Powering battery operated devices with bench supply. - Page 1
Forum section: electronics / beginners

author: registereduser
author_rank: Contributor
timestamp: January 13, 2019, 07:38:09 pm
content: Just wanted to double check before doing something regrettable.I have a TI-30Xa calculator with dead batteries. 2 @ 1.5v buttons of type GPA76.It occurred to me to just wire it straight to the 3.3v on my improvise ATX bench supply. I have two concerns.1. Will the calculator take care of current limiting itself, or do I need to include a resistor. If I do, what value?2. Is the 3.3 OK, or do I need to drop to 3v?

author: floobydust
author_rank: Super Contributor
timestamp: January 13, 2019, 07:50:32 pm
content: I recommend using a current-limiting resistor, like 47-100 ohms.I have had some devices latch up and die because the PSU was more powerful than a battery and the voltage rise was too fast when I connected the leads.You can use a series Schottky diode to drop 3.3V down to 3V. We don't know what the calculator is rated for.EPX76 silver oxide batteriescan be 1.6V max.Maybe use some AAA batteries or something less scary than ATX PSU.

author: registereduser
author_rank: Contributor
timestamp: January 13, 2019, 09:10:57 pm
content: Sounds good. I tried to find info on the power needs online with no success.I'm not too worried about using the ATX, with your suggestions. Thanks for the input.

author: JoeO
author_rank: Frequent Contributor
timestamp: January 13, 2019, 09:47:31 pm
content: No one can tell you since no one knows the characteristics of your improvised ATX power supply. Does it overshoot? By how much? For how long?No one has tested the calculator and specs don't exists to tell you for how long the calculator can withstand the unknown output of your power supply.If you go to a local store, the cost of the batteries will come close to or exceed the cost of the calculator. Presently they sell for $8.69 on Amazon.Good luck!

author: registereduser
author_rank: Contributor
timestamp: January 13, 2019, 10:02:21 pm
content: Quote from: JoeO on January 13, 2019, 09:47:31 pmWhat is meant by overshoot? How can I determine those things?

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 13, 2019, 10:08:42 pm
content: When you first apply power to the ATX supply it is possible to get a voltage spike form the unit into the load, or the thing you want to power without the batteries. This what is meant by voltage spike. It may overload the device and do some damage. This can also occur when you turn the ATX off.

author: registereduser
author_rank: Contributor
timestamp: January 13, 2019, 11:25:09 pm
content: Quote from: joseph nicholas on January 13, 2019, 10:08:42 pmYou say when I turn on the ATX? What if it's already on, then I connect route output through a switch [external to the psu] and turn it on that way? Does that solve the surge problem?Alternatively, or in conjunction, could I not add an inductor and/or capacitor so smooth it out? I have a few scrounged caps and various transformers from junked tvs and radios. I can identify the caps, but not the coils. Well, I can measure resistance on coils, not inductance. Another post coming soon on that whole situation.I can only view these psu surges with an oscilloscope, correct? Which isn't coming any time soon.

author: Electro Fan
author_rank: Super Contributor
timestamp: January 13, 2019, 11:43:57 pm
content: Question: How much overshoot and for how long would the parts in something like a TI calculator (normally 3V) be likely to withstand? For example, could the DUT (such as a TI calculator) withstand a Power Supply that had a turn-on or turn-off over shoot ranging between +/-6 Volts (12 Volt total swing) for a duration of 0.5 to 1.0 ms? Or would that be too much and too long to safely power up and power down such a DUT? Thx

author: Nusa
author_rank: Super Contributor
timestamp: January 14, 2019, 12:41:33 am
content: The correct batteries are cheap enough, especially if you want a bunch of them:https://www.walmart.com/search/?query=gpa76%20batteryBut if you want to use something on hand, I'd suggest a couple of AA's instead of that power supply. We're talking about a device whose current draw is is so low you might not be able to measure it with the meter you have.On the other hand, that calculator is so cheap to buy, go for it. You can't lose much.

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: January 14, 2019, 12:44:22 am
content: what about using a bigger coin cell and a improvised holder, for example CR2032?

author: 6PTsocket
author_rank: Regular Contributor
timestamp: January 16, 2019, 06:28:28 pm
content: Look on ebay they are dirt cheap with free shipping. There are several other numbers for this size battery but be careful, there are alkalines the same physical size and some sellers cross reference both ways. Either will work but the silver oxide will last longer and has a flatter discharge curve. If you want the better silver zinc, read the fine print. Look for sellers that give an expiration date. People do sell old stock. I have bought button cells on ebay for years with no problems. Make sure the seller has a high approval and a lot of sales.Sent from my SM-G900V using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2019, 12:06:42 pm
content: A Schottky diode won't drop 0.3V, at the tiny current taken by the calculator. You could use an ordinary silicon diode which should do the job. A 1k series resistor and 100nF capacitor in parallel with the calculator would also be a good idea.