# Drive an fet with a 555 and a different power source/not the same one that powers fet

#### arthur92710

Joined Jun 25, 2007
307
Ok ill try a 12v transformer.
but i have some wall plug transformer with 12vac. So can i just add the relay(but whats the relay supposed to power? Coil or 555? )

#### Ron H

Joined Apr 14, 2005
7,014
Indeed, on the brigde output to filter current. Electrolytic capacitor are excellent for that. Why don't you use a 12V transformer instead? You just need to use a 12V coil/relay, which is quite common.

The formula to calculate the capacitor's capacitance:

C = Il / (2 x Vdc x ripple_factor x Fm)

Vdc - DC voltage;
ripple_factor - Factor of ripple you want for your project (0.1 or 10% is enought for amplifiers, 0.05 or 5% should be considered for your project);
Fm - Mains frequency (50Hz in Europe, 60Hz in the US).

You will certainly overvoltage the 555 during the low current intervals. Better to use a transformer and totally exclude the voltage divider.
I agree that a 12V transformer would be a good place to start, considering our OP's inexperience. However, he never said anything about needing a relay.

#### bloguetronica

Joined Apr 27, 2007
1,423
Sorry, my fault. I was talking of 12VDC. Since the transformers are rated in AC terms, an 8V to 9V transformer would be enought. Consider that:
Vp = Vac x sqrt (2) (Vp is peak voltage)

I agree that a 12V transformer would be a good place to start, considering our OP's inexperience. However, he never said anything about needing a relay.
I assumed that the coil was from a relay. I had no hints from this thread to assume that. Sorry!

#### arthur92710

Joined Jun 25, 2007
307
OK, I put in a 12vac xformer, filter cap, and voltage regulator.

Was the cap supposed to be in series or parallel to the bridge output?

our OP's inexperience
Whats an op?

Hay guess what i got on my xformers test?
No more
No thats to high the teacher wont give above 100
Ok give up?
I got a 95!! highest grade in the class! second highest was a 85 because all the other kids are "Dumbass's"

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#### thingmaker3

Joined May 16, 2005
5,084
Why is that a problem?
Because of Ohm's Law and Kirchhoff's Laws. Did you read the link I provided?

#### arthur92710

Joined Jun 25, 2007
307
I have a 1000 or 2200uf cap is that big enough?

finally i got it right!

#### thingmaker3

Joined May 16, 2005
5,084
Probably, but no one in the universe can say for certain unless we know your load impedance.

#### arthur92710

Joined Jun 25, 2007
307
But DC does not have impedance..?

do you mean the part before the diode bridge?
would it be inductive impedance or capacitive impedance?

I dont understand impedance very well. It seems really weird and does not make sense.
can you explain it?

#### bloguetronica

Joined Apr 27, 2007
1,423
But DC does not have impedance..?

do you mean the part before the diode bridge?
would it be inductive impedance or capacitive impedance?

I dont understand impedance very well. It seems really weird and does not make sense.
can you explain it?
Impedance is the perceived resistance of a component in a "alternating current" situation. In your case, the current is not an alternating current. Though, it varies with time (it is a pulsed current). If we are talking about pulses with apreciable intervals of 1s, then the coil's impedance won't be much different of it's DC resistance.

By the way, is this the coil of a relay?

#### Ron H

Joined Apr 14, 2005
7,014
You need to make a few changes. Some are necessary, some are only suggested. I have circled them all.
1. Your bridge outputs were wired backwards.
2. You need a GND connection for your 555.
3. Five volts from a 555 will not turn your FET on reliably unless you have a logic level FET. I suggest you use a 12V regulator to solve this potential problem.
4. Unless your coil has lots of turns of fine wire, you will probably smoke your FET. I would add a current-limiting resistor, at least in the beginning. You can always short it out later and see if your FET survives.

