What to do when you dont know input voltage or current? - Page 1
Forum section: electronics / beginners

author: Legion
author_rank: Frequent Contributor
timestamp: February 15, 2014, 11:01:08 pm
content: What do you do when you're following a circuit diagram, but they don't specify the input voltages or current? For example, the one I'm looking at now just shows V+ and ground. Nowhere in the description or anywhere else does it specify the input voltage or current.Do I just start at 0V on my DC PSU and work my way up to 30V or when things turn on, whichever comes first?

author: hamster_nz
author_rank: Super Contributor
timestamp: February 15, 2014, 11:41:23 pm
content: a transistor must have about 0.6V bias, so maybe that could give you a hint. Are threw any ICs? Have a look at their datasheets....

author: Psi
author_rank: Super Contributor
timestamp: February 15, 2014, 11:47:51 pm
content: Are any capacitor voltages given in the diagram?They can give a clue.At least you know it cant be more than the rated voltage of any cap across the supply.

author: Rerouter
author_rank: Super Contributor
timestamp: February 15, 2014, 11:57:15 pm
content: also if there are any active devices on the circuit there datasheet will spec a minimum and maximum voltage which can quickly narrow down your choices,

author: tszaboo
author_rank: Super Contributor
timestamp: February 16, 2014, 12:37:31 am
content: Understand the circuit. Look for a local PSU if there is any. Look for logic ICs , they are mostly 3,3v or 5v unless old 4000 series. It is probably 5/12/15 or 30V if there are only analog stuff. Open datasheets. It is not that hard. Last chance, upload here and we will tell you.

author: zapta
author_rank: Super Contributor
timestamp: February 16, 2014, 01:04:57 am
content: Quote from: Legion on February 15, 2014, 11:01:08 pmDo you have a diagram or a picture? It may give clues. Also, do you know what is the intended use (e.g. car's electronic typically requires 12V)?

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 16, 2014, 02:28:45 am
content: There are a few other hints too, for things like bias resistors and resistor dividers, their values go down as the voltage goes down.Check if there are any linear regulators, generally, the rails supplying them won't be too much above their output (in a good design). Also, if the entire circuit is behind a linear regulator, you can pretty much just put in linear regulator + dropout voltage.

author: Legion
author_rank: Frequent Contributor
timestamp: February 16, 2014, 03:09:46 am
content: Quote from: zapta on February 16, 2014, 01:04:57 amIt's just a small circuit from one of those beginner's electronics books. But I was more curious about the general case.

author: hamster_nz
author_rank: Super Contributor
timestamp: February 16, 2014, 03:19:21 am
content: I would say about 9 or 12V. that would give the correct amount of power to the LEDs (10mA each)

author: IanB
author_rank: Super Contributor
timestamp: February 16, 2014, 11:54:59 am
content: Exactly. You can tell from R2 that the circuit is expecting about a 12 V supply, since that would cause R2 to pass about 20 mA.As a general rule, if a voltage is not specified you could assume it is not critical. Most ordinary transistor circuits will operate from a voltage in the 6 - 12 V range.

author: papo
author_rank: Regular Contributor
timestamp: February 19, 2014, 11:21:58 am
content: Just on a side-note, operating LEDs in parallel with just a single resistoris not really considered good practice, although it might work.RegardsMatt