Difference between revisions of "Talk:DIYbio:Notebook/Open Gel Box 2.0/Power Supply"

From OpenWetWare
Jump to: navigation, search
(adding power supply building stuff Jason Morrison did in kitmakers class)
Line 1: Line 1:
== Gel Box Power Supply Discussion Page  **model spec link: http://www.google.com ==  
== Gel Box Power Supply Discussion Page  **model spec link: http://www.google.com ==  
'''Jan 08, 2009'''
I put together a bridge rectifier + RC, drawing from a variac, today, seems to give pretty stable voltage in the 0-180VDC range.  The variac should give 120Hz oscillations, and the RC should do some decent smoothing.  If you do this, be careful!  We blew a couple bridge rectifiers before putting a resistor before the capactior to slow its [dis]charge cycle.  I'll put it on the oscilloscope in a day or two, and see how well this does.
The bridge rectifier was like $2.50 for 400V/8A rating at radio shack.  Notes and pictures: http://jayunit.net/2009/01/08/kitmakers-class/
'''Jan 08, 2009'''
'''Jan 08, 2009'''

Revision as of 21:29, 8 January 2009

Gel Box Power Supply Discussion Page **model spec link: http://www.google.com

Jan 08, 2009 I put together a bridge rectifier + RC, drawing from a variac, today, seems to give pretty stable voltage in the 0-180VDC range. The variac should give 120Hz oscillations, and the RC should do some decent smoothing. If you do this, be careful! We blew a couple bridge rectifiers before putting a resistor before the capactior to slow its [dis]charge cycle. I'll put it on the oscilloscope in a day or two, and see how well this does.

The bridge rectifier was like $2.50 for 400V/8A rating at radio shack. Notes and pictures: http://jayunit.net/2009/01/08/kitmakers-class/

Jan 08, 2009

See example link at top right (Google) that could be used to permanently link to a local specifications document.

Jan 08, 2009

Comments from Peter Wendt, provider of the power supply schematic in Figure 1 below:

The supply itself is fairly well known and there had been discussions on those already on various newsgroups for instance. It is simple, it is easy to build, it uses only "generic" parts and is relatively cheap - however limited.

The downsides are the transformers (weight, size and losses) and the fact that you should keep an eye on the total power drawn from the "intermediate stage" and the high voltage end as well until the feeding transformer gets overloaded. Results might be better if the driving transformer is significantly bigger than the driven transformer - which adds more legroom for the low voltage supply picked in the middle.

But the circuitry suffices many needs. I use a similar supply for a small tube amplifier:


(which is a link within http://www.mcamafia.de/tubes/keksdose/cookiebox.htm)

It does not work *that* well, since the transformers I picked are a tad too small, but it shows the principle quite well.

Jan 07, 2009

How about something like this:

            Figure 1. 12/230 VDC power supply circuit

(Circuit Courtesy of Peter Wendt)

Nix psu1.jpg

Or this (looks like pulse output, but probably doesn't matter that much):

            Figure 2. Variable HV DC power supply circuit

(Version 1.93, Copyright © 1994-2005, Samuel M. Goldwasser, All Rights Reserved, For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.)



Can anyone get a hold of this paper:

Analytical Biochemistry, Volume 137, Issue 1, 15 February 1984, Pages 156-160, An economic “power supply” using a diode for agarose and polyacrylamide gel electrophoresis, Y. Kadokami, K. Takao and K. Saigo

This is an interesting paper; *jcline@ieee.org 18:40, 8 January 2009 (EST):
They show that a simple unregulated half-wave supply works fine for running gels. This means ripple is not an issue. I guess the gel acts as a big capacitor. The resulting DC voltage across the gel would be Vrms = Vpk / sqrt(2). Where, Vpk = (1/2) Vpk-pk. i.e. if AC input voltage is 120V, then Vrms = 60 * 0.707. (For some reason they state that at AC input voltage of 100V, that the resultant output would be "about 50V" which is not correct. Also, the "power company supplies 100V" is not correct either -- do not trust the wall outlet to ever be more than 15% accurate.)
"1 to 2.5 V/cm" is found to work best for gels. Need to check the Gel Box 2.0 physical measurements to calculate total voltage for the P/S.

Jan 06, 2009

Is there any way to move the power supply discussion from the diybio mailing list to this page? Should we? We could make diybio post inviting the interested parties to come here, and sign in (like on a contributors page -- is there a way to add more pages?). I'd like to see us have a meeting, get organized, and make some progress. Phil...

Prior Mailing list discussions

  • Jonathan Cline, Wed, 07 Jan 2009 00:13:08 -0600
Yes, I think this approach is the best way to go: use an off the shelf
switching supply as the master supply (probably a computer supply - very
ubiquitous), then add step up circuit(s).  Choosing whether LM317 is
appropriate or not is premature, before design criteria are established.
These questions are important:

- what is the maximum noise allowed in the voltage?   (someone mentioned
voltage fluctuations are not important; this could be a factor since different
designs might trade off "perfectly flat DC" for some ripple in the output)

- what is the maximum noise allowed in the current?    

- is the voltage limted to a range?  Yes ; 20v-600v

- is the current limited to a range?   <- this is the important one

- what is the total power dissipation?

Answer all these before proceeding further with the design.   Other interesting
questions might be.. what is the slew rate requirement (how fast does the
voltage need to go from 0v to maximum, once it is switched on, and how fast
does it need to go back to 0v, when switched off); likely this is "slow is ok".
What is the resolution of the voltage steps, i.e. does the setting need to be
+/- 1v, or +/- 10v, or +/- 100v?

