best way to lower minimum voltage of a cheap lab power supply - Page 1
Forum section: electronics / beginners

author: guitchess
author_rank: Regular Contributor
timestamp: March 09, 2022, 04:24:35 am
content: Until recently my electronics experiments were powered with diy PC power supplies or batteries or wall warts of the right voltage. I decided to upgrade for a little more precision and an easier way to limit current. Unfortunately, I made an unwise decision and bought one the cheap 5-30v 10A supplies from Amazon.It's seems to be a fine supply considering, but the control at the lower voltage/current limits is atrocious. I plan to build a reducing circuit so I can have usable voltage down to 1.5v. The question to those with more experience: What would be the best route to create this reducing circuit. Simple voltage divider, buck regulator, etc.?Thanks for your input.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2022, 04:35:51 am
content: Linear lab supplies that work down to zero require a small negative supply in addition to the larger positive supply so that the pull-down resistor (to the negative supply) can get the output to zero.In general, you can't get to zero unless you are able to go negative. (Switching off doesn't count.)

author: guitchess
author_rank: Regular Contributor
timestamp: March 09, 2022, 05:04:29 am
content: Thanks for the reply. I'm not really wanting to go all the way to 0v. I would be happy with 3v, but my goal is 1.5v.I've thought about just setting up a second supply, PC supply, and using the +/- for a linear reminiscent output. But then the same question would still apply. Passive circuit or active?Thanks.

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 09, 2022, 08:18:14 am
content: Quote from: guitchess on March 09, 2022, 04:24:35 amyou should provide more information to get useful advice... at least a link to the PSU you bought.

author: SmallCog
author_rank: Frequent Contributor
timestamp: March 09, 2022, 10:08:43 am
content: How much current? Seems like a good time to learn about linear regulatorshttps://www.electronics-tutorials.ws/blog/variable-voltage-power-supply.html] https://www.electronics-tutorials.ws/blog/variable-voltage-power-supply.htmlOtherwise run one of the little DCDC modules off your existing supplyhttps://youtu.be/Cw2AjcczHg4] https://youtu.be/Cw2AjcczHg4

author: tunk
author_rank: Super Contributor
timestamp: March 09, 2022, 12:38:39 pm
content: More info please, e.g. linear or SMPS, photos of the internals, etc.

author: magic
author_rank: Super Contributor
timestamp: March 09, 2022, 05:25:00 pm
content: Resistive dividers suck for any sort of real power delivery (the output just doesn't stay constant under load), but for currents less than 1A you could put together a simple variable output LM317 regulator and power it from the "big" PSU.

author: rstofer
author_rank: Super Contributor
timestamp: March 09, 2022, 07:54:22 pm
content: Quote from: magic on March 09, 2022, 05:25:00 pmDefinitely! Here is the datasheethttps://www.onsemi.com/pdf/datasheet/lm317-d.pdfIf you want to get a really low output voltage (<1.25V), you need a small negative supply for the adjust pin. Search Google...For a very complex supply with all the goodies:https://electronics.stackexchange.com/questions/244354/making-lm317-output-voltage-adjustable-down-to-0-vMany of these 'Lab Supply' projects wind up as oscillators so I'm not recommending building one unless you have a lot of time on your hands. Current limit seems to be the area where they fail. But that is anecdotal, I buy power supplies, I don't build them.There are a lot of LM317 projects on Google. Pick a simple one to start.

author: guitchess
author_rank: Regular Contributor
timestamp: March 09, 2022, 10:40:17 pm
content: Sorry, I didn't think it was important to give that info for two reasons.1. With the experience present on the forum, I'm sure most can imagine the issues.2. My intention/request is not to modify the supply, merely to create an add on.I'm currently stuck inside taking care of kids, so specs will have to wait a bit.Thanks for taking the time to reply.

author: guitchess
author_rank: Regular Contributor
timestamp: March 09, 2022, 11:02:01 pm
content: This is the option I was originally considering. I just started rethinking the idea after some YT vid inspiration. The linear option definitely seems to be the easier option, a smaller part count as well.Thanks for the reply.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2022, 11:31:33 pm
content: Most questions about such systems or circuits are quantitative, so you should indicate what is "low", "lower", etc. in order to get a useful answer.

author: guitchess
author_rank: Regular Contributor
timestamp: March 10, 2022, 03:16:08 am
content: Quote from: TimFox on March 09, 2022, 11:31:33 pmOk, I'll be more specific. The cheap supply is supposed to be 5v to 30v constant current/constant voltage. However, below approx. 7.4v it essentially becomes one or the other. If I try to limit the current below 250mA or so, the voltage will suddenly drop to 0v. As one could imagine, this is a pain when testing/running lower powered circuits.For simplicity's sake, my original thought was to just build a voltage divider to cut the displayed voltage by 2/3. However, I thought it would be wise to get some more experienced ideas on the subject.Thanks for the reply.Also, since you're a super contrib, another unrelated question. The send thanks button is new to me. Is it better form to use that or a personal thanks for replies.Thanks again.

author: MarkF
author_rank: Super Contributor
timestamp: March 10, 2022, 03:50:48 am
content: I did a small PCB where I could feed a LM317 from either a DC Wall Wart or an AC source.You also feed it from your bigger power supply.