Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations IDS on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

current limiting 3

Status
Not open for further replies.
kenre

kenre

Mechanical
May 23, 2005
300
I once had a circuit diagram for adjustable current limiting using a ua 741 and 2n3055 pass transistors.
No longer to be found. Anyone have a similar one? its to limit up to 30 amp.


Was told it couldnt be done, so
i went and did it!
 
Replies continue below

Recommended for you

Sort by date Sort by votes
Look for supplies by Farnell / Advance, Power Ten and HP / Agilent as starters for high current, low voltage switching power supplies. The really heavy current low voltage stuff is a minority interest so it tends to turn up way below true value. I picked up an 8V 580A HP unit for under £1000 when list is maybe ten times that. There's plenty of big PSUs out there, and most aren't too fussy about 50Hz or 60Hz.


----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Upvote 0 Downvote
I wouldn't exactly say impractical. I have an ACDC EL-300 electronic load that is a similar item in not a very large package. Rated at 60V, 60A, 300W it has six transistors taking all the load. Without the fan it can handle 60W. Any voltage and amperage combination can not exceed 300W. I had searched the anodizing community message boards and people there are using a car battery, lamps, and battery chargers with light dimmers. I have asked for the supply voltage and normal electrode voltage and never received an answer. There was some mention of 30V which may mean he is starting with an unusually high starting voltage. I we don't get better details of the process he will soon be left alone with the dingoes.
 
Upvote 0 Downvote
My Lambda 15v (regulated, adjustable +/-5%) 80A power supply was only $40 Cdn at a local Ham radio flea market. That's about $0.50 per lb of weight.

And since my PS more-or-less covers the highly desirable 13.8Vdc car battery replacement voltage range, it is more of a sellers' market. Units that don't cover this 12 to 13.8Vdc range are a buyers' market in the Ham radio world. I've seen units that were a '2-man lift' being sold for $10-$20 range.

Getting one with a current limit knob narrows the market a bit. But (for example) my company once pushed an old surplus test rack out on the loading dock heading towards the dumpster (skiff). I removed the 0-40A 0-30A HP PS and brought it back into the lab. If I hadn't, it would have been landfilled.

So, there's lots out there if you look in the right places. And they shouldn't cost much if you're lucky.

 
Upvote 0 Downvote
Dingoes wont cause any harm!
the howling does get annoying tho
ebay australia is very very dissapointing
Ok, i WAS hoping to use the one supply for anodizing (decorative color), hard anodizing and hard chrome plating.

I now see that the large range of voltage is causing you blokes all kinds of headaches trying to sort this out.

Decorative anodising, up to 25 volts.
Hard anodizing uses from 25 to 50V,
Hard chrome 5 volts.

i think i need 3 supplies!! Thats ok, i can do that.






Was told it couldnt be done, so
i went and did it!
 
Upvote 0 Downvote
Use the same limiter, just adjust the input voltage going to it. A variac would be ideal to adjust the input voltage to the transformer. Other transformers you have around could be as a buck transformer to lower the line voltage 12,24,36 and 48 volts. These would not have to be large, the secondary current only needs to be that of the power supply primary. Keeping the differential voltage of the limiter to around 10-15V will keep the heat reasonable, minimum 5V. 300W would be about right for maximum watts. Just add two more transistors in parallel and one more to drive them. E of driver goes to base of the parallel transistors. A voltmeter or two small 12V lamps in series from input to output of the limiter would be helpful in adjusting the right voltage.
 
Upvote 0 Downvote
One thing that has bothered me since your first posts is your requirement of little ripple. I have seen most chemical reactions benefit from pulsing, batteries love it. One site uses a PWM speed control modulator they got off ebay for anodizing to adjust average current. There seems to be little agreement about anything on anodizing message boards.
 
Upvote 0 Downvote
Hard chrome needs less than 5% ripple. your description of low ripple may be alot different.
I had a win!! found the original circuit today, will post the circuit tommorow for you viewing pleasure.
UA741 wont take 40 volts so theres a prob already.

I have learnt so much with this thread, thankyou to all!!!








Was told it couldnt be done, so
i went and did it!
 
Upvote 0 Downvote
Hi kenre;
I just looked at this thread. As I read it, I kept thinking, Ohm's law, I=V/R. Use a shunt resistor and control the voltage across the shunt. This will give accurate current control with minimum losses.
VE1BLL said basically what I was thinking.
If your power supply has a voltage regulator, you may need to change only one or two connections in the power supply.
If you use a 1 ohm resistor in series with the load, then the current will equal the voltage. Use a 0.1 ohm shunt (more practical) and the current will equal 10 X the voltage across the shunt.
If you can post the circuit diagram I am sure someone will be able to identify the wire that needs to be reconnected.
With a 0.1 ohm shunt resistor, the loss in the shunt at 30 amps output will be 90 watts. (I^2*R)
This will maintain the set current even when the load resistance changes.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Upvote 0 Downvote
Hi Bill,

That's what the circuit posted above is supposed to do. The non-inverting input of the uA741 is set to a reference voltage by the pot, the circuit regulates the current until the voltage fed back to the inverting input of the uA741 from the shunt resistor is equal to the reference. Looks like there might be a few components missing from the sketch but the underlying principle isn't unreasonable.


