# Are the ratings on a bench supply absolute or do they imply total power output?

If a bench supply is rated as "30V, 5A", does that does that imply a 150W total output (meaning it could supply say, 10A at 15V), or does it mean that 5A is the maximum it can supply at any voltage in its 0-30V range?

If the latter, what will typically happen if I try to draw more than 5A?

• Tthey'd be crap at marketing if a 30V, 5A supply could supply 15V @10A, hence they'd go out of business and we'd be left with the supplies that do no more than what they say on the tin (which of course we are). Electronics products - if the implied blurb seems to good, don't infer. – Andy aka Aug 13 '13 at 21:48
• I had a pretty strong idea whart the answer would b, but it seemed like the kind of question whose obvious answer would be wrong in an interesting way. – Robert Atkins Aug 14 '13 at 21:40

A bench supply rating will usually apply to current output. Although many of the components inside the device may be rated for a higher current at a lower voltage, the current limiting circuitry will kick in based on current. An example is below:

The supply will clip the output in the event of overcurrent. This is usually true with fixed supplies as well as those that have adjustable current limiting (Topward Triple supplies are a great example of this). The voltage will not be able to rise above that which causes the output to exceed the current limit.

Again, this is just typical. I'm sure you will find some cheap supplies that don't protect themselves and just burn out.

edit: for supplies that don't have current limiting, the AC/DC circuitry will most likely begin to ripple because the capacitors aren't rated for that high current output. Your signal will gradually degrade and probably lead to failure.

The answer is "it depends," because some manufacturers limit the current throughout the voltage range, but others specifically are designed for a total power output limit.

What you want to look for is whether the supply output is "Single Range," "Dual Range," "Auto Range," or "Multi Range." BK Precision, for example, has some supplies with dual and multi range, and these are designed to provide VA combinations that offer a greater flexibility.

The BK 9173 is dual range, offering 0-10V, 10A as one range, and 0-20V 5A as the other.

My supply, a Circuit Specialists CSI3005X5, is only single range, and 5A is the maximum current at any voltage.

Edit:

To answer "What will typically happen if I try to draw more than 5A?" - the supply should limit current, and it does that by dropping the voltage. For example on my supply if I short the output the current will shoot to 5A, but the voltage drops to a few millivolts. If the supply doesn't current limit, then it would blow a fuse, or burn up. Make sure it can limit current!

• Holy shit that BK9173 is $1400! As a hobbyist, I'm thinking more about the behaviour of supplies an order of magnitude less than that :-). – Robert Atkins Aug 13 '13 at 19:18 • That's why I linked mine. :) It's a bargain at ~$130. It doesn't provide more than 5A at lower voltages, but it has decent programmable current limiting. – JYelton Aug 13 '13 at 19:20
• Also you might want to see an earlier question I had before I committed to buying: electronics.stackexchange.com/q/52033/2028 – JYelton Aug 13 '13 at 19:20