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FS: ***Selectable test load up to 3A (assembled or DIY kit)*** IN STOCK!

Jufran88

Jufran88

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@jander6442: Oh wow! I think that's the biggest heat sink I've seen put on the test load lol.

@TM Ru22311: :beer: I'm glad you like it! :beer:
 





The Lightning Stalker

The Lightning Stalker

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I am wondering why you decided to go with a 5W resistor. At the full rated 3A, it will be dropping 3V. 3 * 3 = 9W. That is almost twice the rating. It's probably not a big deal since it's not for continuous use, but still what was your reasoning behind this? Resistors can get extremely hot even when run at their rated wattage. Like hot enough to burn boards unless they are heatsinked. With no heatsink, I usually derate them by 1/3. I'm not criticizing or anything because obviously it's proven and works well for what it was designed for.
 
Jufran88

Jufran88

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To me it was the best cost effective choice. Since it's not for continuous long duration usage they are perfect for testing drivers. Also, without having to resort to wire-wound resistors. A simple current check or setting shouldn't take longer a minute or so. Temperature drift will come into play, but shouldn't affect readings drastically.

If there is enough demand I've been thinking of using an axial style resistor with heat sink installed instead for an extra few bucks. Although, there are other heavier duty test loads out there already other than mines. Just keeping mines simple, effective, and cheap. :beer:
 
The Lightning Stalker

The Lightning Stalker

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Makes sense It's tough to find anything over 3W that isn't wirewound and they have poor tolerance. I'm looking at some stud mount diodes here to possibly make a really high current test load for 5-10W diodes.

GZ5My3F.jpg
 
ApexProxy

ApexProxy

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The diodes aren't much of an issue afaik. A little heatsinking on them and they seem to be rock solid for a long time. Its the resistor that can take a beating if you run the load for extended periods. I for one haven't run mine for more than like 30 seconds at a time since it only takes 5-10 seconds to get a good solid reading on a driver anyway. However, I like overkill and actually wouldn't mind having an easy way to torture test my drivers for longer periods.

Jufran, have you looked into using two 5W 500mOhm resistors in series? I found one that is 5W at 1% tolerance for $0.62 each over at mouser. Here it is if you want to check it out:
WHER50FET Ohmite | Mouser
With twice the heat dissipation capability the resistors should be able to hold up to reasonably longer tests.

Unfortunately the 500mOhm 1% tolerance 10W resistors are $2.34 at the cheapest there. If 5% tolerance is acceptable these 10W resistors are only $0.64:
280-CR10-0.5-RC Xicon | Mouser
You could get some seriously long torture tests with two of those lol.
 
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The Lightning Stalker

The Lightning Stalker

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Really, even at 6.5A? Wouldn't they start coming apart? The resistor idea sounds good although it would push the tolerance to 2%. Same as the original resistor though
For even better heatsinking on those diodes, you could try cutting thorough some of that plastic. Carefully though lest you should reach the meaty core.
 
Jufran88

Jufran88

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Unfortunately those are wirewound resistors =[

They tend to be inductive and mess with the switching frequencies of buck/boost drivers. I don't think they affect linear drivers though.

I did find a non-inductive wirewound, but the cost outweighed its functionality.

I did find some radial style resistors with higher power ratings (30W) that can be heat sinked properly. I would add maybe 5-7 dollars more depending on what heat sink and if it would add to the board size.
 
ApexProxy

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Hmm, I didn't know that inductance would be an issue. Now I'm wondering exactly how it effects the driver though. I understand what inductance is but what I wonder about is how it would effect the driver which would be far enough from the resistor not to be within range of the field. Anyone have some insight on this?
 
The Lightning Stalker

The Lightning Stalker

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Switchmode drivers work on the principle of inductance. A simple explanation would be that an inductor resists a change in current. The resistance to change has a value and is measured in henries. Adding another inductance in series with this inductance will effectively increase the resistance to change and therefore the inductance. A higher inductance will cause less current to flow in the circuit. So if you attach a wirewound resistor of unknown inductance, it will reduce the current flowing into the test load by an unknown amount. Then you attach your precious laser diode without said resistor and the driver overdrives it and bad things may start happening. :oops: Even worse, the amount of error depends on the frequency and pulse width of the driver. Basically it's a design nightmare. :banghead:
 
Jufran88

Jufran88

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There's also inductive kick back. Basically the inductor wants to keep current at a constant. Switching frequency drivers (buck/boost) switch on and off. Inductors don't like this and a voltage spike occurs that could potentially destroy the driver and or test load.
 
Jufran88

Jufran88

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Ah those would be ok. However, in series only the ohms adds up, while wattage stays the same. 2 2 ohm resistors with 5W rating would amount to 1 ohm with 10W rating (in parallel)
 
ApexProxy

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Hmm, are you certain? I have heard that the wattage adds for both configurations. It sort of makes sense but at the same time it sort of makes sense that it would add for both configurations.

In the case of setting up ballast resistors for some HeNe tubes I have Sam(Sam's FAQ) told me that using two resistors in series to achieve the required resistance would allow the two resistors to share the heat dissipation.
 
Atomicrox

Atomicrox

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It adds up in both configurations. Here's an example:

@1A:

1Ohm resistor:
1V drop across, 1A through, 1W dissipated

2Ohm resistors in parallel:
1V drop across, 0.5A through each, 0.5W dissipated on each, 1W total

0.5Ohm resistors in series:
0.5V drop across each, 1A through, 0.5W dissipated on each, 1W total
 
ApexProxy

ApexProxy

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Atomicrox said:
It adds up in both configurations. Here's an example:

@1A:

1Ohm resistor:
1V drop across, 1A through, 1W dissipated

2Ohm resistors in parallel:
1V drop across, 0.5A through each, 0.5W dissipated on each, 1W total

0.5Ohm resistors in series:
0.5V drop across each, 1A through, 0.5W dissipated on each, 1W total
Click to expand...

That's pretty much how it worked out in my head. Though I sort of just figured that was the case based on logic. I am not really very well versed on circuitry but I am working on it and picking things up where I can :D

Anyway, I may just build myself a test load with way overkill heat dissipation capabilities so I can run super extended driver torture tests. Like literally run the driver at max unheatsinked to see how long it lives and how it reacts to the excessive heat.
 
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