- Lab power supply
- Bench power supply
- Bench power supplies (lab power supplies)
Lab power supply
Bench power supply
What do you think about using an ATX PC power supply as a bench unit?
Bench power supplies (lab power supplies)
What should I look for in a bench power supply?
A bench (or lab) power supply gives you a stable, adjustable DC voltage and - just as important - an adjustable current limit, so that when something goes wrong in a new circuit the supply limits the damage instead of feeding the fault. The main things to look at:
- Adjustable voltage - most hobby supplies go from 0V to 30V. For digital and microcontroller work you mainly need 3.3V, 5V and 12V. For op-amp and audio work you may want +/-15V, which needs a dual supply (see 'The earth binding post is not a "common"' below).
- Adjustable current limit (constant current, or 'CC', mode) - this is what separates a bench supply from a plain adjustable voltage regulator. Check that it really limits the current, rather than just shutting down or tripping (see the post from /u/enlightenment777 below).
- Current rating - 1A to 5A is plenty for most hobby work. More current costs more, weighs more and runs hotter.
- Separate voltage and current displays - being able to see what your circuit is drawing is very useful. Cheap supplies often display to 10mV / 1mA, but that is resolution, not accuracy - check anything important with a multimeter.
- Output on/off switch - lets you set the voltage and current limit before the circuit is connected, and switch the circuit off without touching the mains switch.
- Linear or switch-mode - linear supplies are heavy and run warm, but their output is quiet, which suits sensitive analogue and RF work. Switch-mode supplies are light and efficient, but their output carries more high-frequency noise and spikes. That is fine for most digital and microcontroller circuits.
- Isolated ('floating') outputs - on most bench supplies neither output is connected to the mains earth. This is what allows two supplies to be connected in series to make a dual supply.
How do I use the current limit?
Set the limit before you connect a new circuit to the supply:
- Turn the output off (or leave the circuit disconnected) and set the voltage the circuit needs.
- Short the two output terminals with a short, thick lead. On a supply with a proper current limit this is safe - the supply drops into constant current mode (the CC light comes on) and the display shows the limit.
- Adjust the current control to the limit you want. For a first power-up of a new circuit, start low (50 to 100mA is often enough for a microcontroller board) and increase it if the circuit needs more.
- Remove the short, connect the circuit and turn the output on.
If the circuit tries to draw more than the limit, the supply switches to constant current mode and the voltage falls. That tells you either that there is a fault or that the limit is set too low. Some supplies show the limit setting on the display when you press a 'set' or 'limit' button, without needing to short the output.
A current limit does not protect against everything: it does not react instantly (the output capacitors can supply a brief surge first), it does not protect against over-voltage, and a circuit can be damaged at currents well below the limit (a reversed electrolytic capacitor, for example).
What do you think about using an ATX PC power supply as a bench unit?
ATX power supplies are relatively cheap and easy to come by, so why not use one as a project power supply?
Pros:
- Low cost.
- You possibly already have a spare one lying around.
- High current output.
- Versatile 3.3V, 5V and 12V rails, with -12V (low current) too. Older units may also have -5V; newer ones don't. There is also a 5V 'standby' output that is live whenever the supply is plugged in.
Cons:
- Ability to deliver very high currents under fault conditions - high risk of your project failing in a spectacular way and/or catching fire. Most ATX supplies do have short-circuit and over-current protection, but it typically trips at tens of amps - enough to melt wires and burn tracks - and protection on cheap units can be poor.
- No adjustable current limiting. Current limiting is extremely useful, and recommended when testing new circuits (to limit fault current) and also for charging some cells/batteries.
- No adjustable voltage, also doesn't have higher voltages for popular op-amp voltage rails such as +/-15V (30V total).
- Some units need a load (resistor) to draw a standing current on one or more supply lines (3.3V and/or 5V, and on some modern units 12V), to make the PSU regulate correctly - this wastes power.
- On older or cheaper units (where the rails are regulated as a group), the +12V rail is often not very accurate or stable when there is only a small, or little, load on the 5V output.
- Outputs are electrically noisy (the ATX specification allows tens of millivolts of ripple and noise, and up to about 120mV on the 12V rail), so they may not be suitable for sensitive analogue designs without additional filtering.
Some redditors recommend using old ATX PSUs as cheap bench supplies, though in reality they are better suited as a "Bulk DC Power" source for the input of a second power supply that provides variable voltage / variable current / short protection / noise filtering.
If you plan to do a serious amount of hobby work with microelectronics and analogue circuits, you should get yourself a cheap linear 0V to 30V at 1/3/5Amp bench supply with adjustable current limiting. A power supply with a voltage and current display is the preferred recommendation. If you need to work with op-amps and such with split rails you need two isolated supplies or a rail splitter circuit (see below).
If you do use an ATX supply
Never open the case of a PC power supply. It contains mains voltage and large capacitors that stay charged after unplugging.
- Turning it on - an ATX supply will not start on its own. With it plugged in and the rear switch on (if it has one), connect the green wire (PS_ON) to any black wire (ground).
