Simple heater question SOLVED
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To halve the power, a 14ohm resistor would do the trick, but will also waste half the power at the resistor. The result would be 10W of heat at the heat block and 10W of heat at the series resistor.
Using a voltage divider network, you could reduce the voltage "seen" by the heater to about 17V (24 * √2) which will halve the dissipated power in the heat block, which is more likely what you want to do. the resistance that the current flows through can be much lower, therefore it will dissipate less power.
A buck/boost supply which will support the appropriate current (about 1.25A) should be useable to supply the positive rail, so long as it's fed a different ground, and is probably the cheapest option.
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Addendum: If you want to make it adjustable power output, then a power regulator is really the only way.
So if you don't mind throwing away the power (Have a high wattage resistor strapped to a heat sink in series) then your 14 ohm one will give you 10W at the heater. You figure the dissipated power for the overall circuit, then calculate the voltage across each device in the circuit, like a standard voltage divider, then work out the power from there.
For example a 10 ohm resistor in series would give a total series resistance of 24 ohm, TPd is 24W with 14W at the heater cartridge and 10W at the resistor. (simple case) this is probably close enough to your 15W case (9.85 ohms gives you close to 15W, if I've done my math right.)
3 Ohm in series gives you 17 ohm total series resistance, TPd is 34W, with 28W at the heater cartridge and 6W at the resistor.
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Seriously guys, all I want to do is a quick and cheap evaluation. Basically just a heater tune with various wattage heaters.
Bunging a series resistor in, is both quick and cheap. It'll cost me about £1.00 to £2.00 per resistor. I don't care about wasting half the power for a quick 5 minute test. I don't want to spend money on a buck/boost supply that I'll never use again. Nor do I want to build an adjustable power regulator. Nor do I want to buy heater cartridges from China that I'll only use for one quick test at £10 a pop.
Not wishing to be too blunt and thanks anyway, but my question was answered by @Martin1454 at 9.30 this morning.
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One last solution (based on @SupraGuy's ideas) which is both cheap and allows you to modify the wattage of the heater would be to buy a 10-pack of LM2596 based buck-down converters together with a 10-pack of small heatsinks.
They come from China and take a while but cost less than 10 bucks altogether (I assumed you wanted to test all the heaters (you talk in plural in your OP) at the same time. The advantage is that you can adjust the power by regulating the voltage instead of swapping out resistors.
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All I was doing there is reminding you that the half the power you waste doesn't go into the heater, so the answer @Martin1454 gave you wasn't correct -- at least not completely. I was assuming that you wanted to simulate the various wattages at the heater block.
High power resistors seem to be cheaper where you are. Around here, as soon as you get past 5W, they start getting about the same price as the odd wattage heater cartridges, and they aren't available in the same kinds of values.
If I were doing this, I'd probably end up using a few 5W devices and series/parallel them to get the resistance that I want. High wattage devices aren't available in better than 5% precision, and if you're lucky, you can get them in E12 series values. (Or completely screwball values that are obviously for specific purposes.) As such, it's going to be somewhat tricky to get the specific values for 5W increments
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Why do you care what power it is? It's all PID controlled anyway, by using a lower power heater it's just going to take longer to heat up and have a harder time holding temperature.
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@surgikill As he said in his OP he needs to evaluate something.
Manuel
Duet 3 6HC (v0.6) with RPi 4B on a custom Cartesian
with probably always latest firmware/DWC (incl. betas or self-compiled)
My Tool Collection -
@wilriker Would help if we knew exactly what was going on.
Also, if all is he looking to do is change power at the heater block he can change the PWM duty cycle and get the same results. If he's looking to increase power draw on a FET or something then he would need some power resistors.
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@surgikill I cannot disclose any details but he needs to find an appropriate amount of heating power that suffices his project.
But you are right. PWM of the heater connectors could really work in this case easiest. Since this is purely about evaluation and not something trying to use a 12V heater on a 24V
VINthis will probably work without any additional hardware.Manuel
Duet 3 6HC (v0.6) with RPi 4B on a custom Cartesian
with probably always latest firmware/DWC (incl. betas or self-compiled)
My Tool Collection -
@wilriker Well, I have the equations here if he's interested. It's cheaper than going out and buying power resistors too.
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@surgikill said in Simple heater question SOLVED:
Why do you care what power it is? It's all PID controlled anyway, by using a lower power heater it's just going to take longer to heat up and have a harder time holding temperature.
How do you know that when I haven't given any details about the application? You know nothing about the thermal masses involved because I haven't given any details. You know nothing about the desired temperatures and desired rise times because again, I haven't given any details. You know nothing about the flow rate and temperature differential of any incoming fluids because I haven't given any details. You know nothing about how many heaters are involved and what their configurations and orientations are, with respect to each other and with respect to the design as a whole.
So how do you know that lower powered heaters are going to have a hard time time holding temperature? Or I just use a high powered heaters and rely on PID? And what happens when a FET fails and the PID gets stuck on full power and burns someone's house down?
