# How much current is required?

#### Otaku

Joined Nov 19, 2008
128
I'm using a 555 to energize a relay with a coil resistance of ~120 ohms. The voltage output from the 555 is 10VDC. I need to know if the current required to energize the relay will exceed the 200mA max of the 555. I've tried using Ohm's Law calculators to get this value, but I'm not sure that I'm getting the value I need to know. Using 10VDC and 120 ohms I get a current value of 0.08333 amps (according to the calculator). All advice appreciated!

#### Mike2545

Joined Mar 26, 2009
116
Use the 555 to drive a transistor and the transistor to drive the relay.

#### mik3

Joined Feb 4, 2008
4,843
Your calculation is correct (83mA) and the 555 can drive the relay. Also, remember to connect a diode in parallel with the relay's coil to absorb the back EMF when switching it off.

#### hgmjr

Joined Jan 28, 2005
9,029
The 200 milliamp maximum output current is only valid for the bipolar version of the 555.

If you are using a CMOC 555, the maximum current is more like 50 milliamps.

I believe you would fair better if you go ahead and use the 555 to drive a BJT or MOSFET transistor and let that drive your relay. That way you can change use either the CMOS or Bipolar 555 devices interchangeably.

hgmjr
hgmjr

#### Wendy

Joined Mar 24, 2008
21,914
Something like this (ignore the actual 555 schematic, it is shown by way of example).

#### Mike Mandaville

Joined May 27, 2009
81
Something like this (ignore the actual 555 schematic, it is shown by way of example).

In a tuneable bistable configuration, I think that a timer-controlled relay would make for an interesting tuneable buzzer.

#### Otaku

Joined Nov 19, 2008
128
I'm still running into some issues with this circuit, and I think they're related to the 555 I'm using. It's a MC1455, supposed to be a direct replacement for the LM555, NE555 etc but when I apply 12VDC to the circuit with pin 1 open, pin 3 goes high. When pin 1 in connected to ground, pin 3 goes low. This behavior is opposite to that of a NE555P in the same circuit. I checked the datasheet for the MC1455, and I see that there may be both a Normally ON and a Normally OFF version of the chip. Has anyone had any experience with the MC1455 that matches this behavior?

#### Audioguru

Joined Dec 20, 2007
11,251
Pin1 is the 0V pin. It is not supposed to be floating. When it is floating then the output cannot go low.

Use the reset pin4 to reset the 555.