voltage divider pot as power supply - Page 1
Forum section: electronics / beginners

author: josephpate
author_rank: Contributor
timestamp: August 21, 2012, 03:24:43 pm
content: I was watching a youtube video where a guy used a 1k pot as a voltage divider. the outside pins were connected to + and - and the wiper pin was his output.I tried this with a 10k pot I had and a 9v battery and it worked great.This leads to the obvious question of why do people spend so much money on power supplies when you can do the same thing with a battery and a cheap pot?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2012, 03:40:01 pm
content: Quote from: josephpate on August 21, 2012, 03:24:43 pmWhich in turn leads to the obvious answer:People want to draw usable currents at the lower voltage,which they can't do if there is,in your case,several thousand Ohms in serieswith the connection they are drawing it from.Plus,the less obvious:As soon as you start to draw current,your voltage divider doesn't have the same division ratio anymore--Any ideas why?

author: baljemmett
author_rank: Unknown
timestamp: August 21, 2012, 03:40:53 pm
content: Quote from: josephpate on August 21, 2012, 03:24:43 pmLet's try a quick thought experiment, which might illuminate -- imagine a battery and a 10k pot connected as you describe, with the wiper at the halfway point so you've got 4.5 volts at the output. Now connect a 1k resistor between your wiper and ground to provide a bit of load; what do you think will happen to the output voltage?

author: SeanB
author_rank: Super Contributor
timestamp: August 21, 2012, 03:44:20 pm
content: Simple reason is power delivery and output voltage. At best you will supply a minute current through the wiper, and if you connect any load to the output it will quickly drop. Try again , but use a plain LED that you do not mind destroying and see the voltage across it as you vary the pot.Another reason is that the output depends both on the input voltage and the load current, and both are variable in a power supply, the pot can be used across a reference supply to supply a high power amplifier to drive the output, basically a power supply.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 21, 2012, 03:46:09 pm
content: QuotePretty simple. They want to get some significant current out of their powersupplywithoutchanging the outputvoltage.But if you put a loadresistor on the output of yourpowersupplyyou will get avoltagedecrease.

author: josephpate
author_rank: Contributor
timestamp: August 21, 2012, 04:39:58 pm
content: Ah I see. Perhaps I need to rewatch the youtube video then.So what's the purpose of a voltage divider then? (sorry I'm at work can't really look it up. Should be working instead of posting this too!)

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 21, 2012, 05:34:57 pm
content: QuoteIn case of adjustable voltage divider (aka. potentiometer):With reference to the powersupply: setting a reference voltage to control the output voltage/limiting the output currentIn this case, there is no load on the voltage divider (because the voltage is feed in an op amp with high input impedance) and therefore, the voltage is stable.Another pretty common purposes is volume control at the radio.In case of fixed voltage divider (and a PSU again):Diving down the output voltage in order to feed it back in the voltage controlling op amp.

author: G7PSK
author_rank: Super Contributor
timestamp: August 21, 2012, 05:56:06 pm
content: Of course you could use one of these. Rated at 3 amps 67 ohms.150mm dia 50mm deep weighs about a one kilo.

author: josephpate
author_rank: Contributor
timestamp: August 21, 2012, 08:33:18 pm
content: Quote from: PSR B1257 on August 21, 2012, 05:34:57 pmI believe the guy in the video was feeding the output into a transistor (the base, maybe?). Does that sound about right?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 21, 2012, 08:55:36 pm
content: QuoteWhich video? A link would be useful.QuoteYes, it does.