And I'm not going to provide that information because this is a prototype design of something that may end up as a marketable product and I don't want to give out any details at this time. The only thing that is relevant to this thread is my first sentence, quote " I have a need to evaluate some heater power requirements." And my question was answered in the first reply.
End of.....
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@deckingman Hope you have a good day. Please remove the stick out of your ass before you sit down somewhere.
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@surgikill said in Simple heater question SOLVED:
@deckingman Hope you have a good day. Please remove the stick out of your ass before you sit down somewhere.
Trap 2
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@calvinx I mean he's arguing with an engineer.The whole point of a FET failing and burning somebodies house down is bullshit. If you wanted to get a heater cartridge that topped out at a certain temp (say 300C), short of using a resistive wire that increases resistance with temp, it's going to be a shit design with not enough power, unless you are only trying to hold 50-80C, which I'm assuming he isn't because he's talking about high temp heater cartridges.
Furthermore, he talks about a lower power cartridge being able to perform the same as a higher power cartridge. This just isn't true. A higher power cartridge will have decresaed rise times and be more responsive to environment variables such as moving air. There's no arguing that, it's just physics. The only reason you would want to use a lower power cartridge is for either size or power requirements.
IDK if he had a bad day or what, but if he's as smart as he is posturing then there was no need to create this post in the first place.
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@surgikill said in Simple heater question SOLVED:
@calvinx I mean he's arguing with an engineer.The whole point of a FET failing and burning somebodies house down is bullshit. If you wanted to get a heater cartridge that topped out at a certain temp (say 300C), short of using a resistive wire that increases resistance with temp, it's going to be a shit design with not enough power, unless you are only trying to hold 50-80C, which I'm assuming he isn't because he's talking about high temp heater cartridges.
Furthermore, he talks about a lower power cartridge being able to perform the same as a higher power cartridge. This just isn't true. A higher power cartridge will have decresaed rise times and be more responsive to environment variables such as moving air. There's no arguing that, it's just physics. The only reason you would want to use a lower power cartridge is for either size or power requirements.
IDK if he had a bad day or what, but if he's as smart as he is posturing then there was no need to create this post in the first place.
As an engineer, have you never heard of a Sanity check ? I'm a Mechanical Engineer, but I would never profess to think I know any more than the next man, we can all learn something from others, and to tout the "I'm an engineer" line smacks very much of arrogance im afraid.
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@surgikill
I don't think I'm the one having a bad day here - in fact I'm pretty chilled.I don't really want to get into a discussion with someone who has to resort to personal insults, nor someone who seems to have some sort of paranoia syndrome, but I think you need to re-read my posts before you go off on a rant.
I never said that a low power cartridge would work as well as high power one. Read what I wrote again. You rightly say that a high power cartridge will have decreased rise time. However, because you don't know anything about the application, you don't know that a rapid rise time is in fact undesirable in this case.
As for the FET failure, as an engineer you will know that they can fail in a way that means they are fully on. So, if you are using a PID to control a heater using PWM, and that heater is too highly rated for the application then you will know that in the event of a FET failing fully on, the heater could reach a dangerously high temperature and cause a fire. This is why when you tune a heater, the predicted maximum temperature is reported and flagged as a potential fire hazard. You think this is bullshit? Then I am singularly unimpressed by your claim to be an engineer.
Suggest you take a chill pill and re-read what has actually been posted before you launch into your next rant.
Oh and BTW, I don't have a Donkey but if I did, why would it have a stick in it and why would I need to remove it before sitting down?
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@deckingman said in Simple heater question SOLVED:
Oh and BTW, I don't have a Donkey but if I did, why would it have a stick in it and why would I need to remove it before sitting down?
Maybe, because once you sat down it will be harder to find the motivation to get up again to finally take that stick out of the donkey - the poor animal should not suffer longer than necessary.
(Sorry, couldn't resist 
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@calvinx Yes I have. I'm sorry but if you read the exchange I had with wilriker I simply threw ideas out there and then had a rant thrown at me. The only reason I question his capabilities is because he became extremely defensive of his decision without even providing anything to back it up. That usually reeks of somebody that has no clue what they are talking about. I'm sorry that I responded with the same level of arrogance I received in his post. Maybe I should just lay down and let him have his way with me?
@deckingman You seemed to want the cheapest way out of it from what I read in the previous posts. You also said that you are going to have your own cartridges made. You could still use PWM for testing purposes then get your own cartridges made, be my guest. All I'm trying to do is help YOU not run around wasting money like I have done before, but apparently you knew what you wanted to do from the start, which was use power resistors. Be my guest. Don't understand why you come here for advice and then get pissed about it though.
I'm also not going to get into the FET failure. If you really want a detailed answer then I'll provide it assuming you aren't an ass about it, but simply put you will have extremely poor performance if you are looking for a heater cartridge to work at 200-300C but not exceed 400C
