Help with 8a driver and load

[paul1598419]

Also, if you use the 0.1 ohm resistor you will need to keep your connections short and robust as they will add resistance to your circuit and throw off your end result. So, only short heavy gauge wire and large thick traces on your board.

 

[kecked]

I use 10 5w 1ohm ceramics in parallel to make up a 50w unit. This is around a threaded rod in a tube with a fan on each end. The tube has addition fins on it. It’s modrled after a brake resistor for a train. I then use the highest current Silicon diodes I can find for the voltage drop....NOT....dude all I do is hook up the diode with a current meter in line with the diode after making sure I turned the current off. These diode take sooo much current this is the best way to do it. It takes multiple turns to get 6a. Would not do that on a 200ma diode but these are huge. You can do what I posted at the beginning but there is no reason. Sorry for the joking but it is sooo easy to go overboard on this stuff.

 

[paul1598419]

The power dissipated in a 0.1 ohm resistor as heat is quite low even at 5 amps that using ten 1 ohm 5 watt resistors to get there is just overkill. I get the use of 20 amp rectifiers heat sinked for this, but only if the connections to them are short heavy gauge wire as to not introduce appreciable resistance to your circuit.

 

[Benm]

Well, if you used a 0.1 ohm resistor, at 5 amps that'd dissipate 2.5 watts in total, so building one out of 'standard' 0.25 watt, 1 ohm resistors is feasible as long as air can flow around them freely (no forced air cooling required).

I've used this approach in the past often enough for RF dummy loads that needed to be 50 ohms and able to dissipate a couple of watts. Putting 20 perfectly standard carbon film 0.25 watt diodes in parallel worked extremely well. For RF application an extra benefit is that you reduce the inductance compared to using one big resistor.

But for laser dummy loads most of the power will be dissipated in the diodes, and when operating upwards of 5 amps it would just be most practical to get them in TO-220 cases and mount them on a suitable heatsink. Such diodes tolerate running at hot temperatures very well, a case temperature of 100 celcius is often not a problem for the diode, but will be for you if you pick the thing up by the heatsink

I've literally had rectifier diodes de-solder themselves (from lead based solder) that still worked fine after cooling down and soldering them back in

 

[diachi]

The power dissipated in a 0.1 ohm resistor as heat is quite low even at 5 amps that using ten 1 ohm 5 watt resistors to get there is just overkill. I get the use of 20 amp rectifiers heat sinked for this, but only if the connections to them are short heavy gauge wire as to not introduce appreciable resistance to your circuit.

Well, if you used a 0.1 ohm resistor, at 5 amps that'd dissipate 2.5 watts in total, so building one out of 'standard' 0.25 watt, 1 ohm resistors is feasible as long as air can flow around them freely (no forced air cooling required).

I've used this approach in the past often enough for RF dummy loads that needed to be 50 ohms and able to dissipate a couple of watts. Putting 20 perfectly standard carbon film 0.25 watt diodes in parallel worked extremely well. For RF application an extra benefit is that you reduce the inductance compared to using one big resistor.

But for laser dummy loads most of the power will be dissipated in the diodes, and when operating upwards of 5 amps it would just be most practical to get them in TO-220 cases and mount them on a suitable heatsink. Such diodes tolerate running at hot temperatures very well, a case temperature of 100 celcius is often not a problem for the diode, but will be for you if you pick the thing up by the heatsink

I've literally had rectifier diodes de-solder themselves (from lead based solder) that still worked fine after cooling down and soldering them back in

Agreed, to everything above.

If you look at the picture of my 20A test load earlier in the thread you'll see several of the tips these guys offered in use. Doubled up all of the wire (only had #18...) and used TO-220 diodes mounted to a CPU heatsink from a server.

Of course, I opted for a 0.1W 100W resistor seeing as I had some from another project. A little overkill but it works.

Benm, that's crazy! Sounds like that diode needed some (or better) heatsinking.

 

[paul1598419]

I've seen the one you speak of, Adam. I might have used copper line cord to connect the diodes with, but your way is likely just as good. I need to build one similar to yours as I have one that uses 10 amp rectifiers with the 0.1 ohm resistor. It works as long as you don't let it run on for minutes at a time.:yh:

 

[Cyparagon]

Using a 1ohm shunt used to be viable in the days where the highest power for DIY stuff was 200mW.
Now it's just stupid. Let's compare against a dedicated ammeter:

Resistor:
Costs $2 or potentially much more.
Has a steep I/V characteristic compared with a laser diode.
Drops 4V at 4A.
Requires external voltmeter to read the value.
May be inductive and cause unwanted operation in the driver.
Most require additional heat sinking at full load.

