# Amperage limiting.

Discussion in 'The Projects Forum' started by superman185, Sep 2, 2013.

1. ### superman185 Thread Starter New Member

Jun 18, 2013
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I have an issue with amperage limiting.

And before anyone tells me too, yes I have searched the forums and google to no avail.

I have found several circuit references on the forum and have contacted the authors with no resolution to my issue.

I have a circuit a 556 timer circuit that I am making for a experiment with a 75amp(for protection) power mosfet on the output side. I also have to have a specific frequency and voltage on the output.
Frequency done by 556 and, 12 volts, the voltage provided by a 30amp 110v AC to DC 12v.

I can not use the 556 to time the on and off cycles due to the specific frequency and timing I am trying to achieve.

I am trying to achieve a 0-30 amp limiter. Maybe even 0-50 amp, I have no issue going to a battery. This is not a long experiment.

I can not have this achieved by voltage trimmer due to the nature of my experiment. 12v must be maintained. And yes a 10% +/- is by nature I know I would have to tweak it.

I would prefer a circuit that limits the amperage through a mosfet or similar so that I can use the smaller resistors.

Also I would prefer to trim the amperage before it goes into the circuit.

Thank you in advance for your help with this topic and time.

2. ### wayneh Expert

Sep 9, 2010
14,851
5,333
If I understand correctly, I believe you want:
A 12V constant voltage AC supply
The frequency is set by your 556
Current to be limited to some value you set, 0-50A
Peak voltage supplied by an existing 12V DC supply

Have you considered the duty cycle of the pulses? Do you want 50%? Varying the pulse width (PWM) would be one way to limit the average current.
How high is frequency?

Depending on specific details, it sounds to me like a standard astable circuit driving the gate of a sufficiently rated MOSFET will do the job nicely. Oops, that won't do current limiting without a few more steps.

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3. ### superman185 Thread Starter New Member

Jun 18, 2013
13
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Let me put it another way...

The "Power Converter" can be a battery for the 50A max.

I can not use the 556 to pulse the "on and off" / "duration" because that is part of the experiment. I need to pulse the frequency with the same duty cycle.

My timing circuit works like as intended, but it draws too much amperage.

I just need to throttle the amperage on the input side, my power supply can't do that and, I don't want to short out my board or battery by creating a dead short if the mosfet fails again.

The voltage HAS TO STAY consistent.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
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I'VE SEEN THOSE BEFORE!

Let me think now....where was it?

Oh yeah, A FUSE!

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Jul 18, 2013
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And if you want one that is fast acting, and intended for semiconductor protection, use a Rectifier fuse.
Max.

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6. ### superman185 Thread Starter New Member

Jun 18, 2013
13
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I have a 60A thermal auto reset-able fuse. To protect the 75A mosfet.

However I need a way to control the AMPERAGE in a VARIABLE way, I need to set the amperage draw myself.

I know it will create heat and I am willing to disperse the heat.

But just using a fuse is not an option.

I need the power to stay on at all times just limiting the amperage. Or else I will not get a clean result in my experiment.

7. ### wayneh Expert

Sep 9, 2010
14,851
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Yeah, that means a fuse is the only option.

You cannot control (reduce) current through a load without the ability to vary (reduce) voltage. If you cannot tolerate reducing voltage when the current limit is hit, you cannot use a current limiting circuit. Of course the fuse will abruptly drop the current and voltage to zero.

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8. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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What he is saying is that when your load wants more current than you 'set' for it to use, a lower voltage will be needed to set the current value.

The only thing that will MAKE less current flow is a lower voltage level. Whether you do this with PWM or with a different type of variable current scheme. The end result will be the same. A lower voltage will be presented to the load. Averaged voltage value of a PWM 12 volt source gives you a voltage LESS than 12.

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9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
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That ^ still seems confusing.

Here:

Voltage = Amps (x) Resistance

You set the voltage and the amperage.

12 V = 10 amps X 1.2 ohms

then

12 V = 20 amps X 0.6 ohms

Then

12 V = 30 amps X 0.4 ohms

Now you can see that when the load (resistance) grows smaller and smaller the amps grow larger and larger.

If you stop letting the amps grow larger as the load(resistance) grows smaller, then the product of the two, the VOLTS will have to get smaller.

It is just how ohms law works.

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10. ### superman185 Thread Starter New Member

Jun 18, 2013
13
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I thank everyone for their time and candor.

I guess I will have to do voltage limiting then.

So what would be the easiest way to control the voltage on the circuit that can handle 50amps.

11. ### wayneh Expert

Sep 9, 2010
14,851
5,333
Do you mean 50 amps average or 50 amps peak (50 - 0 - 50 - 0 as it pulses)?

Either way, you first need a way to detect the current. A low ohms (eg. 0.1Ω or less) power resistor used as a shunt in series with the load allows you to watch the voltage across it, to measure the current. A hall effect sensor is another approach, perhaps better for this high a current.

Then a negative feedback reacts to the increasing current and responds by lowering the output voltage. I'm sure someone will come along with an example schematic for you. Your need is very similar to a current-limited, constant voltage battery charger.

Except for your pulsing DC. I don't think you answered my question about frequency yet. That'll matter to the design of the current limiter.

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12. ### superman185 Thread Starter New Member

Jun 18, 2013
13
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I found a great way around this and can do everything I want and more and I don't need to build it and its cheap to boot.

A "500 Amp Carbon Pile Load Tester", it will keep the voltage at 12 and disperse any heat up to 500 Amps.

Its perfect for what I need.

Just a FYI to all the great people on this forum.

Thank you again for all of your help.

I'm off to harbor freight to buy mine.

13. ### superman185 Thread Starter New Member

Jun 18, 2013
13
0
Also I thanked everyone for their posts.