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Re: Mini variable Inverter?
Posted: Fri Nov 14, 2014 4:39 pm
by Lenp
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!
Re: Mini variable Inverter?
Posted: Fri Nov 14, 2014 10:53 pm
by 1moldmaker
- 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 2229 times
Re: Mini variable Inverter?
Posted: Sat Nov 15, 2014 5:56 am
by sghioto
Try adding resistor R3 as shown, that should protect the diodes and darlington base circuit. Maybe a 1K for starters
Steve G
Re: Mini variable Inverter?
Posted: Sat Nov 15, 2014 6:26 am
by dyarker
"...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,
Re: Mini variable Inverter?
Posted: Sat Nov 15, 2014 9:21 am
by 1moldmaker
Yes it is an inductive load, also I have the power resistors on 8 inch heat sinks.
Re: Mini variable Inverter?
Posted: Mon Nov 17, 2014 5:39 am
by dacflyer
as for variac not being isolated, add in the isolation transformer