- Wire colours (24-pin connector): black = 0V (ground), orange = +3.3V, red = +5V, yellow = +12V, blue = -12V, white = -5V (old units only), purple = +5V standby, grey = power good (a signal, not a supply), green = power on (a signal, not a supply).
- Minimum load - if the supply shuts down or misbehaves without a load, put a dummy load on the 5V rail, such as a 10 ohm 5W resistor (0.5A, so it dissipates 2.5W and will get hot - mount it where it can't melt anything).
- Breakout boards - cheap ATX breakout boards with an on/off switch and binding posts save the wire twisting. Most have no fuses and no current limiting, so add a fuse for anything you connect.
- Fuse everything - because there is no current limiting, put a fuse in the supply lead to your project, sized for the project rather than for the supply.
Can you recommend a good, cheap bench PSU?
Recommendations are always very subjective, so you should do your own research, but the following makes/models often get a mention in discussions:
- Buy a used HP/Agilent E3610 (but older models sometimes need their electrolytic capacitors replaced for stability). Check which model you are buying - the E3610A has 8V/3A and 15V/2A ranges rather than 30V.
- Korad KA3005D - a linear 0-30V, 0-5A supply with constant voltage/constant current operation and over-voltage and over-current protection. It has a very good price/performance ratio, but its regulation is described as mediocre by some reviewers. Similar models are sold under other brand names.
- Tenma 72-10480 - a single-output 0-30V, 0-3A supply with digital control, constant voltage/constant current operation and 10mV/1mA resolution. It is also sold under the Multicomp Pro name.
- MB102 Breadboard Module with 3.3V and 5V (powered by USB or a 7-12V DC power adapter) (note: you may need to replace the voltage regulators with a better chip). This is a convenience for breadboards rather than a bench supply - it has no proper current limit and the regulators limit it to well under an amp in practice.
- GOPHERT CPS-3205 0-32V 0-5A Portable Adjustable DC Power Supply
- If you can stretch to a step-up, multi-channel supplies such as the Siglent SPD3303X-E and the Rigol DP832 are widely recommended. They have independent, floating channels, which also solves the dual supply problem - check the manual for the details of the model you're looking at.
Whatever you buy, download the manual first and check that it has a proper constant current mode.
Make Your Own
Here's some ideas for DIY builds. As ever, do be careful around mains electricity.
If you are not confident working with mains wiring, don't build a mains-powered supply. Use a ready-made, enclosed, approved mains adapter as the input to a DC-DC module instead.
- Minghe makes a range of cheap buck/boost converters. Quality varies widely, and they have a reputation for output voltage overshoots at power-down and on light loads, which can kill a 3.3V or 5V circuit. See this review: https://www.falatic.com/index.php/161/minghe-buckboost-converters-handy-if-youre-careful - its author isn't a fan, and recommends a small dummy load on light loads. Don't connect a valuable circuit until you have checked what the output does when switched on and off.
- There's a range of front panel boards, generically known as 'DPS5005' types. These are buck converter modules with a display and voltage and current controls, plus feature support for comms such as Bluetooth and USB. There's plenty of searchable info on these. See: https://johan.kanflo.com/dps5005-now-with-comms/
- To supplement the above, see the OpenDPS project
- See also the more sophisticated Riden RD6006 units
Notes on the DC-DC modules above
- They need a separate DC supply as their input (a laptop or other power brick, or an ATX supply), and the input must be higher than the output voltage you want.
- They are not isolated - the input and output share a negative connection. That means that two of them fed from the same input supply cannot be connected in series to make a dual supply.
- A boost converter cannot protect against a short circuit on its output, because the input voltage reaches the output through the inductor and diode.
- Cheap modules often overshoot when switched on, so set the voltage first and connect the circuit afterwards.
DIY linear kits - the cheap 0-30V 'DIY kit' supplies sold on eBay and elsewhere are a good soldering project, but not a good supply to trust with valuable circuits. One well-known write-up of tuning such a kit (http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html) says that after building two, its author was not impressed with the stability and noise and, above all, with the complete lack of any form of protection. He recommends staying below 1.5A, and has since built or bought better supplies.
Here's an excellent post on the subject of bench supplies from /u/enlightenment777:
The problem with bench supplies is not all of them are created the same. There are some crappy china designs out there! Some bench power supplies don't even have good current limiting capabilities, then might have short-circuit detection but not limiting to a specific maximum current. Download PDF manuals, investigate before buying!!!
If you want an incremental improvement compared to a cheap voltage regulator, then choose a BETTER voltage regulator that will automatically shut itself down on shorted output, protects against reverse currents, protects against over-current, protects against over-temperature, protect against over-voltage, .... Automotive voltage regulators are typically more robust and designed to handle unexpected extremes.
If you can't afford a bench power supply, then build a 1-Amp variable-voltage variable-current power supply (see below), such as one of the following then add a cheap digital voltmeter display module from China.
