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Is this a good test load?






scumbagatheist

scumbagatheist

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No!

Get one of Jufran's kits. You can buy directly from him..in kit or assembled form. Or, you can get one through CajunLasers. I don't know if they assemble theirs or not. Not only is it seriously sexy PCB goodness, but it's quite functional and well laid out!

Here's the link to Jufran's thread
 
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Pyro Man

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And also its kinda funny because he uses the same diodes. So dont see any difference and plus i can use that kit on ebay to make it into something exact.lt like that
 
benmwv

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That kit looks fine to me. Not good for 3A though. Equal or greater than the jufran test load for current handling.

You are way better off buying from digikey and getting a resistor in a heatsinkable package though. All they are doing is reselling what they bought online anyways. Then your test load will handle WAY more current than the jufran load.

Nothing against his testload. Its great for lower current stuff and has plenty of diodes for any Vf you want, but 3A is a big stretch. 2A is a stretch. If you are putting more than 1A into a resistor that isnt heatsinked it gets very warm and your readings drift. The resistor should be nearly room temp for good accuracy. If you are serious about accuracy get both a 1 ohm and a 0.1 ohm in to220 or something and slap them on a big heatsink.
 
Leodahsan

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benmwv said:
That kit looks fine to me. Not good for 3A though. Equal or greater than the jufran test load for current handling.

You are way better off buying from digikey and getting a resistor in a heatsinkable package though. All they are doing is reselling what they bought online anyways. Then your test load will handle WAY more current than the jufran load.

Nothing against his testload. Its great for lower current stuff and has plenty of diodes for any Vf you want, but 3A is a big stretch. 2A is a stretch. If you are putting more than 1A into a resistor that isnt heatsinked it gets very warm and your readings drift. The resistor should be nearly room temp for good accuracy. If you are serious about accuracy get both a 1 ohm and a 0.1 ohm in to220 or something and slap them on a big heatsink.
Click to expand...

P=IV. Don't forget some voltage is lost on the diodes. The resistor shouldn't heat a lot at all, I've always used a 10W one, even when testing a flexmod, and it handled it well...
At 3A it should heat around 9W uh? (measuring 3000mV on it, 3V, with the driver driving 3A, thats 3A @ each diode, and 3A @ the resistor, because they are wired in series) Thats still lower than max dissipation.

edit: Anyways you won't operate a test load for a lot of time... IMO... If you are worried about precision you should worry about the diodes... for a 0.6 Vf they would heat around 2W each... And their Vf drift with temperature, as they are diodes...
 
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scumbagatheist

scumbagatheist

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I've heatsinked mine for accuracy, anyway. I've used it reliably with short duty cycles to handle loads well above 2.5A with heating, but it was manageable for my needs. Now that it's heatsinked, I have no worries at all.
 
benmwv

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I have used 10W resistors too. Just because the can handle 10W doesn't mean they won't get hot. In this application accuracy is everything and resistance changes with temperature. That is a fact.

P=IV and V=IR, so P = (I^2)R. That is 9W at 3A. Within the specs sure, but if you are looking for accuracy you don't want to be pushing the maximum limits. So the 10W resistor may work correctly for a while but after it heats up (which won't take long with 9W of power in it) the readings will drift. How much depends on the resistor but If you are using an axial resistor they don't dissipate heat well. Now if you have a 1 ohm resistor in the to-220 package and bolt it to a large aluminum heatsink you will find it can take 3A for a long time without even getting warm. If you use a 0.1 resistor instead it won't even be dissipating 1W at 3A and you have much less of a heat problem to deal with.

Also I was under the impression that the resistor on the jufran test load wasn't rated for 10W? Maybe that is wrong though.

Just looked it is a 5W resistor.
 
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scumbagatheist

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Ben, that's all very interesting. But, your conclusion with the other resistor esxapes me at the moment. Any chance for wiring diagram to help a brother out?
 
Leodahsan

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benmwv said:
I have used 10W resistors too. Just because the can handle 10W doesn't mean they won't get hot. In this application accuracy is everything and resistance changes with temperature. That is a fact.

P=IV and V=IR, so P = (I^2)R. That is 9W at 3A. Within the specs sure, but if you are looking for accuracy you don't want to be pushing the maximum limits. So the 10W resistor may work correctly for a while but after it heats up (which won't take long with 9W of power in it) the readings will drift. How much depends on the resistor but If you are using an axial resistor they don't dissipate heat well. Now if you have a 1 ohm resistor in the to-220 package and bolt it to a large aluminum heatsink you will find it can take 3A for a long time without even getting warm. If you use a 0.1 resistor instead it won't even be dissipating 1W at 3A and you have much less of a heat problem to deal with.

Also I was under the impression that the resistor on the jufran test load wasn't rated for 10W? Maybe that is wrong though.

Just looked it is a 5W resistor.
Click to expand...

It don't make sense for me. If a resistor is rated 2%, rated for 10W, I didn't cut his pins (full sized), and I'm dissipating 9W into it,
I'm operating inside the safe operating area. Why should the measurements drift? It isn't made to operate into such high temperatures without drifting (I've seen datasheets of 20ppm/°C resistors!)... It is late here :)tired:), tomorrow I'll put some CV on my resistor and watch the current, and see if it changes.

I think you are talking about the diodes causing any drifts... They heat a lot more than the resistor, for their size. :thinking:

good night all
 
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benmwv

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If you look in the datasheet for the resistor it will say that it is rated for 10W when the temperature is less than 70C or something like that. 9W is a lot of power and without anywhere to go (axial resistors dont dissipate heat well, and are hard to heatsink effectively) the temperature will build. Once the temperature raises out of a certain zone it is no longer bound to be within 2% accuracy and you readings will drift because their accuracy is dependent on the resistor being exactly 1 ohm.

Its a shame matt has closed down his shop for now, but here is the schematic for the mohgasm test load. It keeps the shunt cool and accurate despite using surface mount components because it has a 1ohm for low current and a 0.1 ohm for high current. You just switch the jumper to chose which one you want and adjust your reading to match. with the 0.1 ohm resistor you now read it as 1mv = 10ma.

attachment.php
 

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Pyro Man

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What do you guys mean by a 0.1 ohm resistor in to220? Because from what i see i will build the test load right above me. And where do you measure for the current coming out of the test load right at the resistors?
 
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Pyro Man

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Ok so can anyone tell me what i need to make the test load for the mohgasm schematic. I am going to the store and i need to know soon.
 
benmwv

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The parts in that kit at the top will do fine for you right now. If you start making and testing a lot of drivers, doing a lot of high current stuff, or doing long oscope tests you need high accuracy for then you will want something a little better. Those parts are a great entry level kit though. Probably all you will ever need.
 
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