Mini variable Inverter?

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
User avatar
Lenp
Posts: 1479
Joined: Thu Oct 26, 2006 8:11 pm
Location: Maryland
Contact:

Re: Mini variable Inverter?

Post by Lenp » Fri Nov 14, 2014 4:39 pm

Be aware that a variac DOES NOT ISOLATE the input from the output and as such can become a shock hazard.

Since you only need 24v at 5 amps that is only 96 watts! Why such a large unit?
You still should use a transformer to isolate and drop the voltage to 24, and at less than 100 watts ....try a room light dimmer!
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

1moldmaker
Posts: 53
Joined: Sat Nov 08, 2014 4:38 pm
Contact:

Re: Mini variable Inverter?

Post by 1moldmaker » Fri Nov 14, 2014 10:53 pm

ADJ.GIF
This is the circuit that I am using now, how this works is R2 acts like a governor and R1 adjust the voltage, problem is the circuit is rather sensitive and I am either blowing out the 1n914 diodes or the darlington transistors. Also The darlington transistors get super hot I have them on a heat sink with a fan blowing on them.
ADJ.GIF (9.09 KiB) Viewed 1113 times

sghioto
Posts: 326
Joined: Wed Dec 26, 2007 5:16 pm
Location: Colorado
Contact:

Re: Mini variable Inverter?

Post by sghioto » Sat Nov 15, 2014 5:56 am

Try adding resistor R3 as shown, that should protect the diodes and darlington base circuit. Maybe a 1K for starters

Steve G
Attachments
24 volt variable supply.jpg

dyarker
Posts: 1759
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Re: Mini variable Inverter?

Post by dyarker » Sat Nov 15, 2014 6:26 am

"...blowing...1N914 diodes..." - is the load inductive? With so many in series the sudden transistor cut-off (~4V) on the falling side of each half cycle of AC could be causing a high voltage spike. Why so many 1N914s? Or, is the circuit somehow exceeding the max current of the diodes?

"...transistors get super hot..." - they are going to generate heat because they are being used as linear amplifiers not as switches. Are the heat sinks large enough for the power dropping on the transistors? Transistors correctly mounted on heat sinks?

Cheers,
Dale Y

1moldmaker
Posts: 53
Joined: Sat Nov 08, 2014 4:38 pm
Contact:

Re: Mini variable Inverter?

Post by 1moldmaker » Sat Nov 15, 2014 9:21 am

Yes it is an inductive load, also I have the power resistors on 8 inch heat sinks.

User avatar
dacflyer
Posts: 4578
Joined: Fri Feb 08, 2002 1:01 am
Location: USA / North Carolina / Fayetteville
Contact:

Re: Mini variable Inverter?

Post by dacflyer » Mon Nov 17, 2014 5:39 am

as for variac not being isolated, add in the isolation transformer :P

Post Reply

Who is online

Users browsing this forum: Google [Bot] and 44 guests