Power supplies voltage and run time

A power supply is the instrument that turns a poured agarose gel into a separation. The decision is which voltage to set for the tank in front of you, and when to stop, so the molecular weight ladder opens enough for the size call you need. There is no universal minute count. The photograph on this page is the finished gel under an orange shield, which is the record those settings were meant to produce. How to read that record is covered in agarose gel electrophoresis for DNA. Tanks and supplies are enquiry classes in the molecular biology catalogue, with the requirement attached to the quote request.

Field strength is voltage divided by distance

DNA moves because the field pulls a charged molecule through the agarose mesh. The field is the voltage divided by the distance over which that voltage is dropped. A planning range often cited for agarose is about 5 to 10 volts per centimetre of that distance, until the ladder opens. Measure the electrode gap on your tank. Do not borrow the centimetre from a mini-gel pictured in someone else's method.

A sharper size call, especially when two bands sit close, belongs at the low end of that window. The bands take longer and the gel stays cooler, so the zones stay tight. A quick look at a plasmid that only needs to show "cut" versus "uncut" can sit higher, provided the gel does not warm. "Higher" still means inside the window for that gap, or at the figure the tank maker printed for that buffer. A supply that can deliver 300 volts is not an invitation to use 300 volts on a short tray.

Set constant voltage for ordinary agarose. As the buffer warms, its resistance falls, the current rises, and heating accelerates. That loop is why a run that felt fine at minute ten can smile by minute forty. If the supply is in constant-current mode by mistake, the voltage will wander as the resistance changes and the field will not be the one you calculated. Look at the mode light before you press start.

Put a current limit under the voltage setpoint so a short, a leaked buffer, or a reversed dilution cannot dump the supply's full power into the tank. If the display shows the voltage sagging while the current sits on the limit, you are no longer running the plan. Stop, check the buffer dilution and the leads, and start from a number you can explain.

What the run is allowed to decide

Watch the tracking dyes. They are progress marks, not size standards. Stop when the ladder bands that flank your fragment are separated and the small species you care about is still on the gel. A 200 base-pair product and a primer-dimer need more distance than a 6 kilobase plasmid check. The same voltage can therefore have two honest stop times on two gels poured the same morning.

The molecular weight ladder is the clock that matters. If it has not opened, more time at the same voltage is the next step, assuming the gel is still cool. If the ladder is sharp and the sample is not, the sample is the question, not the supply. If both are curved, the field was too fierce or the gel was uneven. Lower the voltage on the repeat.

Very large DNA and very small DNA set their own patience. Genomic DNA that should stay near the well needs a gentle field so it is not sheared by the way you talk about the gel, and so the well is not heated into a smear. Tiny fragments need a higher agarose percentage and a stop time that respects the dye front. Percentage and voltage are one plan. Changing only the minutes will not rescue a mesh that is wrong for the molecule.

Buffer exhaustion and a supply that was fine last week

The buffer carries the current and resists the pH drift at the electrodes. Tris-acetate and tris-borate buffers do this differently and they heat differently. Use one family in the gel and the tank. A tired buffer has already moved ions for an hour or more. Its capacity is lower, it runs hotter at the same voltage, and ladders start to look unfamiliar. Reused buffer is a common way to buy that failure. Replace it when the current at your usual voltage has drifted, when the tank smells sharp, or when the ladder no longer matches last week's image. There is no honest reuse count that fits every laboratory's volume and duty cycle.

A buffer made at the wrong concentration changes the current immediately. Too dilute and the field is unstable and the gel heats in patches. Too concentrated and the current is high, the supply may hit its limit, and the bands smile. Confirm the dilution written on the carboy before you blame the instrument.

Leads matter. The usual convention is red toward the anode, where DNA is going, and black at the cathode, where the wells sit. A repaired lead can be swapped. If the tracking dye walks backward into the buffer in the first minute, the field is reversed. Stop before the samples are gone.

A power cut and a transfer cassette

If the supply trips or the building cuts, bands start to diffuse as soon as the field is gone. A short interruption, with dyes still sharp, can be finished, and you then judge the ladder with extra scepticism. A gel that sat through a long outage should be repeated when the distances are the evidence. Write the interruption on the image. A restarted programme that quietly reset to a default voltage is a second, separate fault. Read the display after the power returns.

Western transfer uses the same kind of supply and a different plan. The path from gel to membrane is set out in western blot from gel to membrane. Large proteins need enough field and time to leave the gel. Small proteins need a stop before they pass through the membrane. Neither of those times is the agarose minute you remember. A prestained ladder that never appears on the membrane indicts the transfer electricals, the stack order, or the clock. It does not indict the antibody yet.

Polyacrylamide gels often follow a voltage printed for that plate spacing and that buffer system. Mini-gel tanks and large-format tanks are not the same centimetre. Use the figure that belongs to the plates in the clamp, and drop it if the plates are too hot to touch comfortably. This page does not publish a minute chart for those tanks.

Safety around the leads

The hazards are shock, burns from a hot tank, and a gel that fails open or shorted. Dry hands, covered buffer, and leads that are plugged before the supply is enabled are the ordinary controls. A supply with cracked insulation is out of service. Ultraviolet viewing of the finished gel still wants the orange shield. The WHO laboratory biosafety manual frames chemical and electrical risk in laboratories as an institutional matter. It does not certify a homemade lead or a buffer recipe.

Heat, voltage sag, and the afternoon run

In a hot room the same 8 volts per centimetre that was fine in the morning can warm the centre and curve every lane. Sit at the low end of the planning window, and let the run take longer, before you blame the polymerase or the comb. A tired supply on a sagging line can deliver less than the setpoint. If bands suddenly crawl, compare the displayed voltage with the setpoint and with a run from a cooler part of the day. After a cut, confirm the programme reloaded before you trust a precious set of samples to the rest of the clock.

What to put in an enquiry

EVRINTH can take a sourcing question about a supply or a tank as an equipment class. State the electrode gap you need to drive, the maximum voltage and current you actually use, whether the work is agarose, polyacrylamide, or transfer, and whether constant voltage is the mode you standardise on. The nucleic acid analysis pathway is the context when the gel sits between amplification and a clone or a sequencing load. Ask whether a quotation is possible. A watt rating without the gel length is not a specification, and a minute count without the voltage is not one either.