Limiting current

[kurt.brinker]

I have a pair of 5V power supplies that are each capable of providing 6.3A. I am wiring them in series so i will have a 10V 6.3A power supply. I want to connect a few circuits to it but i do not want more than 500mA going through each circuit. So would i put a 20ohm, 5 watt resistor on the return side of each circuit that i only wanted 10V and 500mA going through?


If the the resistance stays the same and the voltage drops, the current will drop as well right?

Thanks

 

[Hero999]

I have a pair of 5V power supplies that are each capable of providing 6.3A. I am wiring them in series so i will have a 10V 6.3A power supply. I want to connect a few circuits to it but i do not want more than 500mA going through each circuit. So would i put a 20ohm, 5 watt resistor on the return side of each circuit that i only wanted 10V and 500mA going through?

What circuits are you powering?

Generally, you don't need to limit the current, a circuit will draw as much current as it needs because its resistance will only draw a certain current, for a given voltage, see Ohm's law.

The only time current limiting i required, is to protect the power supply and wiring against a short circuit. If it's a ready made PSU, the chances are it has over-current protection built-in, if it's a battery then a fuse can be used to limit the current.

If the the resistance stays the same and the voltage drops, the current will drop as well right?

Adding a resistor will drop a voltage proportional to the current, see Ohm's law again.

If the circuit needs 500mA@10V its resistance will be 10/0.5 = 20 Ohm, so if you put a 20 Ohm resistor in series with it, the voltage will drop to 5V so you might as well have used a 5V power supply in the first place, look up potential divider.

This is theoretical because some loads are active which means their resistance changes, e.g. the resistance of a radio will vary depending on the volume and the sound, in which case adding a series resistor can cause it to malfunction.

 

[kurt.brinker]

let me back up and describe the system for you.

The system is wired in series with a 0-10V signal that is used to set the speed for the motors. We bring that signal into our system with an analog input card, we then make the changes we need to and source a 0-10V signal with an analog out card. Does that make sense? The original signal comes in and is branched to the AI card as well as to the a set of closed contacts on a relay. The other side of that relay is the wire going back out. When the system is off, the signal is just looped, when the system is on, the relay is powered on and now the AO is connected to the outgoing wire. Its a SPDT relay.

What i want to do is make a test unit so i can source 0-10V and then monitor the output. The problem is that when the system is off, there will be a direct short between the pos and neg on the power supply. (through the relay). I don't want to burn my relays up so thats why i wanted the limit the current. The AI have high input resistance so i don't need to do anything for those. I can just connect the psu right up.

 

[Hero999]

I think I'm with you.

The 0 to 10V is a signal going to a few high impedance inputs.

When the relay contacts close, the 0 to 10V signal will be short circuited but you don't want to damage the relay.

I recommend working out the impedance of all the inputs connected in parallel, divide it by 100 and use the next standard resistor value under the calculated value. You don't need to limit the current to 500mA, you want to limit it as much as possible, 500mA@10V is 5W which is quite a big resistor, if you can limit the current to say 25mA you can use a small 0.25W resistor.

 

[kurt.brinker]

The impedance of all the inputs? I don't follow.... Are you talking about the AI cards? I shouldn't need to add those into my calculations right.

 

[Hero999]

Please post a schematic, describing a circuit is never enough unless it's something really simple.

 

[kurt.brinker]

the test unit will tie in at the far right for an input. I will use my meter on the output.



**broken link removed**

 

[Hero999]

Sorry but that doesn't make much sense to me.

For a start it's an electrical schematic which I'm not used to.

What do lall the numbers mean SL-3/CR-12?

Where's the 0 to 10V signal?

Where are the two 5V power supplies wired in series?

What loads is the power supply powering? More importantly what's their impedance?

Basically, the current limiting resistor needs to have a high enough value to limit the current to a safe level but still low enough not to drop too higher voltage when loaded. If you can find a suitable value because the loads draw too much current, which will mean too higher voltage drop then you should use a PTC resistor (polyfuse/switch) as a protective device and functions like a resettable fuse.

 

[kurt.brinker]

Sorry but that doesn't make much sense to me.

For a start it's an electrical schematic which I'm not used to.

What do lall the numbers mean SL-3/CR-12?

Where's the 0 to 10V signal?

Where are the two 5V power supplies wired in series?

What loads is the power supply powering? More importantly what's their impedance?

Basically, the current limiting resistor needs to have a high enough value to limit the current to a safe level but still low enough not to drop too higher voltage when loaded. If you can find a suitable value because the loads draw too much current, which will mean too higher voltage drop then you should use a PTC resistor (polyfuse/switch) as a protective device and functions like a resettable fuse.

The SL denotes terminal blocks. So SL4-3 is Terminal strip 4 block 3.
CR denotes the relays.
the 10V signal comes in on 4 channels
Channel 1 comes in on 7 and leaves on 8. the relay determines if it is just looped back or if it comes from the output card.


