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Need help with MAX4172 Precision High Side Current Sense Amplifier Pls

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Something else I forgot to mention is that there are definately two separate windings to the transformer that are split by a nylon divider.

hi Al,
I will use your sketch for a new simulation, will post it shortly so that you can check it, sorry to cause you grief.

Initially I thought were designing a completely new power supply, didn't realise you were trying to upgrade an existing commercial one.:rolleyes:

E.
 
Hi Eric,

At one time I was looking at making a PSU from a PC supply. Plenty of power and a good few different voltages but no variable! As I understand it the switch mode method of the PC PSUs is difficult to reverse engineer to be controlled easily so I put that idea away for good.

Hope the drawing is good enough and you can understand my scribble? If anything seems like a blatant error to you then give me a shout and I will try to straighten it out.

Al
 
hi Al,
Check this now is the same as your sketch, mark the drawing with any errors you may see.

You can join the 0V on the +5V with the Common 0v [ground] on the dual supply.

The dual supplies have a shared ground [0v], but you can connect a 'load' across the +Vout and -Vout if required, but the 0V common must be left floating when you do.

E

EDIT:
Scribble on that drawing where you want the current limit and the current sensing circuits
 
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Hi Eric,

Just had a quick scan of your very elegant schematic, nice! What do you use to draw them?

Also I noticed that you didn't use the common terminal (centre tap) of the second winding?

Is that intentional? All 5 terminals are used on my PSU transformer. Have you done it for a reason?

Al
 
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Hi again Eric,

Just noticed you hace both variable voltage regs as 337s. This has one 337 and one 317. Also R9 is 4k7.

Al
 
Hi again Eric,

Just noticed you hace both variable voltage regs as 337s. This has one 337 and one 317. Also R9 is 4k7.

Al

hi Al,
That top LM337 is just a typo it should be LM317, if I make R9 = 4k7 and add the missing CT to 0V, is that close to what you have.

E.
Scribble on that drawing where you want the current limit and the current sensing circuits
 
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Hi Eric,

Here is your schematic with my proposals for current limiting. Yes with the ct to 0 and the 4k7 then it is pretty much what I have here.

Please note that though I have shown pots on the 317s they would be actually 12 different resistors on a 12 way rotary switch. Unless you know how to do it with a pot?

I discounted pots due to working out the wattage carried by the resistor at load being about 1/2w which is bigger than any of the pots I have for sure.

Al

P.S. Please forgive the crudity of the alterations. They make your nice neat drawing look much worse, sorry. The missus says " why can't you do pretty drawings like Eric's". Al ;)
 
hi Al,
Looks possible, only thing, I thought you wanted the 5V to have current measurement circuit and not current limiting.

E.
EDIT:
Noticed you have made the -Vreg limiter a LM317, it will have to be a LM337
 
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Hi Eric,

Yes I do want to measure the current. I thought that the best way was to have a set limit to test things with e.g. 50mA on a circuit that should use less than that and then read the actual current being used.

The limit would be just for safety's sake really, and the odd time for testing power LEDs and such.

As to the 317 or 337, I wasn't sure how it be? What if I used rail to rail on the variables? Would one current reg be enough if it was only on the high side? I hardly ever need the negative voltage and really only use it as a ground when I need more than 15v to test something. New laptop screens are mostly lit by LEDs now and the little buggers tend to be in series of 7, 8 or 9 so you need up to about 30v to try them. That is when current limiting is really handy. But if I only had limiting on the high side wouldn't that be sufficient as the high side could only let pass the set amount of current. Or am I as usual confused?

My wish list is:

Display current used from 5v
Display current used from + Variable v
Display Voltage of +Variable
Display Voltage of -Variable

Do you think this is viable and do you know any way of using the say 317 to limit current using a standard pot?

Thanks again, Al
 
hi Al,
Look at this.

I have also posted a larger diagram without the plots so its easier to read.

E.

R10 and R11 on your build will be switched resistors to suit the current limits you require.

Current Limit = 1.25v/R
 
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Hi Eric,

Well looks like I can almost get cracking. Just another couple of questions.

Firstly do I need a current limiter on the low side at all? I hardly ever use the pos and neg together(with CT) so if I use just the pos to CT then it would limit it ok and if I used pos to neg without the CT then wouldn't the high side reg be enough to limit the current? I'm thinking that even from pos to neg it could only use the limit set by the high side reg?

Also is it viable to use a 317 on the 5v line the same as in the pos variable on the schematic? It would be handy to have the limiter on the 5v line too. I have more 12 way switches! ;)

And finally, would you be kind enough to choose me an op-amp from the ones I have? I don't know enough about them to be honest and would hate to make the wrong decision.
LM107H, LM358N, ZN416, UA741, LM748, LT1101, LM1458DP, MC11458 AND NE5534P.

Thanks again Eric, maybe one day I will finish a project without you but I'm not holding my breath! ;)

Al
 
hi Al,
Will look it over, the LM358 is about the best from that group.

Post tomorrow.

E,
 
hi Al,
I think this now includes all the points on your wish list.;)

E
 
Morning Eric,

Just had some more bad news. My sons ex has just phoned to say that if he wants to see his son again it has to be through court! She really is a nasty piece of work.

Enough of my personal life anyway, perhaps I shouldn't burden others with it but it seems to help telling mates.

Back to the circuit. I take it the LM358 is still the best bet then? What gain should I expect from it? Is it a fixed formula or suck it and see? ;)

Thanks again for all the help mate. Its good to know there is someone I can count on. Al
 
Morning Eric,
Back to the circuit. I take it the LM358 is still the best bet then? What gain should I expect from it? Is it a fixed formula or suck it and see? ;)
Al

hi Al,
I would use the LM358 for starters, its the best in your list.

The gain is determined by the loop around the OPA and the 2N3904, you dont have to set it.

I would wire that 15K pot in the emitter of the 2N3904 so, one end of the pot to the emitter, the other end to 0V and use the pot wiper output to the ADC. You can then tweak the Vadc as required.

E.
 
Thanks Eric,

I'm starting the build now. I will keep you up to date and post some pictures when its done.

Al
 
Thanks Eric,

I'm starting the build now. I will keep you up to date and post some pictures when its done.

Al

hi Al,
I would suggest a resistor/ LED on each of the supplies, after the current limiters, to show that the supply is OK.

Also if you have space on the front panel an On/Off switch for each supply +15V, -15V and the +5V.

E.
 
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