Hi all, iv got a no name PSU which outputs 29.31v(im guessing that would drop to 24v under load) and i need to work out its max amperage output. but alas, my cheap china multi-meter doesn't do Dc current, only Ac via a clamp. Is there a way of doing this without a multi-meter?
P.s i also need to work out the MAX current draw from my project, would i need to check the current from each individual component or just measure it ALL at once.
Best Regards
Dave.
Probably not, and it is not a good idea to guess.
Without a data sheet, there is no way to know what the maximum, safe level of current draw might be, other than long term experimentation.
Not t worry, just found this. I writing was so fait. have to enhance on photoshop.
im guess that 2A is max?
It would be a good idea not to draw more than 2A.
ABSOLUTE maximum, I would not try to pull more that 1.5 Amps!
Ok cool. Is there a way to limit the current draw on my psu? Like what i can do with my stepper driver?
A fuse will blow to protect it but there's no easy way to limit it.
I'm guessing not... With all of that circuitry, I'm guessing it's regulated and it will hold fairly-stable.
You can't really measure "maximum" current... You can, but you want to measure the maximum rated current. If you stick an ammeter across the output with nothing in series you'll get excess current and "bad things" can happen!
You might want to get another meter... Most "cheap" meters can measure current. But there is one thing I'd look for - Most multimeters have a separate connection for measuring current and some cheap meters don't. That's important because in current-mode the meter is a short-circuit and if you accidently leave it in current mode you'll short stuff out and maybe blow something or blow the fuse in the meter.
Some meters have a separate connection for high current (Amps) and for low current they use the regular Volt & Ohm connection. That doesn't seem "safe" to me but maybe they have some kind of current limiting (or maybe there's a separate internal fuse).
My wife got me the metre years ago, is there a post that can suggest a good meter thats not over £30.
Dave
The recommended, and much, much better approach, is to use a power supply than can provide at least 2x the maximum expected current draw of your circuit.
But im unsure how to work out the maximum expected current draw. as i dont have a multi-meter that can measure dc current.
Datasheets and arithmetic.
Motors and servos will be the bulk of it. Look up the stall currents, add them up, and double the result. Order a power supply that can supply at minimum that value.
If there aren't any motors and servos, follow the advice in the post above for the other parts.
Thanx peeps. 2 of my motors are not named. No markings what so ever. All i know is that a 29v psu powered them, but not at the same time.
Dave
One very successful approach to using salvaged stuff of unknown characteristics is to power it up, and run it for a while under full load.
If it blows up, toss it in the recycle bin. Nothing lost, nothing gained.
Universal input, oddball output voltage. Yes 2A is the max, X29V = ~60W delivered to load.
how'd you work that out mark?
Ohms law. If the voltage is 29V and the current is 2A then the power is Pw = Ia * Ev or 2x29 = 58 or ~60W to LOAD.
The input is 100V-240V 50-60Hz. US is 120V/60Hz, Europe is 240V/50Hz so it will operate on most any wall power worldwide. This is common on computers. The power inlet is that funny beveled rectangle shape but the plug can be anything so you can ship the same power supply world wide and just have to toss in whatever cord set the locals use.
Input is 1.5A max. Worst case is 1.5A (max current) at lowest voltage (current & voltage are inverses, higher voltage = less current), 100V or 150W input. So with input of 150W and output of 60W the efficiency is 60W/150W or ~40%. At peak output you can cook on it! The "missing" 90W is dissipated as heat.
Fortunately this is the limit, most power supplies don't spend a lot of time at that operating point.
I guess the 1.5 A will not be needed at 60W. That would be a pretty bad efficiency...
Perhaps some safety margin is built in here. For inrush current or so. If you would put a 1.0A fuse in the primary lead, it might very well blow when you plug in the cord at the moment of highest voltage of your AC source. The rating will therefore be higher than needed.
You would need to check though what voltage is delivered at 2 Amps and at 0 Amps. That will tell you something on the limits of the regulator.
With those numbers in the photos, it might be a Staigo power supply meant for an electric chair. Then it could be very noisy.
I thought Old Sparky ran at a much higher voltage?
