Amperage Specifications of a Radio Shack Transformer - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: August 18, 2016, 04:15:57 am
content: Background: I found a Radio Shack, Model MPA-200 Amplifier in the trash. All the electronics were a loss, except for the transformer. I am having trouble finding a data sheet for it. The Radio Shack part number is 11097052. The "NP" part number is TA0779. The EI number is also given; EI-114, which I think is the core size.Requests:- Mostly, I just need the amperage ratings for the two secondaries. I would even take a best-guess-value. The main secondary outputs 79v and is center-taped; 39.5v each side. The other secondary outputs 6.5v.- A data sheet would be great, if anyone knows where to find it.- What is a "NP" number?I did my homework, but found very little on this transformer, other than the Radio Shack parts list information. Testing, or calculating, the amperage rating seems to be beyond a DIYer's abilities.Thank you for your help.

author: whitevamp
author_rank: Regular Contributor
timestamp: August 18, 2016, 04:27:48 am
content: Quote from: t1d on August 18, 2016, 04:15:57 ami be-leave its.National part number.

author: N2IXK
author_rank: Frequent Contributor
timestamp: August 18, 2016, 06:11:16 pm
content: Good luck. This was likely a custom made transformer, manufactured to spec for Tandy. Unless you can somehow get hold of their internal design documents, you will not get a datasheet on this transformer.You can always come up with a fair estimate of a transformer's power rating based on the size/weight of the iron core. Formula can be found in older editions of the ARRL handbook, IIRC.Another "rough guesstimate" can be made by applying a variable load to the secondary, until a drop of 10-15% in secondary voltage is seen compared to the unloaded voltage.

author: rsjsouza
author_rank: Super Contributor
timestamp: August 18, 2016, 07:12:39 pm
content: Some documents from Radio Shack are still available, including the parts list.http://support.radioshack.com/support_audio/14637.htmLooking at the specifications and the parts list you can guesstimate a few parameters:1) The primary is most probably a single voltage 120V (no 220/240V selector)2) The sustained amplifier outputs a maximum of 120W (4 ohms). If the amplifier was ideal, the transformer would be 120W (zero loss) - however, if you consider the losses on the circuit are between 30~50% (the latter being more realistic), the transformer must sustain 170~240W. That yields a primary current of 1.4 to 2A, which is under the 6A fuse.3) The filter capacitor is rated to 63V, which must be enough to sustain the peak voltage of the rectified output. If you consider an output DC voltage of 60V, the Vpeak= VRMS* 1.414. VRMS= 42V.4) The VRMSabove is at the output of the rectifier, which drops about 1.2V from the secondary of the transformer. In this case, the Vsecondary= 43.2V. That is a somewhat unusual value, which could be a bit high (40 or 42V could be a more realistic value)5) Considering the power in 2) above, the output current would range anywhere from 4A (170W/43.2V) to 6A (240W/40V).6) These current values are compatible with the CM1002 rectifier bridge, which is rated for 10A.In general these linear power supplies don't fall much further from these typical calculations, unless the original designer applied huge safety margins to the components themselves.One reference for calculations:http://www.bristolwatch.com/ele/basic_ac_rectification.htmThe ultimate test is to have a Variac, a few meters and apply power to the transformer gradually and evaluate its parameters.http://pe.org.pl/articles/2012/9a/3.pdf