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# Limit DC amps

## Limit DC amps

#1
09-21-23, 08:26 PM
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Limit DC amps

Is there such a device to limit the amperage to a draw? Example I'm using a battery that's 12 volt 100 amp hour. My device can easily draw the full 100 amps but I don't want it to how can I limit the amperage but not the volts? Is there some sort of limiter or equivalent that would only allow so many amps to pass through without it tripping or burning out when the end source can handle the whole load.

#2
09-21-23, 08:33 PM
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A device will only draw the current it needs to run properly.
You just need to supply protection.

If you have a 12v 100a power supply and you have a 12v 3a fan.
Install a 4A fuse to protect the fan and wiring.

If this isn't helpful.... what are you trying to restrict current to ?
If you restrict current.... the voltage must drop. Ohms law.

#3
09-22-23, 07:52 AM
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Is there such a device to limit the amperage to a draw? Example I'm using a battery that's 12 volt 100 amp hour. My device can easily draw the full 100 amps but I don't want it to how can I limit the amperage but not the volts?
H3re2Learn - The current in a circuit is inversely proportional to the resistance, therefore when resistance is increased the current is decreased and vice versa.

If you add load resistance then the current reduces while voltage remains constant. Another way of thinking about it is; when voltage is constant by doubling the resistance the current is halved and vice versa. Will adding resistance work to achieve your need? We'll find out shortly!

Ohm's law states that the electrical current (I) flowing in an circuit is directly proportional to the applied voltage (V) and inversely proportional to the resistance (R). Therefore, if the voltage is increased, the current will increase provided the resistance of the circuit does not change. Accordingly, current will decrease when the resistance of the circuit is increased.

Here's the catch - a resistance in series causes a decrease in voltage. A resistance in series doesn't change the
current flow, the current in a series circuit will remain the same. So, adding resistance in series with the load will not change the current. So where can a resistance - maybe a variable resistance possibly - be added in the circuit to decrease the current? Sadly, nowhere!

You see, your load, whatever that load may be, is THE LOAD and it is not going to be changed! For THE LOAD to operate as designed it must have the correct design voltage, so we can't be changing the voltage by adding resistance in series. The fact of the matter is this; the load draws a certain amount of current. The load's current (when operating as designed with the correct design voltage) simply cannot be changed or altered by adding or changing anything in the circuit.

The current that your load draws can indeed be 'limited' (and protected) by a current limiting device, such as a fuse or a circuit breaker. However, the load's current cannot be limited or reduced in any way (fixed or varied) while keeping the voltage constant.

#4
09-22-23, 01:06 PM
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A resistance in series doesn't change the current flow, the current in a series circuit will remain the same. So, adding resistance in series with the load will not change the current.
That’s not correct. Resistors decrease the current in a series circuit.

#5
09-22-23, 02:23 PM
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That’s not correct. Resistors decrease the current in a series circuit.

Since there is only one path for electron flow in a series circuit, the current is the same magnitude at any point in the circuit. The total current in a series circuit is the same as the current through any resistance of the circuit.

Add 3 resistors in a series circuit and the total circuit current would remain exactly the same through all the individual circuit resistors. Therefore, as I said; a resistance in series doesn't change the current flow, the current in a series circuit will remain the same.

Adding resistance in series with the OP's load will not change or affect the current draw as he wishes to accomplish.

This may help: https://pressbooks.bccampus.ca/basic...cuit-breakers/

#6
09-22-23, 02:59 PM
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We need to keep our replies ON TOPIC.
Every thread is turning into a blog and that is not what this site is about.

Please direct your responses directly to the OP question without the wiki help.
I'll start deleting posts that don't apply directly to the OP's question.

#7
09-22-23, 03:45 PM
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Wanted to find an answer without mentioning why to avoid picks and pokes at what I am doing. - it's a "sparking rail" for a haunted attraction. The truly best result is with a battery and shorting it out on the rail. With a smaller lets say tractor battery with less amps it dies very quickly. 2 hours +/-. Bigger battery lasts all night (6 hours) but the "wand" gets hotter than wanted and the best correction I can think of is if there were less amps being drawn. Then the battery lasts even longer yet and don't have to worry about heat. If I try the voltage drop = amp drop, how do i go about doing that? Let's say for example 12v 100amp so 8v 60amp* (or w.e the math is) I will assume it will still spark/arc fine since its the amps that's causing the show and not the volts, so do they make a dimmer switch of sorts that won't burn out when the loop is shorted?

#8
09-22-23, 06:58 PM
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Installing resistor in series will limit the current. However, it also weakens your spark effect.
So, this isn't really the correct solution in your case.
You don't want to limit current in your case because you want spark.

I think using a large 24V transformer might do the trick. May be 100VA ?
You can also try cheap AC stick welder with voltage set very low. Welders are essentially a high current transformer.

#9
09-22-23, 08:13 PM
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Unfortunately a battery won't take that abuse for long.
I agree with the welder idea if you have AC power available.
You need low voltage and high amperage. A welder can deliver that.

My other thought is to use carbon brushes.
Carbon is a natural resistor and may just deliver the effect you are looking for.
It would be experimental...... brush example
If you try this... be careful as the brushes will get hot.

