Low Voltage Inverter For OEM Device

[tecspm]

Anyone have experience with any suppliers of either custom or off the shelf low votage inverters? We are in need of an inexpensive 12 vdc to 24 vac inverter in the 75 watt output range to be powerewd from a compact rechargeable 12vdc power pack.

Bob Smith
Operations/Engineering
Northeast Building Controls

 

[cbarn24050]

What waveshape do you need? How many do you need? On the face of it it seems pretty simple.

 

[sreid]

Look at Xantrac products such as

http://www.xantrex.com/web/id/179/p/1/pt/32/product.asp

 

[tecspm]

This device will power a portable digital controller so true sine wave will be preferred but modified sine could possibly be tolerated(never tested). Also I would like to keep THD as low as possible, less than 4 % would be acceptable(again not tested only manufacturer recommended). Because of size and weight the scenario of a 12 vdc to 120vac common inverter coupled to a 120 vac to 24 vac step down transformer is not acceptable. I have done exhaustive research on the web and thru our dealer network and cannot find the specific voltages I require (other than custom built) at a reasonable price. Once developed this device would be offered to a restricted market of HVAC DDC application engineers/technicians thru professional networks, not the general public. (There really wouldn't be a general use for it!) Maybe someone would have advice on modifying an off the shelf 12vdc to 120vac inverter for this use.

 

[nbucska]

Why sinewave ? it would be significantly less efficient.

 

[tecspm]

Sine Wave recommended by manufacturer. Probably because this is out of the realm of normal use of the controller. I am not a circuit designer but find it hard to beleive this is the first time a power supply of this type is required. Considering the amount of 24VAC devices used in the HVAC controls market, we cannot be the first with a need to power these from a portable source. I would definately appreciate any information on modifications to a standard 12vdc to 120vac inverter.

 

[itsmoked]

While often bucking the norm myself I would say maybe you should step back and look at the problem again..

Why would a controller needing less than 75W need AC?
Does it output 24Vac? Does it have a motor in it? It probably doesn't have weird AC logic inside it. Internally it requires DC. It probably rectifies the 24VAC immediately. Why not feed it with 24Vdc? 12V to 24Vdc step up power supplies are as common as pennies.

 

[tecspm]

Great advice, but the problem continues.....
The controller utilizes 10 watts internally and the remainder is for driven loads.The controller does immediately rectify to 24 vdc, also 5.12 vdc, sources both 0-10vdc and 0-20 ma dc analog outputs plus drives hot switched triacs. Additionaly any peripheral devices controlled by the device require 24 vac. We do not want to modify the controller used because it is already available to over 120 of our brother dealers worldwide (our intended market). Maybe I just have to give in to the weight and space problem and utilize a 12vdc to 120vac inverter, available locally at about $19 US for 75 watt and drive a 120 vac to 24 vac transformer available for around $15 US. Thanks to all who reply. Please keep up the attempt at a solution. Your advice is greatly appreciated.

 

[cbarn24050]

You could use a bridge amplifier/oscillator, in reasonable volume you should be able to get those price figures but efficiency is low <50% which could be a problem. A switcher would be better but more expensive.

 

[ScottyUK]

Another possibility worth considering is to locally generate a sinewave and drive an audio amp IC feeding a step-up transformer. There are quite a few ICs which can meet your power requirements - many are intended for the car radio market, so you might need to drop the 24V supply a little to 15V or 18V, but that is a trivial requirement.

There are few ways to generate the sine too: function generator IC; EPROM & DAC; Wein bridge; etc. The method you use will depend on the accuracy you need. The function generator IC might be the best option since you will need neither crystal frequency precision nor low THD.

If you are worried about efficiency, a class D amplifier design would be a little more complex but have much better efficiency.


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I don't suffer from insanity. I enjoy it...

 

[zeitghost]

I've just spent an interesting (?) couple of hours generating sinewaves using a PIC16F84, a DAC0800, and a 741.

Gives nice (ish) sinewaves which are crystal controlled.

 

[waross]

A UPS with a small 24 volt transformer plugged into it will probably give you a good proof of concept. If it is found practicable you may be able to tap the power transformer for 24 volts. In North America there are always UPSs on sale somewhere at very good prices.
yours