma @ TTL LEVEL the max the voltage however is 18v so in your case add 2 -3 diodes in series withe source to get you down to a safe marging do not add a resistor in series

6. ### JeffreyNHouston Thread Starter Member

Feb 16, 2009
26
0
Thank you!

Under load, the transformer tested 12.8v. All is well...