Power supply going into CC - Page 1
Forum section: electronics / beginners

author: sy
author_rank: Regular Contributor
timestamp: February 29, 2024, 02:38:19 am
content: Hello,I am trying to build a non-inverting amplifier.To get the +5V and -5V rails I am connecting the Master+ and Slave- together and using this as the ground point. -5V would be Master- and +5V would be Slave+.However, when I connect the power to my circuit the voltage drops to 0.8Vish and enters constant current mode.I've tried increasing my current limit, however, even at 700mA it still stays in CC. I would've thought at some point it switches over to CV but 700mA seems like a lot of current for a circuit like this.Any help would be appreciated.Thanks!

author: donlisms
author_rank: Frequent Contributor
timestamp: February 29, 2024, 03:08:04 am
content: Have you double-checked your power connections, slowly and carefully? Something doesn't seem right on the board. I would expect to see a red wire for the positive supply, a black wire for the negative supply, and (whatever) for the ground connection. I haven't traced everything; it's just that the supply wiring seems a little odd.When you're quite sure it's perfect, put your voltmeter on the connections and the board, and confirm. If you would like, you can do that without the rest of the circuit connected, so all you are verifying is the supply connections.And just a comment... I would have expected "master" to be used for the positive supply, not because it really matters, but because it seems more conventional to control things from the positive side. Usually.

author: Jwillis
author_rank: Super Contributor
timestamp: February 29, 2024, 03:17:41 am
content: The 10uF cap looks backwards on the negative side. For the negative power source the polarized caps positive should be connected to common and the negative should be connected to the negative supply.

author: pqass
author_rank: Super Contributor
timestamp: February 29, 2024, 03:49:26 am
content: I'm not sure this will make much difference, but I like to park any unused op amps as buffers (-ve to out, +ve to gnd) rather than both +ve and -ve to gnd, out left open.Also, confirm that you have connected pin 4 to +5V and pin 11 to -5V (seedatasheet). This seems reverse to what you have just judging by your chosen wire colour. That is, the upper red power rail should be attached to the -5V source and the lower red power rail to the +5V source. And that the upper electrolytic cap is backwards; negative band lead to the -5V source not the blue gnd rail.

author: sy
author_rank: Regular Contributor
timestamp: February 29, 2024, 04:16:47 am
content: Quote from: pqass on February 29, 2024, 03:49:26 amHi pqass, just looking at the package how do we find out where pin 1 is?I believe there's usually a circular notch in one of the corners, but both cut-outs seem to be aligned in the middle of the component.I took pin 1 using the side with the circular divet.

author: MrAl
author_rank: Super Contributor
timestamp: February 29, 2024, 04:22:32 am
content: Quote from: sy on February 29, 2024, 02:38:19 amHi,As others have said, it could be just that one or both of the electrolytic caps are backwards. You should be able to do a rough test without those caps, then put them back when done. One or both may be blown out now though so you may have to use new ones now.A problem like this has to have something like that wrong or just a short in one of the power supplies.You could do a test with just the positive power supply connected and make sure that works first. You might get a weird output but at least you would know it works like that to start with. You don't have to use an AC input to test right away you can use a DC adjustable source like with a potentiometer.

author: pqass
author_rank: Super Contributor
timestamp: February 29, 2024, 04:31:03 am
content: Quote from: sy on February 29, 2024, 04:16:47 amBeware of injection moulding marks; that circle depression on the right side. That's NOT marking pin 1.See attached for different styles of pin 1 markings.In dual-in-line packaging, it's usually to the bottom left of the ink/etched lettering being right-side up.In your case, the 1st picture below. When in doubt, see thedatasheetpage 22.

author: minsik
author_rank: Contributor
timestamp: February 29, 2024, 04:39:38 am
content: Check the datasheet. Sometimes the circular divot is a mould mark in manufacture and not an orientation thing. Usually the datasheet for that brand will clear up any problems.Also for a +- supply at top looks ok, but negative of top supply should go to positive of the bottom supply -+. Can your power supply do this or is it just a dual supply with both sections negative grounded.Just put power supply into the board rails (with nothing else) and sort out that first. When you have a good +- and -+ supply, where with the meter reference lead=black is connected to the common of the two supplies, which will be the negative of the top supply, and the positive of the bottom supply, and your multimeter shows a positive voltage in the positive rail ie top, and the probe connected to the negative rail of the bottom supply where the meter must show a negative voltage.Then think about constructing the rest of the cct. Until power supply is generating the positive and negative (with a common) correctly then set current limiting to a modest amount like 100ma maximum, you are safe to proceed to making a cct work.Good luck from Minsik, South Australia.

author: sy
author_rank: Regular Contributor
timestamp: February 29, 2024, 04:56:27 am
content: Okay after making the changes everyone suggested above I managed to get it working.Quote from: minsik on February 29, 2024, 04:39:38 amMy other question is how do we determine the max current to set to ensure the IC is not damaged?Is there something in specific I should be looking for in the datasheet?

author: pqass
author_rank: Super Contributor
timestamp: February 29, 2024, 05:30:32 am
content: Quote from: sy on February 29, 2024, 04:56:27 amGreat to hear.Glad you didn't blowup the chip or caps (that would have been exciting).QuoteMake a rough assessment of all the loads in the circuit that can draw power.Like: the op amp IC itself (seedatasheetIcc page 5), resistor dividers (22K+10K), (22K+4.7K), output load (10K); which amounts to 3.2mA (*4 op amps) and <1mA for the rest.If it's this low (<15mA) then just double (or triple) the value. You're shooting for a low enough value that you're almost certain that the circuit would work. The lower you can get away with the better since if you did make a mistake and there was a low resistance path, that the PS set current limit would limit the power drawn. The chip may be able to cope with that for a few moments and not instantly let out the magic smoke. This gives you enough time to power-off.

author: minsik
author_rank: Contributor
timestamp: February 29, 2024, 11:16:50 am
content: As been said, once calculate the needed maximum current (i prefer an educated guess, much quicker) set the cc current limit adjustment to its lowest, then short together the + and - wires, set the current to say 100ma, very safe and low enough to prevent wires burning up etc, and then return to the correct connections. From then on no touchy the CC adjustment! and cct is protected from disasters.I have used a home made 0-30V 0-30A power supply in a workshop repairing small radios, to uhf 5W transmitters and vhf 25-30W transmitters all by setting a realistic maximum current.If the current limiting is good the foldback will limit the output to a fraction of a volt at the overload.good work on the progress.

author: MrAl
author_rank: Super Contributor
timestamp: February 29, 2024, 06:09:25 pm
content: Quote from: sy on February 29, 2024, 04:56:27 amHi,You look up the data sheet(s) and find out what the maximum voltage and current is, and also the maximum power dissipation and maximum temperature which tells you if you need a heatsink. If you need a heatsink then you have to calculate the size you need.