Ammeter:
Costs $1.59 with free shipping.
Has negligible contribution to I/V load characteristic
Drops negligible voltage at any load.
Does not require an external voltmeter to read the value.
Non-inductive.
Does not require heat sinking.

 

[kecked]

Here here we’ll said. Amp meter rules these days. Just curious why we never used a high power led. Would it not be very similar?

 

[paul1598419]

That has got to be the cheapest DMM I have ever seen. Wonder how accurate it would be used as an ammeter. The cheapest DMM I have cost me $135.00. :thinking:

 

[Benm]

Be careful with those really crappy multimeters though - the range is rated for 10 amps, but some of them are shitty enough not to be able to take that current over a minute or so.

Also the contacts in them and leads supplied with them are usually not suitable to handle 10 amps... those lead might look decently thick, but it's often mostly plastic insulation and very little metal (not even copper...) inside.

If you want to use a crappy meter like that on your dummy load you should just take it apart and solder thick wires directly onto the PCB.

Agreed, to everything above.
Benm, that's crazy! Sounds like that diode needed some (or better) heatsinking.

Hehe, what heatsinking? It was just one of those 1n400x diodes in an experimental device - it worked fine conducting a few amps instead of the rated 1 amp, until it just fell out of the pcb (mounted traces side up). What surprised me most is that it still worked after that ordeal, though it was replaced with a 1n540x type

 

[paul1598419]

I got my first Black Buck 8M driver in the mail today. Very impressive driver. Came with its own thermistor to attach to the diode or heat sink. Has a place to add a 10K pot to adjust the power after the limit is set with a multi-turn pot on the board. Can also add a switch to turn it off and on without the current having to go through it. Don't have specific plans for it yet, but I know I will.

 

[Cyparagon]

some of them are shitty enough not to be able to take that current over a minute or so.

I assume you're referring to the leads, and I wouldn't trust those leads with much over 2A, personally. But as you said, the idea is to just solder the device onto the test load directly. Leads add complication, bulk, and voltage drop, all for no good reason.

The ammeter shunt itself is typically just a length of copper or copper-based conductor. There's really no chance something like that will overheat at a mere 10A.

That has got to be the cheapest DMM I have ever seen. Wonder how accurate it would be used as an ammeter.

2% according to the specification list, which is believable. Certainly better than the 5% (+ external voltmeter tolerance) that is common with low value resistors.

Here here we’ll said. Amp meter rules these days. Just curious why we never used a high power led. Would it not be very similar?

Meh, the vast majority are around 3-3.5V. Close enough? Maybe for some diodes. It might end up costing more and the light may be distracting. Diodes are easy to find and harvest from scrap boards. Also, LEDs are less resistant to user error and power spikes than simple PN junctions. For example, even the lowly 1 amp 1N4007 is RATED to take 30 AMP PEAKS for up to 8ms and 10 amp peaks for 1 second according to the datasheet.

 

[paul1598419]

Like I said earlier, I use a 0.1 ohm 5 watt 1% resistor as my sense resistor in my dummy load. I do have another with a 1 ohm resistor for lower current drivers, but it has the same tolerance as my 0.1 ohm. The traces are thick and very short.

 

[thehans]

Hello, I'm in the same situation as the OP: 8A driver, same diode, and same application(CNC).

I happen to have a ton of 1N5407 diodes, would these be suitable to simulate the NUBM47?

These seem to have a drop of ~0.85-0.9V at 2.5A so I'm thinking a 6S2P configuration would be able to approximate the laser at 4.5-5A.

edit: By the way I have an ammeter capable of 10A max, so I'm not planning on using an external sense resistor.

 

[paul1598419]

Since you are not using these as rectifiers it is important to get over rated diodes when using them in DC mode. Mine are rated at 20 amps each. I also have my smaller dummy load using 10 amp diodes. Heat sink them too.

 

[thehans]

Well they are rated 3A each, that's why I was thinking of doing them 6S2P (2 in parallel for each series stage), or is that a bad idea? I can do more in parallel if needed, I have hundreds of them...