Web surfing today, I saw some comments on a blog (presumably from a bio lab
guy) that "I always run my gels current limited to 100mA in the lab".     It
seems different people set different current limits; other guys said they were
setting 250mA, or 300mA, or 400mA.

P total = V * I to calculate maximum power needed for the master supply (add
30% to 50% for overheat loss; which is mostly heat).

If 100mA and 600v, then it's ~60W.
If 100mA and 100v, then it's ~10W.

At the range of <20W, then a much smaller supply could be used as the master.
For example, optimal might be a laptop supply brick, which are very common, and
price -vs- value makes it very cheap; you won't be able to build a good master
supply for what they now manufacture in china & sell computer or laptop
supplies at discount..

If current limiting is a factor in electrophoresis, then you will want a
constant current circuit as well as voltage doubler after the master supply.  

I dont think the PDF you quoted is applicable, once you answer the "how much
current" question; it seemed applicable for small milliamp range only (" the
maximum output current reaches 1 mA").  Define the Specs first, then design

Why is variable voltage even needed?

If "DIY, Inc." controls the entire process (gel box + power supply + agar kit +
buffer kit) then the voltage should not need to be varied, and can be fixed.

There are times when it is suitable to force a standard into place, just by
picking one, and not allowing choice.  Standardizing the process may be one way
to simplify the protocols, remove "choice" from the user's feature list, and
simplify the design too.

Just an idea - I don't know if this is applicable in this case.   Variable
voltage might be needed in labs to optimize protocols, or as a multi-use
supply, or maybe "because I like 245 volts and I've always used that voltage
anyway, and I ain't changing it".   If the entire process is already optimized
by the manufacturer (i.e. Jason), then why need to change the voltage?   Thus,
the p/s is part of the gel box 'system design'.  Everything can be optimized to
work together.  The drawback is that the components may not be interchangeable
with other boxes or protocols.   This drawback may be a good tradeoff.

  • JonathanCline, Tues, Jan 6 2009 8:56 am

On Jan 5, 10:11 pm, "Meredith L. Patterson" <clonea...@gmail.com>
> > On Mon, Jan 5, 2009 at 7:26 PM, Jason Morrison
> >
> > <jason.p.morri...@gmail.com> wrote:
>> > > A timer also seems to be a good feature.
> >
> > I'd just use a microcontroller for that. Prototype it on an arduino,
> > use pushbutton switches or a potentiometer to set the time (you can
> > even use a couple of 7-segment LEDs, or a $10 LCD, to display the time
> > and a countdown). I've already prototyped something nearly identical,
> > also on an arduino, for the DIY electroporator; for the final version
> > it'll be a $3 Atmel AVR. Add a piezo speaker to make it beep when it's
> > done or whatever.

Although microcontrollers might be cheap nowadays, using a
microcontroller should always be avoided if possible, using analog or
discrete electronics (non-software parts) instead.   The
microcontroller adds a lot of complexity to the circuit and a lot of
opportunity for software bugs.  Especially in a power circuit.  A
simpler to build, more foolproof to error, timer circuit is to use a
555 timer chip in one-shot mode.  Example circuits are widespread.
Use 556 for two timers in one chip.   The time can be modified with a
potentiometer (volume knob).  The resolution and repetition is set
with fixed components (resistors + capacitors).

The design criteria are:

- the timer doesn't need high resolution
- the maximum time can be fixed in the circuit, and time control can
vary linearly in that range
- the timer can be automatically set by the power supply voltage
(click the "600v" setting and the timer is set to X mins; click the
"100v" setting and the timer is set to Y mins)
- the potentiometer can be calibrated so that the scale marking reads
approximate times (i.e. "45 mins" might mean 45 mins ± 1 minute)

The "Faster Better Media" also mentions that running a gel with a
stepped voltage supply can decrease total time.  i.e. run at 500V for
X mins, then run at 50v for Y mins.  This could be done with a dual
timer circuit.

> > The fancy-schmancy (but broken) Bio-Rad electrophoresis power supply I
> > have does two outputs, and each can be programmed separately to run a
> > timed sequence. This sort of logic is not difficult to program and I
> > really don't get why the supply costs thousands of dollars. The
> > green-and-white box is pretty, but not that pretty.

What is the total power requirement for running gels.  Some of the
commercial supplies are listed as 25W or 50W.  It would be a bad idea
to "hook the gel up to the wall outlet" as someone mentions later.
Bad, bad, bad.  The user might end up researching the life-after-death
experience.   The main purpose of a regulated supply is to regulate
voltage and current: limit voltage for proper operation of the circuit
and limit current to limit safety issues.  Internationalization is
also important.  A supply which plugs into AC120V/60Hz for north
america is worthless in europe and much of asia, since those countries
are AC200V/50Hz.    It might be a better design to use a common
international switching supply (a small 60W computer power supply) and
add a voltage multiplier to step up the voltage to the desired gel
voltage; this allows the wall-side to plug into international power.

The power supply could also be physically tied to the gel itself for
thermal regulation.  In the "Faster Better Media" references, it
mentions that running high voltage gels for fast operation has the
drawback of excessive heat dissipation, and too much heat can ruin the
experiment.  The power supply can be thermally regulated by
mechanically coupling the gel box to the power supply regulator (that
means, bolt a block of metal to both the gel box and the regulator
chip).   Because the regulator chip has a built-in thermal shutoff
(some of these are programmable as well, by voltage input),  the power
supply can be made to decrease in voltage as temperature rises, or
stop supplying after a voltage threshold.  This is a discrete circuit
design, not microcontroller based (which is better, because it won't
have software bugs).

## Jonathan Cline