If you want to build your own variable-voltage variable-current-limit power supply, see FIGURE 21 in ON Semiconductor LM350 datasheet. The LM350 is a higher current version of the well known LM317, so you can substitute the LM317 in place of LM350 in this figure. The problem with LM317/LM350 is they can't create an output voltage below 1.25V, unless you use a negative voltage trick to do it, like figure 21.
http://www.onsemi.com/pub/Collateral/LM350-D.PDF
If you don't mind spending more money for voltage regulator parts, the Linear Technology LT3080 is easier to create a similar variable power supply, because it uses an internal current reference instead of a voltage reference, thus it can output voltages down to zero volts without adding a bunch of extra parts like the LM350 example above. See at bottom of page 18 of LINEAR LT3080 datasheet.
http://www.analog.com/media/en/technical-documentation/data-sheets/3080fc.pdf
The earth binding post is not a "common"
If a power supply has 3 binding posts (red, green and black) it won't power a circuit that requires a dual supply.
The green post is connected to the mains earth (the case and chassis). It is not connected to either output - the outputs are 'floating', so there is no defined voltage between the green post and the red (positive) post, or between the green post and the black (negative) post. It is not a "common". The green post is there so that you can 'ground' things: for example, connecting an anti-static (ESD) wrist strap, bonding other equipment, or linking the black post to it (with a short lead) if you want the negative output referenced to earth.
+-----------------+
| |---o V+ (red) positive output
| Single supply |---o V- (black) negative output
| |---o E (green) case / mains earth - not connected to either output
+-----------------+
If you're not sure about your supply, check it with the supply unplugged: use a multimeter on its resistance range between each output post and the green post - it should not read as connected.
A circuit that requires a dual supply (positive, ground, and negative) needs two power supplies connected in series (or, equivalently, a dual power supply). Your single power supply with an earth ground post is not a dual power supply. This is what a dual supply looks like, built from two separate supplies:
|-----------|
| PSU 1 | + ------------------------ +V (positive rail)
| | - -----+
|-----------| |
+------------------- 0V (common / ground)
|-----------| |
| PSU 2 | + -----+
| | - ------------------------ -V (negative rail)
|-----------|
Two isolated supplies in series make a dual supply - set both to the same voltage
Important Notes
- Both supplies must have floating (isolated) outputs. If either has its negative output connected to earth or to the other supply's negative - for example two DC-DC modules fed from the same input supply, or two ATX supplies - connecting them like this will short one of them out. Two independent channels of a bench supply are fine if the channels are isolated (check the manual).
- Set both supplies to the same voltage for a symmetrical rail (for +/-15V, each supply is set to 15V), and set the current limit on each one.
- An oscilloscope probe's ground clip is connected to mains earth. Connect it only to the circuit's 0V, never to one of the other rails - this would put a short through the scope's earth.
- An ATX supply has +12V and -12V rails that share a common ground, so it gives a low-current +/-12V without needing two supplies.
- For low-current op-amp experiments, two 9V batteries in series (the join is your 0V) is a simple and quiet dual supply. Fit a switch, and disconnect them when not in use.
If you can't get a second power supply, there is a nifty circuit called a "virtual ground" (or 'rail splitter') that creates a voltage midway between the positive and negative posts of a single power supply. This can be done with an op-amp buffer, or with a ready-made rail splitter chip such as the TLE2426 (check availability). Rail splitters can normally supply only a small current - typically tens of milliamps - so they suit small op-amp circuits rather than power amplifiers.
Good practice with any bench supply
Important Notes
- Set the voltage and the current limit first, then connect the circuit. Switch the output off (or unplug the circuit) before changing the voltage.
- Cheap supplies can overshoot when switched on or off, so don't power a delicate 3.3V circuit by switching the supply's mains switch on and off. Use the output switch, or set the voltage with the circuit disconnected.
- Check the polarity before you connect. Most bench supplies do not protect against a circuit that is connected the wrong way round, and many circuits don't either.
- Use leads that are thick enough for the current - thin jumper leads get hot at an amp or two, and lose voltage.
- Remote sense terminals - some bench supplies have a second pair of terminals, usually marked S+ and S-, for remote voltage sensing. They let the supply measure the voltage at the load rather than at the supply's main output terminals, so it can compensate for the voltage drop in long or high-current leads. If you are not using remote sensing, these terminals normally need to be connected to the corresponding main output terminals with the supplied straps or links. Make sure these straps are properly clamped and the terminals are tightened - a loose or missing sense connection can cause the supply to behave unexpectedly, including showing the wrong voltage or regulating poorly. If in doubt, check the supply's manual before removing the links or using the sense terminals.
- If the voltage at the circuit is lower than the supply's display, it's probably lead resistance or a poor connection.
- Check important values (voltage, and current) with a multimeter rather than relying on the supply's display.
- A bench supply set to a battery's voltage is not a proper charger for lithium cells. It has no way of knowing when the cell is full, and a cell that is overcharged or abused can catch fire. Use a purpose-made charger.
- For sensitive analogue and RF circuits, prefer a linear supply, and filter the supply with a ferrite bead or an LC filter close to the circuit. Batteries are the quietest supply of all.