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Old 29th Mar 2024, 2:33 pm   #1
Syrinx1
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Default Suitable Schottky Diode for DC motor?

Hi,

I decided to buy one of these (see picture) with the intention of making a homebrew vacuum desoldering iron.

The motor is rated for 12v at approximately 4amps. As a rule of thumb would a 10amp 24V Zener across the terminals be sufficient to bleed off the flyback voltage when the motor is turned off?

Thanks
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Old 29th Mar 2024, 6:37 pm   #2
kalee20
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Default Re: Suitable Schottky Diode for DC motor?

I'm not quite sure what you're trying to achieve!

If you have a switch in series with the 24V supply and you switch it off, you'll get a brief negative-going inductive spike (which you can clamp with a conventional diode connected anti-parallel with the motor if you really want to save your switch contacts) and this will settle to a positive voltage of nearly 24V, reducing to zero in a few hundred milliseconds as the motor runs down.

No Zener or Schottky diode is necessary!
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Old 29th Mar 2024, 10:24 pm   #3
Ed_Dinning
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Default Re: Suitable Schottky Diode for DC motor?

Hi for fastest dissipation of the turn off spike a series resistor the same value as the motor resistance (a few ohms) will work , but as Peter says, if operation is by a switch and no semiconductors are involved, why bother?

Ed
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Old 30th Mar 2024, 9:59 am   #4
Syrinx1
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Default Re: Suitable Schottky Diode for DC motor?

Hi Chaps,

I'm probably getting my knickers in a twist here. I was thinking in terms of running the motor with PWM, so was concerned about the power dissipation in a slower anti parrelel diode with a higher forward voltage drop at higher frequencies.

Thanks,
Jonathan
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Old 30th Mar 2024, 11:47 am   #5
factory
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Default Re: Suitable Schottky Diode for DC motor?

The Pace ones use a diode on the pump motor, not sure which it is, they also start at a higher voltage of 29V then run at 19V, no PWM used, I can't remember the voltage of the motor they used, but I've had two fail open circuit at work after years of use.

The vacuum pump you've found is interesting, the first Pace I bought for myself, the vacuum pump had been butchered by the previous owner/user, the genuine replacement motor/pump assembly is way too expensive for me.

Does it really use 4A at 12V? would be interesting to know as some of the ratings seem excessive on some I've just looked at, from various places selling them online.

David
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Old 30th Mar 2024, 1:18 pm   #6
kalee20
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Default Re: Suitable Schottky Diode for DC motor?

Quote:
Originally Posted by Syrinx1 View Post
I'm probably getting my knickers in a twist here. I was thinking in terms of running the motor with PWM, so was concerned about the power dissipation in a slower anti parrelel diode with a higher forward voltage drop at higher frequencies.
Ah. That makes sense now.

Almost any 40V Schottky diode rated at a few amps would do - I'd reach for an 80SQ040 (8A 40V wire-ended).
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Old 30th Mar 2024, 1:53 pm   #7
Syrinx1
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Default Re: Suitable Schottky Diode for DC motor?

Thanks for the advice.

Regarding the motor ratings, they may well have been somewhat exaggerated.

The pump cost less than £20 delivered so it's not the end of the world if it doesn't quite meet the advertised specification
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Old 30th Mar 2024, 2:26 pm   #8
Syrinx1
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Default Re: Suitable Schottky Diode for DC motor?

Just found a few STPS1545D which should fit the bill. Not as convenient as they are TO220 package, but given they are 15amp rating I'm assuming they'll be ok without heatsinking.
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