General purpose current limit circuit

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NeilP   10 GW

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General purpose current limit circuit

Post by NeilP » Jun 08 2011 1:03am

http://www.vidisonic.com/2008/07/10/cur ... g-circuit/

Could anyone with more knowledge in electronic design look at the page above, and let me know if they feel a circuit like that could be built to work with currents up to 20 amps or so,
Better if it was variable, so one of the resistors a potentiometer, to allow a lower current

Looking for a way to limit current coming out of a PC PSU string for bulk charging
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Re: General purpose current limit circuit

Post by nieles » Jun 08 2011 6:23am

if we could just get the charging part of this BMS it would be awsome.

http://www.tppacks.com/proddetail.asp?p ... E-1BMS-PCB

the circuit in the BMS is a switching current controller. so the wasted power will be minimal


the circuit you linked circuit will act as a series connected resistor. (lots of power will be wasted when the voltage need to be dropped down)

edit: made my post somewhat clearer
Last edited by nieles on Jun 08 2011 7:38am, edited 1 time in total.

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Re: General purpose current limit circuit

Post by Ricky_nz » Jun 08 2011 6:41am

Not ideal as the sense resistor must drop near 0.6V at 20A resulting in 12W dissipated in the sense resistor which needs to be a precision device.

There is a better circuit involving an OPAMP driving a MOSFET(s) or Transistors.
The opamp has gain meaning that less voltage needs to be dropped accross the sense resistor.
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Re: General purpose current limit circuit

Post by NeilP » Jun 08 2011 7:27am

ok, thanks

is that on that site or elsewhere
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Re: General purpose current limit circuit

Post by grindz145 » Jun 08 2011 7:37am

Operating anything linearly to dissipate current is going to be a problem when you start to need any appreciable current. Need to use more of a PWM type of circuit, as a basic topology.

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Re: General purpose current limit circuit

Post by Ricky_nz » Jun 08 2011 7:48am

NeilP wrote:ok, thanks

is that on that site or elsewhere
I didn't look.
I haven't got the time at the moment to draw one up but its basically part of a linear power supply design of which there are hundreds on designs on the net normally fed from a transformer but could be fed from a switch mode supplys output.

Grinds is right about losses. If the circuit is dropping 3V @ 20A you will have 60Watts of heat to get rid of, if it drops 5V to limit the current thats 100W in heat. You may find the cost of building the limiter climbs to the point it is better to buy the correct power supply.

That said I have a linear power supply that supplies some of my amateur radio equipment thats capable of 25A output and drops similar voltages and its well heatsinked . I built a 40A one and it definitely needed to be fan cooled and used 8 * 2N3055 transistors.
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Re: General purpose current limit circuit

Post by grindz145 » Jun 08 2011 8:08am

Ricky_nz wrote:
NeilP wrote:ok, thanks

is that on that site or elsewhere
I didn't look.
I haven't got the time at the moment to draw one up but its basically part of a linear power supply design of which there are hundreds on designs on the net normally fed from a transformer but could be fed from a switch mode supplys output.

Grinds is right about losses. If the circuit is dropping 3V @ 20A you will have 60Watts of heat to get rid of, if it drops 5V to limit the current thats 100W in heat. You may find the cost of building the limiter climbs to the point it is better to buy the correct power supply.

That said I have a linear power supply that supplies some of my amateur radio equipment thats capable of 25A output and drops similar voltages and its well heatsinked . I built a 40A one and it definitely needed to be fan cooled and used 8 * 2N3055 transistors.
I have a big old linear supply like that for my ham gear too. It weighs like 15kg.. It's amazing how small a 300W switcher is by comparison. :mrgreen: Ahh the good old days.

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Re: General purpose current limit circuit

Post by bigmoose » Jun 08 2011 8:52am

Neil that circuit would work to keep the supplies from tripping off. The PWM approach may not, as the current spike may be greater than your supplies can deliver.

Perhaps try to scrounge up a lab type supply with voltage and current pots to set their values. One lab supply in series that is current limited will limit the string's current; and the variable voltage may let you tune your set point.
Last edited by bigmoose on Aug 07 2011 7:01pm, edited 2 times in total.
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Re: General purpose current limit circuit

Post by grindz145 » Jun 08 2011 8:58am

If you're going to go linear, just get a bunch of PTCs and put them in parallel until you get the desired charging current. Cheap and easy. Not terribly precise, but I don't think precision is what you're going for here. Just a tangential suggestion...

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Re: General purpose current limit circuit

Post by NeilP » Jun 08 2011 3:19pm

Ricky_nz wrote:
NeilP wrote:ok, thanks

is that on that site or elsewhere
I didn't look.
I haven't got the time at the moment to draw one up but its basically part of a linear power supply design of which there are hundreds on designs on the net normally fed from a transformer but could be fed from a switch mode supplys output.

Ah, ok, i mis understoodI thought you menat you had seen one somewhere there, no problem


[quote="Ricky_nz"
That said I have a linear power supply that supplies some of my amateur radio equipment thats capable of 25A output and drops similar voltages and its well heatsinked . I built a 40A one and it definitely needed to be fan cooled and used 8 * 2N3055 transistors.[/quote]

I had started building a 12 volt 20 or 30 amp variable supply many years ago for a 70cm rig...(was it a Pye Vanguard...cant remember), and that too had a bank of 2N3055's as the output...but there again I was never good enough with theory to get the design correct, and all our ham radio club members were all in to the modern Icom and Yeasu black boxes, so the project was never finished.

If you're going to go linear, just get a bunch of PTCs and put them in parallel until you get the desired charging current. Cheap and easy. Not terribly precise, but I don't think precision is what you're going for here. Just a tangential suggestion...
PTC's? but no, I already have these supplies and the idea is to use them as a second charger to suplement my pair of Meanwells that are at home. I have a good charge system , just wanted to have a backup for free or nearly at the farm workshop as well as one at home.

Thinking just maybe a big rheostat from a theatre or similar, so I can have a variable resistance...start it high and bring it down, once the initial connection current surge that trips the supplies out has passed
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Re: General purpose current limit circuit

Post by www.recumbents.com » Jun 08 2011 4:07pm

Ok, this is either really simple or impossible.

1) Find a zener diode with the proper cutoff voltage
2) Use a 12V multiple pole relay to switch the power to the PSA and from the PSU to the battery. A momentary push is in parallel with the pole that switches power to the PSU. The button starts the PSU, which energizes the relay. PSU starts charging. when voltage goes high zener diode disables power to the relay and everything shuts off. Maybe a pot thrown in in series with the zener to provide some adjustment?

-Warren.

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NeilP   10 GW

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Re: General purpose current limit circuit

Post by NeilP » Jun 08 2011 4:34pm

I cant see how that limits the current though? I need to have a low current to start with, and then the ability to turn the current up once the PSU is running
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Re: General purpose current limit circuit

Post by NeilP » Sep 19 2011 2:24am

It has just struck me how to do this...buy a single Meanwell..say a S-350-12 and do a SVR2 R33 mod with say a 5 or 10k pot...the 12 volt meanwell will do up to 29 amps or so , downwards
So just stick one of these in series with my string of PC PSU's


So simple..not a standalone circuit but it will work
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Re: General purpose current limit circuit

Post by Spicerack » Sep 19 2011 5:57am

....if you can find a proper S model meanwell/clone....with the svr and scr circuits....and not a NES model.... :?
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Re: General purpose current limit circuit

Post by NeilP » Sep 19 2011 6:37am

I know, already blown a NES
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