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#### arthur92710

Joined Jun 25, 2007
307
By the way, is this the coil of a relay?
No its not for a relay its kind of the primary of a air core transformer. The secondary is another air core coil.

watch this video http://web.media.mit.edu/~xercyn/sight/levitation/resSmaller.mov

Read Wireless Power Transfer with a Tuned Resonant Air-Core Transformer:
at http://bea.st/sight/levitation/

You need to make a few changes. Some are necessary, some are only suggested. I have circled them all.
1. Your bridge outputs were wired backwards.
2. You need a GND connection for your 555.
3. Five volts from a 555 will not turn your FET on reliably unless you have a logic level FET. I suggest you use a 12V regulator to solve this potential problem.
4. Unless your coil has lots of turns of fine wire, you will probably smoke your FET. I would add a current-limiting resistor, at least in the beginning. You can always short it out later and see if your FET survives.
1- ok i will fix that
2- GND ? where would that be? isn't it enough to connect it to negative?
3- ok i will use a 12v regulater. Can i use the 5v regulator to output 12v?
4- I have a 45 watt fet will that also burn?

#### Ron H

Joined Apr 14, 2005
7,014
No its not for a relay its kind of the primary of a air core transformer. The secondary is another air core coil.

watch this video http://web.media.mit.edu/~xercyn/sight/levitation/resSmaller.mov

Read Wireless Power Transfer with a Tuned Resonant Air-Core Transformer:
at http://bea.st/sight/levitation/

1- ok i will fix that
2- GND ? where would that be? isn't it enough to connect it to negative?
You don't need to connect anything to earth GND. You did not have a common connection between your 555 and the source of the FET. Look at your previous schematic.
3- ok i will use a 12v regulater. Can i use the 5v regulator to output 12v?
You can. See below.
4- I have a 45 watt fet will that also burn?
Sure, especially if you don't have a heatsink. Do you have a part number for your FET? A thousand pardons if you already posted it.

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#### arthur92710

Joined Jun 25, 2007
307
Cool that was the same circuit I have!

You don't need to connect anything to earth GND. You did not have a common connection between your 555 and the source of the FET. Look at your previous schematic.
Oh! wow i totally missed that!

Ok i actually have 2 fets
The first one is a SUB85N02 85A 20v
the second one Is a E45NK80ZD(its an automotive fet! its kind of big) 45A 800v

What do you use to draw you circuits? I use Electronics workbench its really a pain XD

#### Ron H

Joined Apr 14, 2005
7,014
If you use the SUB85N02, use the regulator at 5V. 12V will probably destroy it, as the gate is only good for 8V.
The 12V regulator would be best for the E45NK80ZD.
I use Linear Technology's SwitcherCAD III for simulations, and generally also for schematic drawing. It's totally free and uncastrated.

#### arthur92710

Joined Jun 25, 2007
307
Ok here are the changes.
Also Can i add a current increasing transistor to send more current to the coil?

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#### Ron H

Joined Apr 14, 2005
7,014
Ok here are the changes.
Also Can i add a current increasing transistor to send more current to the coil?
Why did you switch to a 2N7002? Do you know what the resistance and/or the inductance of your coil is?
You can't increase current by adding a transistor. It's obvious from your schematic errors that you don't even have this running yet. Don't think about improvements until you have something to improve on.
Oh, yeah, you had C1 and C2 in the wrong places also.

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#### arthur92710

Joined Jun 25, 2007
307
Day job -.- ... mumble.... i got to go to school...
I thought I fixed the diode bridge.
I didn't mean to switch it to a 2n7002 it was by electronics workbench.
No its not running. I first want to draw a circuit then I will build it.

Thanks for the corrections Ron!

#### Ron H

Joined Apr 14, 2005
7,014
Day job -.- ... mumble.... i got to go to school...
I thought I fixed the diode bridge.
I didn't mean to switch it to a 2n7002 it was by electronics workbench.
No its not running. I first want to draw a circuit then I will build it.

Thanks for the corrections Ron!
School IS your day job. Don't quit! (I'm not implying you were thinking of it.)
You're doing great. I remember when I was in labs as a junior and senior at San Jose State back in the early 1960's (yep, they had electronics even then), most of the guys in there had never laid a finger on hardware, and they were 20 years old! The hands-on experience I had as a hobbyist and in a summer job gave me a huge advantage over most of them.

#### arthur92710

Joined Jun 25, 2007
307
Ok so now i hope this is a working circuit.

If I build this right will it work?

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#### nomurphy

Joined Aug 8, 2005
567
Your resistors R1 & R2 don't appear to be standard values. Also, they are rather low in value.

Attached is a sheet of standard values.

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