Author Topic: Alternative size current sense resistor  (Read 1584 times)

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Offline queennikki1972Topic starter

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Alternative size current sense resistor
« on: September 23, 2019, 09:20:46 pm »
I have over 40 current sense resistors but of a different size than the diagram. I'm trying to learn to have a current limit on my linear supplies.. I was wondering if this is right if I use
Current Sense Resistor 3W .05ohm or if the mentioned 10ohm is recommended..

minimum current = 1.25/R1  (.05ohm)   means 25ma minimum   ??????

R2= 220ohm resistor
R3= 5kohm pot
Maximum Current is 1+(R3/R2) x the minimum means 593mA maximum   ?????

These are not real numbers just checking the math
« Last Edit: September 23, 2019, 09:34:21 pm by queennikki1972 »
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Offline MosherIV

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Re: Alternative size current sense resistor
« Reply #1 on: September 23, 2019, 09:39:13 pm »
Hi

I would suggest that you do NOT try to use LM317 to implement current limit.

In a linear supply, current limit is done by ORing together the voltage setting with the current limit setting. Best done by ORing together the output of 2 op-amps.
When the current limit is reached, the current limit op-amp drags the output voltage down to limit the current.
This would be very difficult to do with LM317.

Have a look at the 2nd psu circuit from this link
http://www.bowdenshobbycircuits.info/page4.htm
Not the clearest current limit circuit to understand but clearly shows how to OR the output of 2 op-amps together


 
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Offline queennikki1972Topic starter

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Re: Alternative size current sense resistor
« Reply #2 on: September 23, 2019, 09:51:20 pm »
Unfortunately its a little hazy for me to read at my level of experience.... I'll study it and see if i can get the idea.
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Offline queennikki1972Topic starter

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Re: Alternative size current sense resistor
« Reply #3 on: September 23, 2019, 09:55:34 pm »
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Offline MosherIV

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Re: Alternative size current sense resistor
« Reply #4 on: September 23, 2019, 10:12:04 pm »
No problem.

I searched for ages looking for DIY bench PSUs. I studied them until I understood.
Eventually, I found this one :

http://www.microsyl.com/index.php/2010/03/31/bench-power-supply-0-25v-0-5amp/comment-page-2/

I found it by the easiest to understand.
The current reading is done with a ZTX1009, feeds an op-amp which compares it with a current limit setting set by R11.
When the current limit is reached, the op-amp U1B pulls the voltage op-amp down via the diodes D4 & D5.

Personally, I alway prefer high side current sense and power drive.
Additionally, use Bi-polar transistors, then yo can add a switch to the base of the power transistors for a simple low current, output on/off switch. You cannot do that with FET based PSU designs.
 

Offline queennikki1972Topic starter

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Re: Alternative size current sense resistor
« Reply #5 on: October 10, 2019, 01:45:10 am »
How do you feel about this circuit?

5A constant current constant voltage regulator

http://www.circuitstoday.com/few-lm317-voltage-regulator-circuits
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Offline MosherIV

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Re: Alternative size current sense resistor
« Reply #6 on: October 10, 2019, 07:07:32 am »
How do I feel about it?

 :-/O
Difficult.
I am impressed controlling both V and current limit can be done with LM317.

Do you understand how it works?

It needs 3 voltage rails, Vin, V+ and -6V.
The volt adjust and current limit is not that easy to understand.
It may not be easy to add panel meter to show the set voltage and/or set current.

If you just want to build a bench psu - this may work.

Does it help your understanding - only you can answer that.
 
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Online MarkF

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Re: Alternative size current sense resistor
« Reply #7 on: October 10, 2019, 01:41:12 pm »
If you use a rail-to-rail or single supply op-amp, I believe you could make it work with lower output voltage range.  Maybe worth trying anyway.

« Last Edit: October 10, 2019, 01:53:26 pm by MarkF »
 

Offline Kleinstein

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Re: Alternative size current sense resistor
« Reply #8 on: October 10, 2019, 04:17:55 pm »
The current limit is usually set by the resistor and the set voltage to drop at the resistor at most. Going from 10 Ohm to 0.1 Ohms is a large change, getting current 1000 times higher. So the math in the initial posting is wrong.

Adding a variable current limit to a LM317 or similar voltage regulator is difficult to get good. It is usually easier to start with a combined constant current / voltage regulator with an external power transistor. So more like using an LM723 or 2 OPs and a  reference. A lab supply is a rather common topic - with plenty of poor quality circuits around.
 

Offline AVGresponding

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Re: Alternative size current sense resistor
« Reply #9 on: October 10, 2019, 04:55:24 pm »
Using a single supply certainly makes things easier; the original design would have needed 4 power rails, with the +/- rails for the op-amp needing to be fairly well regulated if you want to avoid oscillation, which given the AC coupling of the feedback on the op-amp is likely to make your load regulation, shall we say, 'poor'  :scared:

It may be also a better idea to use something like an L200 instead of an LM317, as it's specifically designed to have variable current regulation, and uses the same TO220 package, albeit a 5-pin variant. It also has a TO3 version, much as the LM317/337 etc do with the K series.

The ST datasheet includes example circuits with extended current capability using a pass transistor, and doesn't include AC coupling in the feedback circuit...
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