5v jump up to 9vs

[soupnatizzle]

Hey, i am in need of a simple solution for a simple problem.

I keep forgetting to turn off my DMM and 9V batteries are expensive. so i want to create a way to plug it in when needed.

I have a few old USB cables around and i always work with a computer right next to me.'

How can i jump the voltage up to 9v from the usb's 5v, im not worried about the current as the DMM uses next to nothing.

do i need to use a transformer? and if so im going to need some help with the stepup, 5 to 9vs is a 55% increase so a 1:1.5 ratio transformer?

 

[MikeMl]

A charge-pump chip, or build one out of 555

 

[soupnatizzle]

i actually just starting using 555 timers this semester and have only been using them as frequency generators.

I am at a lose for how i could use one to increase voltage

 

[hdc090360]

A transformer only works with AC, whereas a battery and your DMM are DC, so we can rule that out. The quickest way is to buy a 9Vdc plugpack. If you are keen on learning you are going to need some sort of step up switching power supply. Have a look at webbench on the National Semiconductor website. www.national.com

 

[soupnatizzle]

how could i do this with a 555 timer?

 

[MikeMl]

i actually just starting using 555 timers this semester and have only been using them as frequency generators.

I am at a lose for how i could use one to increase voltage

Look at this simulation. Note the current drawn from the USB supply. Delivers ~9.2V @ 10mA.

 

[MikeMl]

A thought just occurred to me. The circuit I posted is not isolated from the computer's third-prong ground. Not a good thing to have hooked up to a battery-powered DMM. You can no longer do floating measurements, plus you have a serious shock hazard. Now that I think about it, your idea should be dropped. Just learn to turn off the DMM or buy one like my Fluke which just goes to low battery drain sleep mode...