Isssues with power supply

Thread Starter

ConstructionK88

Joined Jul 25, 2018
282
I'm using a 110vac to 19vdc 2.1amp supply to power 3 hair dryer fans for a forge air supply. The issue is when I try to quickly increase or decrease the output through my lm317 voltage controller the fans begin to pulsate slowly instead of blowing. I can mitigate this by very slowly adjusting them, although this isn't a practical way to use it. Previously I didn't have this issue but I accidentally destroyed that controller and I don't believe over heating is an issue. What would cause this?
 

Papabravo

Joined Feb 24, 2006
17,240
I'm using a 110vac to 19vdc 2.1amp supply to power 3 hair dryer fans for a forge air supply. The issue is when I try to quickly increase or decrease the output through my lm317 voltage controller the fans begin to pulsate slowly instead of blowing. I can mitigate this by very slowly adjusting them, although this isn't a practical way to use it. Previously I didn't have this issue but I accidentally destroyed that controller and I don't believe over heating is an issue. What would cause this?
Your verbal description does not convey what is happening in the circuit. Can you provide a schematic diagram of how you are using the LM317?
It sounds like the transient demand changes as the fan comes up to speed.
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
282
My apologies. Circuit as shown with the exception of 2 being used in parallel since it can only handle 1.5amps. 2 seems to split the load well and prevents overheating and internal protection from activating. The circuit itself is correct. The only change is when quickly adjusting the pot; the fans pulse in approximately 3second intervals instead constantly turning. If I adjust slowly, very slowly this doesn't happen. But slowly adjusting defeats their intended purpose.
 

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Thread Starter

ConstructionK88

Joined Jul 25, 2018
282
Your verbal description does not convey what is happening in the circuit. Can you provide a schematic diagram of how you are using the LM317?
It sounds like the transient demand changes as the fan comes up to speed.
Previously I did only use 2 fans instead of the 3. Do I need to adjust the value of the electrolytic cap on the output or is this more likely the power supply simply can't keep up with demand?
 

Papabravo

Joined Feb 24, 2006
17,240
I did attach a schematic on a previous response. It may not have loaded. It is the standard TI diagram with 5k pot to adjust output voltage
It can be hard to recognize things that are hard to see. Schematics from books are often missing important details like what output voltage you are using and how much current each fan requires. they are schematics it is true, but they are really not much help. It sounds like the part cannot do what you are asking it to do. with a significant difference between the input and output voltage the part will dissipate a large amount of heat and my be going into thermal shutdown or thinking there is a short circuit.
 
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Thread Starter

ConstructionK88

Joined Jul 25, 2018
282
Ahhhh AAC doin something I didn't approve of. But the circuit I built is the standard TI with 5kpot. Exception of using 2 lm317s in parallel to split the load. I verified both before and after soldering. It works well with 1 or 2 fans. It works but only if adjusted very slowly and that has me confused
 

crutschow

Joined Mar 14, 2008
28,158
So how much current do the fans take?
If the total is more than about 1.5A (depending upon the input-output voltage difference), the LM317 will go into current-limit mode, which would likely cause the behavior you are seeing.
 

dendad

Joined Feb 20, 2016
3,939
It does sound like the start up current is too high for the regulator.
You may do better to use a switch mode regulator in place of the linear one.
This is a 3A max one, but higher power units are readilly available for a few $$$.
LM2596Buck.jpg
Either way, a higher current regulator will be needed I think.
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
282
@crutschow @dendad to clarify this has worked well before. I can guess that the extra fan is demanding more current than is available but shouldn't that mean that it wouldn't work at all instead of working if it's slowly adjusted? Also I can't tell for sure how much the amp draw is. These motors have no easily discernable markings. They do run easily on nearly any voltage and even down to <5v@150ma that I've noticed. I had thought heating was an issue but I can absolutely rule that out.
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
282
I feel I should add it does pulsate regardless when directly connected to power supply. Each fan separately works great with a direct connection. The lm317s are allowing all 3 to work without pulsating but ONLY if I slowly increase/decrease power
 

Papabravo

Joined Feb 24, 2006
17,240
I feel I should add it does pulsate regardless when directly connected to power supply. Each fan separately works great with a direct connection. The lm317s are allowing all 3 to work without pulsating but ONLY if I slowly increase/decrease power
I don't foresee that we can be much help to you in figuring this problem out. I've tried, but I'm done.
 

dendad

Joined Feb 20, 2016
3,939
Most motors draw quite a lot more current to start up than they do when running. so the power supply probably overloads when the fans are turned on, the fans start turning but the excess current causes the regulator to current limit, dropping the voltage, the fans slow and when the current drops, the reg tries again.....
So, as has been mentioned a number of times, use a bigger power supply so the current will not be a problem.
 

dendad

Joined Feb 20, 2016
3,939
Do you have a multimeter?
If so, look at the power supply voltage to see if it is pulsing.
These are very good value. Not the cheapest, but excelent devices...
AN8002.jpg
There are a few in the family.
 

crutschow

Joined Mar 14, 2008
28,158
The lm317s are allowing all 3 to work without pulsating but ONLY if I slowly increase/decrease power
A motor draws a large current when the load is increased (as when the fan speeds up) so it's not surprising that the LM317 may limit the current, causing the observed pulsations, if it goes in and out of current limit when the voltage is rapidly increased but not when it is slowly increased.
But without knowing how much the fans actually draw (do you not have a multimeter?) we can only guess as to the problem.

Adding another LM317 in parallel, or a boost transistor added to one LM317 would likely solve the problem.
 
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