# Should I buy a diode with higher maximum ratings if it costs the same?

I need a diode that prevents surge currents from a DC motor flowing back into the power supply. The motor draws a continuous of 10A at 12V. Was checking a store for a rectifier diode and found out, that they cost about the same. So why should I not pick the one with the highest maximum ratings?

Also a side question: Can I really pass 10A continuously without cooling?

• Do both diodes have the same forward voltage? What do their specs say about 10A operation, package power dissipation, thermal resistance? Oct 15, 2020 at 20:10
• If the motor draws 10A continuously, what is causing these 'surge currents' in the opposite direction? Oct 15, 2020 at 20:37
• I have a problem with the motor creating a lot of noise in signals and I believe it is due to power surges whenever the brushes of the motor hit something. More details to that here: electronics.stackexchange.com/q/527513/103350 Oct 15, 2020 at 20:46
• Datasheet days enough about per and temperature. You have to learn to follow it. Rth = temperature rise per Watt is 12 degrees K (,or C) per Watt under stated conditions. Forward voltage at 10 A is 0.8 V. These are all typical values. So temp rise above ambient under stated conditions is power X Rth = VF X I X Rth = about 115 C rise above ambient. Nov 6, 2020 at 5:48

Yes, you can substitute a higher spec part for a lower spec one for a case like this.

The second question, however, is a bit trickier. The data sheet you referenced has this graph:

So as you can see, the part's ability to pass the rated forward current depends on the ambient air temperature.

• That makes sense. In the datasheet it's not written like the heat is something to worry about. So I suppose this says it all when it comes to its current carrying capability. Oct 15, 2020 at 20:52
• I'm guessing they assumed that you'd read the entire document. Datasheets are known to be "concise", even terse. ;-) Oct 15, 2020 at 20:55
• @RIJIK In my experience, heat is rarely something you have to worry about right until the moment you stop worrying about it. Suddenly you have to worry about it again. So always keep it in the back of your head that heat may/could/will be relevant.
– Mast
Oct 16, 2020 at 6:35

The compressor will draw 10x it’s rated current on startup, so your supply may be over stressed.,If not add a 10% rating dummy load such as a bulb then it will be more stable.

But the duration will be much less than a single 50,60Hz cycle if rated for 400A single surge current on a 15A diode.

Your problem is lack of specs on power of source and load with a 10x surge with possible radiated or conducted EMI transients interfering with stability getting back into feedback loop. the filter may help for conducted filtering but not radiated if your cable acts as an antenna and couples into the regulator with dI/dt air gap arc noise.

The thermal resistance can be 2’C/W to the leads and 12’C/W to the case.

So if the back EMF on turn off drives the supply the diodes will clamp the over voltage spike safely. If you wanted to test the diode temp. and feel it is getting too hot to touch, you may clamp the body to the chassis with a copper strap shaped in a U around the body and screwed to the chassis with heat shrink on the leads for accidental short protection.

Adding a remote plastic Cap across the ,tor will reduce the RF bit if the 12 V load referred back to the 5V regulator is unstable with a step response the best treatment is the dummy preload for unknown PSU’s. the key point is the surge current on start and BEMF on stop.

All PC PSU’s regulate on 5V AMD the others track by transformer ratios and very tight (high mutual coupling. This means you can also preload the 5V instead, but may be easier for you on the 5V supply as these tend to be higher power rating and the 12V is for or less steady GPU’s loads

If you can list the power ratings of compressor and supply, that would clear up your problem with photo layouts. Hopefully you are using twisted pairs to the motor!

## Update with new info

10mF will help for a few milliseconds and appear as a short until charged up. No good. the motor May appear like 0.12mOhm rising to 1.2mOhms running on 12V you can measure the start R with a good ohm meter.

This is an automotive style compressor . It may start with 50~ 100A and decline until full RPM. Which could be a 1 kW so it is design for car batteries not PC PSU’s !! Good luck ,

but I think you will need a separate regulator for your Analog/ Logic from 12V down , the 5 V will also be unstable.

The steady state noise is a shielding problem that demands single or double shielded heavy wire, not common with a large CM filter or easier , shield and isolate the offended system with cables at right angles.far apart.

Still need more details on system, layout and problems.

• That's a very informative answer as is it directly tailored to the problem I am trying to fight. The compressor is very cheap. The only other data to voltage and current (&pressure) is the frequency of the turns max 500/min. Since the servo is going crazy even after startup, I am not sure if preloading will help. What do you mean by twisted pairs? Like with USB Data cables? Oct 15, 2020 at 21:45
• What is your compressor spec. Yes if low current to USB.. specs man gimme da specs and nuthin but da specs. ( and a photo) then add a plastic cap across motor. this is effectively a DCR, L C issue for conducted and low impedance coax or twisted pair solution. Oct 15, 2020 at 21:49
• The exact name of this compressor is a "cartrend 10924 compressor, 12VDC, 10A, 10bar, max 500Turns/min". That is all the relevant info written on it. I tried to find a Datasheet with more info, but really the best one I could find is the manual (contains an image) and there is not much info either: asset.conrad.com/media10/add/160267/c1/-/gl/002176868ML00/… Oct 16, 2020 at 8:44
• I am currently waiting for 10mF caps so I can build a Pi Filter. My wires are not twisted and are actually coiled up like in this picture: bauhaus.info/kompressoren/cartrend-kompressor-manometer-klein/p/… Oct 16, 2020 at 8:51
• 10mF will help for a few milliseconds, compressor Wil start at the locking current which could be a kW so it is design for car batteries not PC PSU’s !! Oct 16, 2020 at 9:12

For silicon diodes there no downside generally to buying a higher PIV rated device within reason, up to 400V anyway. Schottky diodes tend to have a higher Vf if they are rated for more voltage so there is a downside (they run hotter and you lose some voltage drop).

Can it pass 10A continuously? I would say you need to have a fairly good heat sink on those leads to safely get 10A through it, probably solder at least one side directly to a heat sink with very little length. You need to hold the temperature of both leads to about 90°C at 10mm from the plastic, and that is running the junction at 175°C which is not conducive to long life. Their definition of "ambient" as the temperature of the leads 10mm from the plastic is not congruent with my definition of "ambient". Photo from here

Personally, I would prefer a TO-246 or similar part bolted to a heatsink unless cost control is an overwhelming concern and all the necessary testing can be done to ensure the junction temperature is within acceptable bounds.

• Wow, that's some interesting way to mount axial component. Oct 15, 2020 at 20:18
• @weaknespase It's pretty common in low-cost switching power supplies. Oct 15, 2020 at 20:18
• That voltage drop is a good argument. Also my neighbor always suggests me to buy something like a TO-246 and now I know why. Oct 15, 2020 at 20:50
• @weaknespase I believe that's an assembly for this particular application. I've not seen any off-the-shelf diodes like this but I have seen this type of mounting before. They may have sourced it from the heatsink vendor. Oct 15, 2020 at 20:58
• Wouldn't Schottky diodes also have a higher leakage current if they are rated for higher voltages? Oct 16, 2020 at 10:06

A couple of other people have mentioned Schottky diodes, but could I please make one point:

NEVER REPLACE A ZENER DIODE BY ONE OF HIGHER VOLTAGE.

And yes, I've seen it done in the field.

• Are you talking about zener or schottky diodes? Oct 16, 2020 at 12:38
• I said Zerner. In capitals. Oct 16, 2020 at 12:52