#10
09-22-23, 08:27 PM
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We've used the same battery now for 3 seasons and its still kicking, would like to have a better set up though. I will look into the carbon brushes. Is a stick welder DC? I thought it was ac and I'd prefer if we can still grab the rail without consequence. If I tried a 24v 100va transformer you think it will hold up to the abuse? The "pros" use 10amp battery chargers but its not a very big effect as well as the 2 I tried had a safety that shut it off when crossed.

#11
09-22-23, 10:44 PM
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shorting a battery does not really seem very safe to me not sure what your doing or what your rail is even made of but I would expect it to get very hot and the battery to also get hot if you was pulling 100 amps.
would probably be safer to use a small coil or transformer that is much higher voltage but not much amperage small neon sign transformer may work could potentially shock you but really not a significant amperage still may have high concentrations of ozone if not used in a well ventilated area though.

#12
09-23-23, 08:07 AM
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what I am doing - it's a "sparking rail" for a haunted attraction.
H3re2Learn - I recommend that you look into using ferrocerium rods to produce the sparks effect you're after. It's easy and probably safer than dealing with a 12 volt battery.

Ferrocerium rods are made from a combination of cerium and iron. When ferrocerium is scratched the alloy gives off sparks. Think of an old Zippo flint lighter sparking to light the lighter fluid on fire. What was referred to as 'flint' is actually ferrocerium!

However, just for you - when ferrocerium is struck by a hard and sharp object, such as a carbon steel blade that can be handheld, tiny little shavings are oxidized (become combined chemically with oxygen) and actually ignite by the friction of the strike. These tiny fragments burn at up to 5,000°F, thus producing a lively array or cluster of sparks (as shown in the image below).

What's nice for you is that no batteries or cables are needed! The larger the diameter of ferrocerium rod and the harder the strike from a sharp carbon steel blade the more shavings become oxidized and the larger the array of sparks it will make.

If you don't want to manually strike the ferrocerium rod then you should be able to come up with some mechanical spinning or rotating device that substitutes the strike for you.

I hope this may help!

Last edited by Kooter; 09-23-23 at 10:14 AM.
#13
09-23-23, 09:34 AM
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You can also easily make a spark shooting ferrocerium cannon by putting a ferrocerium rod into a barrel tube made from a steel brake line and connect it to a PVC tube air chamber. Use a propane torch's flame to heat up the ferrocerium rod in the steel brake line tube to a molten state, then hit it with a blast of compressed air - and you'll release a massive display of sparks!

#14
09-23-23, 10:19 AM
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The rail is steel and is touched by people so the sparks can not be AC driven. I will buy some ferrocerium rods to do a test run but as I said in a previous post, there are haunt prop companies that sell battery chargers of such for the same effect but no where near as brilliant as more amps. Low amp and 1k + volts dc I think will also not be as great. I ordered a cheap dc stick welder to try but I think it will shut off after so much use. The "wand" is actor opperated for a scare and fire is definitely not an option.

#15
09-23-23, 08:59 PM
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AC or DC doesn't matter. What matters is the voltage. At low voltage, both AC and DC cannot conduct enough current to electrocute a person.

You don't want high voltage for the effect you want. High voltage will cause an arc, not sparks.

#16
09-24-23, 09:17 AM
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I have definitely felt a little shock from a 24v ac transformer for my doorbell vs I can grab both posts of a DC battery and nothing happen. I think our best result will come from the stick welder as long as it doesn't auto shut off.

#17
09-25-23, 06:15 AM
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Limiting the current draw (AC or DC) without causing unnatural voltage changes can only be done by cutting back on the load. For example not turning on all of the stove burners at the same time. Or by having a circuit breaker trip when the amperes draw exceeded a particular amount.

For arc welding you can hold the wand to manage the arc to limit the current draw but the strategy for getting a good weld might not be compatible with the strategy for preventing a breaker trip.

A series resistance cuts the amperage because the total resistance (of the original load plus the series resistor) is greater than just the resistance of the load while the overall supply voltage is the same. But with the series resistor taking fraction of the original supply voltage, the performance of the load with the remaining reduced voltage will (likely) be degraded.

It is possible for the resistance of the load to vary with the voltage applied (its share of the voltage with a resistor) so predicting the total resistance and therefore the amps draw is more complicated than you would think. But all the while Ohm's Law (described earlier) still applies within every portion of the circuit (such as, here, the welder by itself) as well as to the entire circuit..

Be careful when using a battery for applications (arc welders? flame throwers?) it was not designed for. Even though the ampere hours might make sense, a "car" battery rated at 100 A-H most likely cannot take a sustained 100 amp draw (even well under one hour) without danger of overheating internally.

Last edited by AllanJ; 09-25-23 at 06:59 AM.
#18
10-01-23, 06:22 AM
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Update* - we tried the DC arc welder last night for 5 hours and it seemed to perform well except for one possible issue. It's a 110 welder with a dial from 20-130 amps. I have it on its lowest (20) and it emits just as many sparks as the battery did except where the "wand" (3/8 X 6 standard bolt) makes contact on the rail, it's a super bright blue for .05 seconds. The battery didn't have the same hot spot I would call it and i am wondering if someone will go blind of sorts.

#19
10-01-23, 08:02 AM
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well yes its a welder would imagine all the normal ppe is required to safely use it like welding gloves , helmet etc not really a safe application for people viewing it they do make large tinted screens though. if someone used it for 5 hours without it they may have some vision issues but hopefully they will be temporary but still may be painful.