Easiest method to get -12V @ 0.5A for HF Waveform Generator??? - Page 1
Forum section: electronics / projects

author: Pathfinder0311
author_rank: Newbie
timestamp: April 05, 2015, 07:09:21 am
content: Hi guys!This is my first post! I've watched several of Dave's videos and my favorites are the one about the white van speaker scam and the Bob Widlar poster! I love his videos and they have helped me out a lot in my quest to better understanding electronics. I'm a Junior studying EE at the University of Akron in Ohio and I want to pursue analog IC design :pI'm making a HF Waveform Generator and I need +/- 12 V to power the circuit. +/- 12V for the last OPAMP at the output, and thru +/-5V regulators for chip logic and voltage buffers. With two 9V batteries they are drawing about 125 mA each.Q: Using a +12V wall wart, what is the most cost-effective solution to getting -12V?

author: tggzzz
author_rank: Super Contributor
timestamp: April 05, 2015, 07:24:16 am
content: Quote from: Pathfinder0311 on April 05, 2015, 07:09:21 amProbably a DC/DC module with a negative output. Here's a list of 489 possibilities:http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000004440&st=dc%2Fdc%20converter%20-12V&pageSize=25&showResults=true&pf=110177797Search terms was "dc/dc converter -12V"

author: Strada916
author_rank: Frequent Contributor
timestamp: April 06, 2015, 09:38:46 am
content: Two 12v wall warts one positive end and one negative end joined. These become + - 12volt on other wires with respect to joint ends make sence?

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2015, 10:10:40 am
content: A 12VAC wall wart plus a voltage doubler.http://www.bristolwatch.com/ele/power_supplies.htmIf it needs to be regulated, use a 15V wall wart and add the LM7812 and LM7912.

author: Ian.M
author_rank: Super Contributor
timestamp: April 06, 2015, 10:22:04 am
content: ... but make sure they don't have a ground pin that connects to output 0V otherwise the -ve side one will elect a pope (or if its short-circuit protected, shutdown).Its also a clumsy PITA to use and can result in latchup of the ICs in your circuit if they are'nt switched on at the same time.A better solution is *NOT* to start with 12V DC. If you have to, it gets ugly, and you need either a switched mode -ve rail generator circuit (not switched capacitor - it wont be beefy enough) or an isolated 12V DC-DC converter that can have its +ver output strapped to 0V + MOSFET switches in both + and -12V rail with a monitoring circuit so that the load does not get connected until both 12V rails are stable.If you have the choice, a 12V to 15V AC wallwart into positive and negative half wave rectifiers, followed by large reservoir caps and linear regulators is a far better choice for powering analog circuits where supply noise rejection may be an issue. If you use a 12V AC wallwart you are a little bit marginal on headroom so use Schottky diodes, LDO regulators and overrate the wallwart so the transformer's regulation factor is in your favour.I see Hero99 has just posted the basic circuit while I was typing this. Don't leave it unregulated as the reservoir caps will charge to sqrt(2)*12V -Vf_diode, which may cause problems if your circuit cant stand 16V (or more if the transformer has poor regulation). R1 seriously harms regulation and should be as small as possible consistent with the repetitive peak If of the rectifiers or should be shorted by a relay powered from the output. In many cases, the transformer source impedance is high enough to do without it if you use diodes with an adequate rating.

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2015, 10:25:14 am
content: Quote from: Ian.M on April 06, 2015, 10:22:04 amYes, I'd even get rid of R1 altogether (this isn't my circuit). You could replace it with a PTC resistor (pollyfuse/switch) to give some short circuit protection.

author: paulie
author_rank: Frequent Contributor
timestamp: April 06, 2015, 06:19:33 pm
content: I agree the resistor serves no purpose. Regarding current protection generally that is inherent in the secondary winding. And note that only a common 12v transformer is required, not expensive hard to find 15v.Personally I think a DC-DC is better solution than that "vintage" circuit. I've modified several of the 80 cent LM2596 Ebay converters to supply negative instead of positive. Takes less than a minute and costs nothing.

author: Ian.M
author_rank: Super Contributor
timestamp: April 06, 2015, 06:32:57 pm
content: .... that's if you can stand or filter out the switching hash on your supply rails. If not, the old-skool half wave split supply setup followed by linear regulators becomes quite attractive for one-off or very small volume production. With choke input filters and RFI suppression caps across the diodes, feeding linear regulators you can get the supply noise far lower far more easily than with any switching solution. It was quite commonly used for high speed external modems etc.Its far far less common now due to the availability of cheap good low voltage single supply rail-to-rail OPAMPs and other analog chips removing most of the need for negative rails in low noise analog applications.

author: paulie
author_rank: Frequent Contributor
timestamp: April 06, 2015, 06:49:40 pm
content: All true. I almost always go the single supply route these days. However some times it's not practical. Like in a recent project that required low cost, low noise, low offset, op amp (op07) and also a precision peak detector both requiring dual voltage.

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2015, 07:50:15 pm
content: Quote from: paulie on April 06, 2015, 06:19:33 pmIf you're going to use the old LM7812 and LM7912, 12VAC is pushing it. 12VAC is 17V peak, minus the rectifier losses, gives 15.8V, then you need 3V or so for good regulation, minus a few V ripple. 15V would be optimal and isn't that hard to find.

author: Ian.M
author_rank: Super Contributor
timestamp: April 06, 2015, 07:57:11 pm
content: Yes. I pointed out earlier that a 12V AC input doesn't leave enough headroom for old-skool regulators and would need Schottky rectifiers and LDO regulators to get a clean +/-12.0V DC out.

author: paulie
author_rank: Frequent Contributor
timestamp: April 06, 2015, 09:44:22 pm
content: I agree on schottky because they are only a penny or so more but not the 15v transformer which cost a lot more and not as easy to find. Rectifier loss would be only about .2v not 1.2v so even old school regs have plenty of headroom. If one is seriously worried about that then LDO come to the rescue. We are still talking about pennies difference unlike with transformers that run dollars more. 12v electronic transformers can be had for a dollar and change unlike 15v at 5x more.But as I mentioned that whole approach is foolish when negative DC-DC converters run 80 cents on ebay and .half that on aliexpress. Smaller, cheaper, and OP does not have to redesign the whole circuit.

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2015, 10:17:18 pm
content: Yes Schottky diodes would be ideal and avoid the need for a higher voltage transformer. I notice you're in the USA which may be different you where I live where 15V and 12V transformers are the same price.The problem with switched mode power supplies is noise and I'd be dubious about anything that cheap.

author: paulie
author_rank: Frequent Contributor
timestamp: April 06, 2015, 10:38:18 pm
content: Actually I was talking about Ebay/Aliexpress so location shouldn't matter. But I see you fly the black flag so maybe pirates get them even cheaper.Having purchased dozens of chinese DC-DC for myself and hundreds for customers I'm convinced reliability is not an issue. As far as noise they work fine out of the box for most applications and a cheap 220uf will suffice for most other more sensitive circuits.

author: Smokey
author_rank: Super Contributor
timestamp: April 07, 2015, 04:40:18 am
content: These things rock... A little expensive, but if you just need some voltage, it's a really small package for 1Whttp://www.digikey.com/product-detail/en/VASD1-S12-D12-SIP/102-1343-ND/989798