Need recommendation for rheostat or potentiometer

[NyeSpy]

Weird question, but bear with me...

I am an 8th grade science teacher and my students are building a hovercraft. They want to use two 7.2v DC NiCd RC racing batteries as the source of power. One battery will be connected to a computer fan that generates a lot of static pressure (approx 1200 pascals). This will be the "lift" fan. The other battery will be connected to a computer fan that moves A LOT of air (205 cfm). This is the "push" fan.

Here is where it gets tricky. The students want to be able to control the speed of the "push" fan. The craft should be adjustable, from moving at a slow crawl to screaming down the track. INTEGRATED CIRCUITS are NOT allowed, so PWMs are excluded. They want to use a rotary resistor to adjust the fan speed. Is there a particular rheostat or potentiometer that I should be targeting? I am not very electronics oriented and any parts we try I will have to purchase. Trying to avoid spending a small fortune on a bunch of stuff that either won't work (best case) or that will burst into flames (obvious worst case).

Battery - https://www.amazon.com/Racers-Edge-7-2V-1500mAh-Battery/dp/B000JCNAR4
Push Fan - http://www.digikey.com/product-detail/en/mechatronics-fan-group/MD1238X12B-FSR/1570-1122-ND/5209819

Thanks for all of your help!!!

 

[davenn]

INTEGRATED CIRCUITS are NOT allowed, so PWMs are excluded.

so do a discrete transistor PWM circuit

 

[NyeSpy]

The rule states "Brushless motors and integrated circuits are not permitted" and the craft is subject to impound and inspection. Competitors at the event forums are saying PWMs would violate the rule against ICs. Would a discrete transistor PWM be subject to scrutiny and possible disqualification?

Pardon my ignorance. Electronics is not my gig.

 

[davenn]

Competitors at the event forums are saying PWMs would violate the rule against ICs.

I said use discrete transistor circuit ... NOT IC's
do some googling on transistor PWN circuits

 

[NyeSpy]

I am seeking a clarification on the rules. I am pretty sure they are going to say this is not allowed.

Until I hear back, what do I need to be looking for in a rheostat or potentiometer? We want to have as much control over fan speed as possible (a lot of variation) without too much fear of burning out the resistor. An amazon link would be amazing (but now I'm probably being too greedy!).

 

[BobK]

12V 2.5A.

If you dialed it down to 6V with a rheostat (not potentiometer,) it would be 1.25A at 6V in both the fan and the rheostat. So the rheostat would need to be 15W minimum, probably 25W to be safe. That is going to be large and expensive.

This one would be about right:

http://www.mouser.com/ProductDetail...GAEpiMZZMtC25l1F4XBU5flxUv%2bkc1oBgXZKbr86GA=


Bob

 

[NyeSpy]

Yikes, $50 is definitely hard to swallow (considering I've already dropped $65 on fans). If it matters, the longest run time will be 25 seconds. Does that allow us to downsize any to a less expensive part?

 

[davenn]

Does that allow us to downsize any to a less expensive part?

not really ... it has the same current running through it from the moment you switch on

The above reasons you have been given are the exact reason why I aimed you in the direction that I did

have you googled for transistorised PWM circuits yet ??

 

[NyeSpy]

Have you googled for transistorised PWM circuits yet ??

I did. My head is still swimming. Is that something I can buy or will it need to be built?

Still waiting for the judges to weigh in. I would be shocked if something this complicated would be allowed. The rules seem to indicate safety and SIMPLICITY.

Here's a few more rheostats I found on Amazon. Any of these workable for what we are trying to do?

https://www.amazon.com/Scientific-L...UTF8&qid=1486501599&sr=1-12&keywords=rheostat

https://www.amazon.com/Electronics-...UTF8&qid=1486501599&sr=1-10&keywords=rheostat

https://www.amazon.com/Wirewound-Po...UTF8&qid=1486502193&sr=1-24&keywords=rheostat

 

[Leggyownz]

I do a lot of Ebay sales so here is the Rheostat previously mentioned. Always copy the part number and google it. always worth it to save some money. The company selling this switch is dependable. We buy from them a lot and they buy from us a lot too. This is just if the ones you mentioned won't work. I'll let someone else be the judge of that.
http://www.ebay.com/itm/OHMITE-RHS1...380624?hash=item4883e3a950:g:S0MAAOSwXeJYGPWq

 

[BobK]

Here's a few more rheostats I found on Amazon. Any of these workable for what we are trying to do?

The first one would do, but, be aware it is 6 inches long.
The second (100Ω) is too high
The third (2Ω) is too low

Bob

 

[(*steve*)]

An astable multivibrator can be used to create a simple PWM circuit.

You can also use a power transistor and a few simple components to make a simple linear regulator to limit the voltage to the fan.

You could also mehanically limit the airflow through the fan.

 

[NyeSpy]

Thanks for all of the input! I think I may go with that 6" bar rheostat. We'll have plenty of space on board the craft.

I did hear back from the event coordinators regarding the transistorized PWM. They stated that there could potentially be issues with any design involving transistors or circuits of any kind. In many cases, volunteers act as judges and our entry might be disqualified if they feel that the rules have been violated. It's a chance I'd rather not take considering the time, cost, and effort invested in this.

You folks are awesome!!!

 

[Leggyownz]

Well good luck to you and your students!

 

[JWHassler]

The requirements you cite are very restrictive, especially if batteries are to provide the lift.
Rheostats at those power levels are big, heavy and will run hot
Also: a 12-volt computer fan is almost certain to a brushless motor.
Can you use line power, perhaps with a very light tether/cord?

 

[NyeSpy]

The requirements you cite are very restrictive, especially if batteries are to provide the lift.
Rheostats at those power levels are big, heavy and will run hot
Also: a 12-volt computer fan is almost certain to a brushless motor.
Can you use line power, perhaps with a very light tether/cord?

The lift fan can get full power. It will be on a separate battery and we won't need to adjust the voltage for that. It is the "push" fan that needs to be adjustable. We expect that most contestants will vary the amount of thrust by mechanically varying to flow of air to/from the push fan. We hope by utilizing a resistor we can really dial in the thrust we need... whether we need to get to the finish line in 5 seconds or 25 seconds.

The only ICs allowed are any that might be built-in to a computer fan. They had to release an official statement a few months ago to clarify that.

Line power is not allowed. Batteries only, and we are capped at 9v. I had the R/C racing batteries and charger in storage and figured they would far outperform a commercial square 9v battery.

 

[davenn]

The rule states "Brushless motors and integrated circuits are not permitted"

so you have a real problem ... ALL computer fans are brushless

 

[NyeSpy]

so you have a real problem ... ALL computer fans are brushless

Brushless computer fans are OK also.

Sorry for all the confusion... I could have posted all of the rules and regulations, but wanted to shorten my post by two pages.

 

[davenn]

Brushless computer fans are OK also.

Sorry for all the confusion... I could have posted all of the rules and regulations, but wanted to shorten my post by two pages.

so why the contradiction .... you say they are not, then you say they are ... ....

 

[NyeSpy]

I cut and pasted from the original rule set which included the bit about brushless motors. In September, they released the following -

"Hovercraft 4.i. should read: "Brushless motors and integrated circuits are not permitted, unless they are an integral part of and are imbedded into a commerically available electronic or computer cooling fan." (9/12/16)"

Sorry for the confusion. I didn't really focus on the fans because we are all set on that front. We just need an effective and safe way to control the speed of one of them.