----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Upvote 0 Downvote
Scottyuk, no components missing other than the obvious transformer and bridge rectifier.
What has me stumped now is the ua741 only handles about 18 volts, can i supply this with 18 v via a regulator and still have it work? Speaking to an anodiser recently, they start at 30v and ranp up to 80v to supply the required current.

Ive been looking for a PWM circuit, but nothing with higher current limiting.




Was told it couldnt be done, so
i went and did it!
 
Upvote 0 Downvote
Kenre,


Not as it stands - the uA741 supply shares the same reference as the main power circuit, and it has to be able to deliver an output on pin 6 which is two Vbe drops greater than the output voltage, so the supply range of the uA741 limits the output voltage available. That circuit is going to need quite a few mods (i.e. re-design) to work over a larger voltage range. If this is anything other than an interesting hobby project I would seriously look for a commercially built lab power supply. There have been a few on UK eBay which would have done this job since this thread started - wrong continent I know, but there's bound to be one sooner or later.


----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Upvote 0 Downvote
This circuit really isn't any different than the one with the LM371. The LM371 just builds the op amp voltage ref and driver transistor into one package. The transistors in your circuit won't share the load evenly and you could use LM347 with a single supply.
 
Upvote 0 Downvote
I doubt you will find an off the shelf design for a 30A, 0-70V adjustable current PWM supply. Honestly, if you are having some difficulty with the linear design I don't think designing a switching supply is for you. Not trying to offend you but a PWM will require more engineering than a linear.

I don't agree with Scotty's assessment of the power supply though. You can use a separate supply for the control circuitry. For 30A in the load the op-amp needs to generate about 3V or so on it's output. As you turn the current down the voltage required also goes down. You also should be able to find a better opamp that will run single ended at about 5V to do what you want. The transistors all need to be rated for the supply voltage plus some margin so they don't fail (I'd use at least 1.5 times personally).

I do agree with Scotty when he says you need more circuity to be able to parallel the transistors and get them to load share. You generally can't just parallel bipolar transistors because they do not share well. I think a shunt for each emitter and then take the feedback off each shunt with a resistor (say 10k) all joined at the opamp would likely work. Like combining a summing circuit and the control in one opamp.

 
Upvote 0 Downvote
Oops, there is something missing! there should be a .1 ohm resistor (5watt) on each emitter of the 3055s.

No offence taken, i didnt design this circuit myself. I wish i was able to hehe.




Was told it couldnt be done, so
i went and did it!
 
Upvote 0 Downvote
That sounds more likely to work. Combining them to one resistor to measure the current will likely be more accurate.

I'd say find a single ended opamp that will run on something between about 5V and 12V. Then, build a power supply to match the op-amp and the rest of the circuit as you've shown and see how it works.

Is this for the RC business?
 
Upvote 0 Downvote
Hi Lionel,

Regarding the opamp supply... yes, you're absolutely right that a separate supply could be used which gets around my 'non-problem': I'd mentally inserted something which isn't there in the drawing. I've even been back to check!


----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Upvote 0 Downvote
Lionel ,
Eventually i will add this to the business, as part of another much larger project. Im no good at designing electronic circuits, but really enjoy putting them together from a circuit diagram or even a kit.

Scotty, are u reffering to my post about supplying the opamp a lower voltage?










Was told it couldnt be done, so
i went and did it!
 
Upvote 0 Downvote
Did you ever try putting a battery bias in the common of the LM317 to subtract off the reference voltage and make it adjustable? I don't think you can beat using the LM317 that adds some current and thermal protection with that low a part count.
 
Upvote 0 Downvote
I havent had a chance to get back to this yet, been busy doing what im better at hehe, making metal swarf.

Do you have a sketch?




Was told it couldnt be done, so
i went and did it!
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

mavila2012
  • Locked
  • Question
Voltage Divider (current not stable) 1
Replies
10
Views
148
TNMaster
TNMaster
RyreInc
  • Locked
  • Question
Contactor performs better at switching current than carrying current 3
Replies
14
Views
116
IRstuff
IRstuff
ultrasoundguy2
  • Locked
  • Question
current limiting
Replies
13
Views
127
gorlandheli
gorlandheli
gmarocco
  • Locked
  • Question
Doubt on copper pour area width for current capacity 1
Replies
5
Views
115
itsmoked
itsmoked
itsmoked
  • Locked
  • Question
Super Cap leakage current
Replies
17
Views
116
MacGyverS2000
MacGyverS2000

Part and Inventory Search

Sponsor

Back
Top Bottom