The positive from my power supply will go to terminal block 7 and the negative will go to 8.

 

[Diver300]

CR1 to CR4 are relay contacts. 11, 12 and 14 are standard numbers for common, NC and NO contacts.
After that I'm guessing.

 

[Hero999]

I remember being an electrical engineer for a couple of years and doing the drawings both annoyed and confused me.

I never understood why they draw switch contacts like capacitors or why every single
terminal block and wire number needs to be on the schematic. Isn't that what assembly drawings are for?

Anyway, I've replaced the capacitor symbols with switch contacts and removed all superfluous information such as terminal blocks and wire numbers. This information might help when assembling or testing it but it's just distracting when trying to figure out how it works. All I need to know is what's connected to what, the wire number, terminal block isn't important at the moment.

Are the four 0 to +10V channels going to be connected together for the purposes of the test?

If so, we might as well draw them as one power supply for now otherwise they'll have to stay separate.

You still haven't answered my questions.

I can't help you if you don't tell me the impedances the 0 to 10V supply is driving.

Do you know?

Have you looked on the datasheet for the module?

The chances are the impedance is really high i.e. >100k so you can just put the supply in series with a 220R resistor.

EDIT:
Is my schematic right?

It does seem odd that you're switching the negative rail or have I got that wrong?

 

[kurt.brinker]

Sorry for my absence, I drove over 4K miles last week so i was a little out of the loop. I am back now and ready to work on this. I will be answering your questions shortly!

 

[kurt.brinker]

Ok first of all, i prefer your schematic over the one our guys developed. I know theirs is necessary for wiring it up but yours just cuts to the heart of it and lets you know whats going on.

These 10V channels are used as a reference for speed control. Under normal operations it is a simple 10V circuit with a pot used to adjust the voltage and in turn, control the motor speed. We come in and tie in series with that signal. As you can see on your diagram, when our system is off, the signal is just looped right back out to them. We are monitoring it so out outputs are already at the correct voltage in case they switch our system on. Once you turn on our system, instead of looping right back to them, the signal is now sourced from our analog output cards. (Based on the input we are still receiving)

What i want to do is connect a 0-10V source into the system so i can send different voltages to the analog cards and then meter the outputs to make sure they are changing. I would like to be able to turn our system on and off so i loop that power right back and make sure all my relay contacts are good?

Does that make sense?

 

[Hero999]

Sorry, that doesn't make that much sense.

Don't forget, I can't see the rest of the system. So far I understand that it's a four channel motor controller with the speed controlled by 0 to 10V inputs.

What do the different slots actually do?

With the system drawn above, with the relays in the unpowered state, all the 0 to 10V voltage sources will be shorted to 0V. The inputs to slot 4 will be floating which might lead to instability if they're high impedance and don't have internal pull-down resistors.

When the relays are activated, the 0 to 10V sources will be connected to slot 2 and slot 4 will be connected to 0V.

I'm not sure you understood me before when I talked about input impedance.

Each input will have a resistance which takes a tiny electrical current.

The 0 to 10V outputs will also have a source impedance.

In the schematic below, I've drawn the impedances on the schematic for the first three channels of slot 2 and the 0 to 10V voltages, I've assumed 10k and 100R respectively but it'll probably be much different. As the 0 to 10V sources are pots, the impedance will depend on the value and the wiper position, it will be very low when the wiper is set to either the maximum or minimum settings and highest when it's in the middle.

If the 0 to 10V channels already have a high enough impedance not to draw to higher current, then they can just be short circuited without any problems. If the impedances are low, short circuiting any of them will cause a high current to flow which will cause overheating.

If the input impedances as high and the inputs can't be short circuited, then you can limit the current by putting a resistor in series with the 0 to 10V voltage sources. Of course the resistor will need to be low compared to the input impedance but still high enough to prevent too higher current from flowing. For example if the input impedances are 100k then you could add a 470R resistor which will have negligible voltage drop.

 

[kurt.brinker]

The analog input has an input impedance of 100K ohms. So that means my current is 1mA@10V and less than that at lower voltages?


The input slots are so we can monitor the current system voltage so our outputs are ready to go in case they decide to switch on our system and let us control the motor speed. The output slots are the new values we send out for motor speed.


If you want, i can email you the full schematic...
I hate that i can't explain it correctly.

 

[Hero999]

No, the current will be 100µA@10V but yes, it will be lower at lower voltages.

Just add a 470R resistor in series with the 0 to 10V signal.

I've edited my previous schematic to show what I meant by input/source impedance. I know you know now, but I had already prepared it so it might help those who don't know.

 

[kurt.brinker]

Thanks for all your help. I will be putting this together soon and testing it out.

Be prepared to double your post count over the next few months.