Stepping down voltage?

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lasershark1
Posts: 143
Joined: Sun Apr 22, 2012 1:52 am

Stepping down voltage?

Post by lasershark1 » Tue May 01, 2012 1:18 am

Anyone experienced with a good way to step down voltage? I was looking at a 24v high amp power supply but I need to get the voltage down to 18volts to run my Ryobi tool - not sure what happens if I pump 24v into it - I dont want to burn it up.

Is there an affordable way to step down that voltage?

Enraged
Posts: 230
Joined: Mon Apr 09, 2012 8:29 pm

Stepping down voltage?

Post by Enraged » Tue May 01, 2012 2:15 am

Why not just buy an 18v supply from eBay? I linked one in that other thread

DevinTX
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Location: Central Texas

Re: Stepping down voltage?

Post by DevinTX » Tue May 01, 2012 2:27 am

I'm just a noobie when it comes to electronics, but I found this...http://www.ehow.com/how_5794977_step-do ... ormer.html do you think this would work?

DevinTx
ShapeOko # 207 / Dual Y Drive / Acme Upgrade / Belt on outside / Working on expansion

Enraged
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Re: Stepping down voltage?

Post by Enraged » Tue May 01, 2012 3:38 am

I think so, but in all honesty, if you can afford the $30 on the new supply, it is the simplest solution.

lasershark1
Posts: 143
Joined: Sun Apr 22, 2012 1:52 am

Re: Stepping down voltage?

Post by lasershark1 » Tue May 01, 2012 3:48 am

Enraged wrote:Why not just buy an 18v supply from eBay? I linked one in that other thread
Because it's 10A - by my math, I could hit that 10A... 5.6A from the Ryobi, and close to 5.5 from the steppers if all running under load... thats jut over 10. While it SHOULD be safe I was hoping to find something with a higher amp rating so I didnt pop the power supply in the middle of a long, intense job.

18v power supply search on ebay only shows up with 3, 6, and 10A models.

Enraged
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Re: Stepping down voltage?

Post by Enraged » Tue May 01, 2012 5:22 am

I was under the impression the supply would drive the spindle only, sorry

calica
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Re: Stepping down voltage?

Post by calica » Tue May 01, 2012 7:03 am

I would NOT recommend following the suggestions on the link DevinTX provided. The current consumed by a DC brushed motor will vary widely based on the load placed on it. As the current changes, the voltage drop across the resistors will vary as well.

I assume you're going to take your Speedsaw apart to make a direct connection for the power supply. If you are, could you see what motor it uses? It will likely be RSXXX. Once we know the XXX we can look up proper motor specs and know what the stall and no-load currents are. I doubt the 5.6A value. This type of motor is often used in 18V drills and something similar is probably used here. The current range is 2.7-130 amps.

minorthreat
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Location: Boston, Ma

Re: Stepping down voltage?

Post by minorthreat » Tue May 01, 2012 7:14 pm

I measured the 5.6A draw @20V under no load... under normal cutting conditions it'll need a few more amps, that being said, if you're approaching the stall current and need 200+ amps to keep it going you are doing something very very wrong.

I would suggest using a motor controller to control the speed off of the a 24v supply. 10-15A peak should give you enough margin to cut through most things.

lasershark1
Posts: 143
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Re: Stepping down voltage?

Post by lasershark1 » Tue May 01, 2012 8:10 pm

Well here's a question - this motor runs off an 18v battery. Will using a motor driver (pololu 18v15 driver, just ordered today) with a 24v supply be ok as long as I control the speed at reasonable levels? Or should I really have an 18v source?

How does a motor driver control the speed? By varying the voltage? Or pulsing the full voltage through?

minorthreat
Posts: 43
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Location: Boston, Ma

Re: Stepping down voltage?

Post by minorthreat » Tue May 01, 2012 8:36 pm

The pololu 18v15 board supports 5.5- 30 V voltage range and can deliver a continuous 15 A without a heat sink. So that will be fine @24v.

The motor driver effectively controls the voltage by switching on/off at the PWM signal's frequency (up to 40kHz). The ryobi will be fine through the entire voltage range. Don't forget to solder the large caps across the power supply pads when the board comes in.

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